Sun Microsystems Java 5.0 Java programming language Technical documentation
Below you will find brief information for Java programming language Java 5.0. This document covers the fundamental concepts of Java programming, including data types, variables, arrays, operators, control flow statements, classes, inheritance, packages, interfaces, exception handling, multithreading, string handling, I/O, and various API classes. It provides a comprehensive overview of the Java technology and its applications.
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Index
Index ................................................................................................................................................
1
Chapter 1 : Java Technology ........................................................................................................
7
1.8 Why Java is important to Internet?
..................................................................................
1.15 Application Structure and Elements
Chapter 2 : Data types, variables and Arrays ...........................................................................
20
2.5 Rules for naming an identifier
2.8 Declaring constants – Final variables
Chapter 3 : Operators .................................................................................................................
33
3.2.2 Increment and Decrement Operators
1
Chapter 4 : Control flow statements ..........................................................................................
57
4.1 The while and do-while Statements
Chapter 5 : Class Fundamentals and OOP ...............................................................................
73
5.8 Declaring Variables in a Class
5.9 Instance and Class Members
.........................................................................................
............................................................................................
2
5.12 Pass by value and Pass by reference
..............................................................................
Chapter 6 : Inheritance .............................................................................................................
128
Chapter 7 : Packages and Interfaces ........................................................................................
158
Chapter 8 : Assertions and Exception handling .....................................................................
175
3
8.9 Creating Your Own Exception Classes
Chapter 9 : Multithreaded programming ...............................................................................
206
.................................................................................................
9.8 Using the Thread yield method.
.....................................................................................
Chapter 10 : String Handling ...................................................................................................
237
Chapter 11 : I/O .........................................................................................................................
259
11.7 Scanning text with java.util.Scanner
............................................................................
11.9 Working with Random Access Files
...........................................................................
4
Chapter 12 : API classes in java.lang package ........................................................................
292
Chapter 13 : Utility & Legacy classes ......................................................................................
314
...............................................................................................
13.17 Timer and TimerTask classes
Chapter 14 : Regular Expression Processing ..........................................................................
340
14.1 What is a Regular Expression?
....................................................................................
Chapter 15 : API classes in java.text
........................................................................................
346
5
Chapter 16 : Collections Framework and Generics ...............................................................
358
...................................................................................................
16.3 What Is a Collections Framework?
..............................................................................
Answers .......................................................................................................................................
402
6
Chapter 1 : Java Technology
1.1 History of Java
Around 1990 James Gosling , Bill Joy and others at Sun Microsystems began developing a language called Oak. They wanted it primarily to control microprocessors embedded in consumer items such as cable set-top boxes,VCR's, toasters, and also for personal data assistants
(PDA).
To serve these goals, Oak needed to be: o o o
Platform independent (since multiple manufacturers involved)
Extremely reliable
Compact.
However, as of 1993, interactive TV and PDA markets had failed to take off. Then the Internet and Web explosion began, so Sun shifted the target market to Internet applications and changed the name of the project to Java.
By 1994 Sun's HotJava browser appeared. Written in Java in only a few months, it illustrated the power of applets, programs that run within a browser, and also the capabilities of Java for speeding program development.
Riding along with the explosion of interest and publicity in the Internet, Java quickly received widespread recognition and expectations grew for it to become the dominant software for browser and consumer applications.
However, the early versions of Java did not possess the breadth and depth of capabilities needed for client (i.e. consumer) applications. For example, the graphics in Java 1.0 seemed crude and clumsy compared to mature software developed with C and other languages.
Applets became popular and remain common but don't dominate interactive or multimedia displays on web pages. Many other "plug-in" types of programs also run within the browser environment.
So Java has not succeeded at development of consumer applications. However, Java's capabilities grew with the release of new and expanded versions and it became a very popular language for development of enterprise, or middleware, applications such as on line web stores, transactions processing, database interfaces, and so forth.
Java has also become quite common on small platforms such as cell phones and PDAs. Java is now used in several hundred cell phone models. Over 600 million JavaCards, smart cards with additional features provided by Java, have been sold as of the summer of 2004.
7
1.2 What is Java?
The term Java actual refers to more than just a particular language like C or Pascal. Java encompasses several parts, including :
•
•
•
A high level language – the Java language is a high level one that at a glance looks very
similar to C and C++ but offers many unique features of its own.
Java bytecode - a compiler, Sun's
javac
, transforms the Java language source code to bytecode that runs in the JVM.
Java Virtual Machine (JVM) – a program, such as Sun's
java
, that runs on a given platform and takes the bytecode programs as input and interprets them just as if it were a physical processor executing machine code.
Sun provides a set of programming tools such as javac, java and others in a bundle that it calls a
Java Software Development Kit for each version of the language and for different platforms such as Windows, Linux, etc.. Sun also provides a runtime bundle with just the JVM when the programming tools are not needed.
Note that because of the open nature of Java any or all of these parts can be replaced by non-Sun components. For example, just as many different languages can create machine code for a given processor, compilers of other languages have been created that output bytecode to run in the
JVM. Similarly, many JVMs have been written by groups outside of Sun.
Java, Open or Closed?
Java is not quite an open language but not quite a proprietary one either. All the core language products - compiler, virtual machines (VM), class packages, and other components - are free.
Detailed specifications and source code are made openly available.
The Java Community Process (JCP) leads the development of new standards for the language.
Other companies and organizations can legally create a clean sheet compiler and/or a Virtual
Machine as long as it follows the publicly available specifications. Microsoft did this with the
Version 1.1 JVM that it used in its Internet Explorer browser.
Sun, however, does still assert final say on the specifications and controls the copyrights to logos, and trademarks.
1.3 Versions of Java
Since its introduction, Sun has released a new version of the Java language every two years or so. These new versions brought enhancements, new capabilities and fixes to bugs. Until recently, the versions were numbered 1.x, where x reached up till 4. (Intermediate revisions were labeled with a third number - 1.x.y - as in 1.4.2.) The newest version, however, is called
Java 5.0 rather than Java 1.5.
8
Below is a timeline of the different versions of the basic, or Standard Edition (SE), of Java along with some of the new features that each one introduced. This edition contains the core language packages (the name for code libraries in Java) and is aimed for desktop programming.
•
•
•
•
•
•
1995 : Version 1.0 of the Java Development Kit (JDK) was released for free by Sun.
o o o
8 packages with 212 classes
Netscape 2.0-4.0 included Java 1.0.
Microsoft and other companies licensed Java.
1997 : Version 1.1: o o
23 packages - 504 classes
Improvements include better event handling, inner classes, improved JVM. o o o
Microsoft developed its own 1.1. compatible Java Virtual Machine for the
Internet Explorer.
Many browsers in use are still compatible only with 1.1.
Swing packages of greatly improved graphics became available during this time but not included with the core language.
1999: Version 1.2
, also called the Java 2 Platform o o o
59 packages - 1520 classes
Code and tools distributed as The Software Development Kit (SDK)
Java Foundation Classes (JFC), based on Swing, for improved graphics and user o interfaces, now included with the core language.
Collections API included support for various lists, sets, and hash maps.
2000 : Version 1.3: o
76 packages - 1842 classes o
Performance enhancements including the Hotspot virtual machine.
2002 : Version 1.4: o o
135 packages - 2991 classes
Improved IO, XML support, etc.
2004: Version 5.0 (previously numbered 1.5): o o
165 packages, over 3000 classes
Faster startup and smaller memory footprint o o o o
Metadata
Formatted output
Generics
Improved multithreading features
Other Editions of Java
In the late 1990s, Sun split off two other more specialized branches, or editions, of Java. One is aimed at small, embedded applications and the other for large scale middleware applications:
Micro Java
9
Embedded systems such as cell phones and device controllers typically offer reduced resources as compared to desktop PCs. This means substantially less disk space or no disk at all, and less of other types of nonvolatile memory. It also usually means a smaller display or perhaps no display at all.
For such systems Sun offers slimmed down versions of Java.
JavaCard - extremely limited Java for systems with only 16kb nonvolatile memory and
512 bytes volatile
EmbeddedJava - based on Java 1.1 for 32 bit system with about 512kb each for ROM
and RAM. Individual packages, classes and even methods in the core language can be thrown out to make room.
PersonalJava - based on Java 1.1.8 for larger systems with 2MB ROM and more than
1MB RAM.
Java 2 Platform, Micro Edition (J2ME) - based on the Java 2 Platform, J2ME replaces
the Java 1.1 based systems (EmbeddedJava and PersonalJava but not JavaCard). The developer will choose from different configurations to suit the capacity of a given system.
J2EE - Java 2 Platform, Enterprise Edition
With the Java 2 Platform came a separate version with enhanced resources targeted at enterprise applications. The Java 2 Enterprise Edition now provides a wide array of tools for building middleware software such as for database access applications, online storefronts, and other services.
Naming Conventions
All of these editions and version numbers can be a bit confusing for newcomers. Also, terms change such as Java Development Kit becoming Software Development Kit. For this course, however, you can just use the latest version - Java 5.0 - and not worry about all these historical issues.
1.4 Code Compatibility
Sun has maintained good compatibility among codes written with the different versions.
Generally, the newer versions maintain compatibilty with older code. The approach has been to add new features without subtracting any older features. (In some cases, such as the event handling system introduced in Java 1.1, code using newer classes and techniques should not be mixed in the same program with older version code.)
10
Thus far, Java maintains backwards compatibility. A program written according to Java 1.0 will compile with a Java 5 compiler. (Though some obsolete methods will generate
"deprecation" warning messages from the compiler.) The bytecode from a Java 1.0 compiler will still run in a Java 5 virtual machine.
1.5 Java 5.0 Improvements
Java 2 Platform, Standard Edition 5.0 (J2SE 5.0) was launched as the official Java version by
Sun on September 30, 2004.
Most of the changes fall into the ease of development (EoD) category. With a few important exceptions, the changes do not add new functionality but rather provide an easier way of doing the same things you could do before but with less code and better compiler-time error detection.
The most important changes to the platform include the following:
Quality, Stability, and Compatibility
The designers of J2SE considered quality, stability, and compatibility to be the most important aspect of the new release. Release 5.0 is the most tested release ever. Great efforts were made to ensure compatibility with previous versions of Java. The Sun engineers made a public plea for users worldwide to test their code with the 5.0 Beta releases and to report any problems that appeared, especially any code that worked with earlier versions of Java but failed under 5.0.
Performance and Scalability
Faster JVM startup time and smaller memory footprint were important goals. These have been achieved through careful tuning of the software and use of class data sharing.
Ease of Development
It is in the EoD area that the most significant changes appear. In most cases, no new functionality was added in the sense that almost anything you can do with 5.0 you could do with
1.4, it just sometimes took a lot more boilerplate code (i.e. code that is repeated frequently) to do it. The exception to this general statement has to do with the new multithreading and concurrency features that provide capabilities previously unavailable.
In many cases, the new EoD features are all about syntax shortcuts that greatly reduce the amount of code that must be entered, making coding faster and more error free. Some features enable improved compile-time type checking, thus producing fewer runtime errors.
Monitoring and Manageability
The 5.0 release includes the ability to remotely monitor and even manage a running Java application. For example, it is now much easier to watch memory usage and detect and respond to a low-memory condition. Many of these features are built right in to the system, and you can add additional monitoring and managing features to your own code.
11
Improved Desktop Client
The last great theme of the 5.0 release was an improved experience on the desktop client. In addition to better performance because of a faster startup time and smaller memory footprint, there is a new, improved Swing look and feel called Ocean, and a new easy-to-customize skinnable look and feel called Synth in which you can use XML configuration files to specify the appearance of every visual component in the system. In addition, the GTK and XP look and feels introduced in J2SE 1.4.2 have received further improvements. There is support for
OpenGL and better performance on Unix X11 platforms. The Java Web Start and Java Plug-In technologies (both used to run Java applications downloaded over the Web) have been improved.
Other new features in J2SE 5.0 include core XML support, improvements to Unicode, improvements to Java's database connectivity package known as JDBC, and an improved, highcompression format for JAR files that can greatly reduce download times for applets and other networked applications.
1.6 Features of Java
The Java programming language is a high-level language that can be characterized by all of the following buzzwords:
Simple
Object oriented
Distributed
Interpreted
Robust
Secure
Architecture neutral
Portable
High performance
Multithreaded
Dynamic
With most programming languages, you either compile or interpret a program so that you can run it on your computer. The Java programming language is unusual in that a program is both compiled and interpreted. With the compiler, first you translate a program into an intermediate language called Java bytecodes —the platform-independent codes interpreted by the interpreter on the Java platform. The interpreter parses and runs each Java bytecode instruction on the computer. Compilation happens just once; interpretation occurs each time the program is executed. The following figure illustrates how this works.
12
You can think of Java bytecodes as the machine code instructions for the Java Virtual Machine
(Java VM). Every Java interpreter, whether it's a development tool or a Web browser that can run applets, is an implementation of the Java VM.
Java bytecodes help make "write once, run anywhere" possible. You can compile your program into bytecodes on any platform that has a Java compiler. The bytecodes can then be run on any implementation of the Java VM. That means that as long as a computer has a Java VM, the same program written in the Java programming language can run on Windows 2000, a Solaris workstation, or on an iMac.
Java was designed to be easy for the professional programmer to learn and use effectively.
Java frees you from having to worry about many of the most common causes of programming errors.
Because Java is a strictly typed language, it checks your code at compile-time. It also checks the code at run-time.
Java virtually eliminates the memory access by managing memory allocation and deallocation.
In fact, deallocation is completely automatic, because Java provide Garbage Collection for unused objects.
Java supports multithreaded programming which allows you to write programs that do many things simultaneously. The Java runtime system comes with an elegant yet sophisticated
13
solution for multiprocess synchronization that enables you to construct smoothly running interactive systems.
Operating system upgrades, processor upgrades and changes in core system resources can all combine to make a program malfunction. The Java designers made several hard decisions in the
Java language and Java Virtual Machine in an attempt to alter this situation, thus making Java architecture neutral.
Java is designed for the distributed environment of the Internet, because it handles TCP/IP protocols. Accessing a resource using a URL is not much different from accessing a file.
Java supports dynamic programming where small fragments of bytecode may be dynamically updated on a running system.
1.7 Java Applets and Applications
The most common types of programs written in the Java programming language are applets and
applications. An applet is a program that adheres to certain conventions that allow it to run within a Java-enabled browser. An applet is an application designed to be transmitted over the
Internet and executed by a Java-compatible Web browser.
An application is a standalone program that runs directly on the Java platform. A special kind of application known as a server serves and supports clients on a network. Examples of servers are
Web servers, proxy servers, mail servers, and print servers. Another specialized program is a
servlet. A servlet can almost be thought of as an applet that runs on the server side. Java
Servlets are a popular choice for building interactive web applications, replacing the use of CGI scripts. Servlets are similar to applets in that they are runtime extensions of applications.
Instead of working in browsers, though, servlets run within Java Web servers, configuring or tailoring the server.
1.8 Why Java is important to Internet?
Java provides a firewall between a networked application and your computer. When you use a
Java-compatible web browser, you can safely download Java applets without fear of viral infection or malicious intent. Java achieves this protection by confining a Java program to the
Java execution environment and not allowing it access to other parts of the computer. Another reason why Java is important to Internet is due to its portability i. e. Write once run anywhere feature.
With Java technology, the Internet and private networks become your computing environment.
Coupled with the power of networking, the Java platform is helping computer users to do things that were previously unimaginable. For example, users can securely access their personal information and applications when they're far away from the office by using any computer that's connected to the Internet; soon they'll be able to access tailored applications from a mobile phone based on the Java platform, or even use smart cards as a pass key to everything from the cash machine to ski lifts.
14
Why Java technology? Networks require software that is portable, modular, and secure -- all areas where Java technology shines, because it was designed for use on networks from the beginning.
Businesses are using Java technology because it connects easily to existing computing systems, lowers computing costs, and speeds software development. It also lets businesses use the
Internet to securely connect to their customers, suppliers and partners.
And consumers benefit from Java technology because it brings personal, business, and entertainment services to them -- easily and securely -- in many locations and on many different kinds of appliances and devices at home, at work and on the road.
1.9 The Java Platform
A Java platform is the software environment in which a program runs.
The Java platform has two components:
•
•
The Java Virtual Machine (Java VM)
The Java Application Programming Interface (Java API)
The compiler, javac
, takes your source file and translates its text into instructions that the Java
Virtual Machine (Java VM) can understand. The compiler converts these instructions into a bytecode file.
The Java interpreter installed on your computer implements the Java VM. This interpreter takes your bytecode file and carries out the instructions by translating them into instructions that your computer can understand.
The Java compiler and interpreter are case-sensitive.
The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.
The following figure depicts a program that's running on the Java platform. As the figure shows, the Java API and the virtual machine insulate the program from the hardware.
15
Native code is code that after you compile it, the compiled code runs on a specific hardware platform. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability.
1.10 What Can Java Technology Do?
Every full implementation of the Java platform gives you the following features:
•
•
•
•
•
•
•
•
The essentials: Objects, strings, threads, numbers, input and output, data structures,
system properties, date and time, and so on.
Applets: The set of conventions used by applets.
Networking: URLs, TCP (Transmission Control Protocol), UDP (User Datagram
Protocol) sockets, and IP (Internet Protocol) addresses.
Internationalization: Help for writing programs that can be localized for users
worldwide. Programs can automatically adapt to specific locales and be displayed in the appropriate language.
Security: Both low level and high level, including electronic signatures, public and
private key management, access control, and certificates.
Software components: Known as JavaBeans
TM
, can plug into existing component architectures.
Object serialization: Allows lightweight persistence and communication via Remote
Method Invocation (RMI).
Java Database Connectivity (JDBC
TM
): Provides uniform access to a wide range of
relational databases.
The Java platform also has APIs for 2D and 3D graphics, accessibility, servers, collaboration, telephony, speech, animation, and more.
Java 2 Runtime Environment (JRE) consists of the virtual machine, the Java platform core classes, and supporting files. The Java 2 SDK includes the JRE and development tools such as compilers and debuggers.
Applications are standalone programs.
1.11 Writing a Program
The easiest way to write a simple program is with a text editor. So, using the text editor of your choice, create a text file with the following text, and be sure to name the text file
ExampleProgram.java. Java programs are case sensitive, so if you type the code in yourself, pay particular attention to the capitalization.
//A Very Simple Example class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
16
}
1.12 Compiling the Program
A program has to be converted to a form the Java VM can understand so any computer with a
Java VM can interpret and run the program
The Java compiler is invoked at the command line on Unix and DOS shell operating systems as follows:
javac ExampleProgram.java
1.13 Interpreting and Running the Program
Once your program successfully compiles into Java bytecodes, you can interpret and run applications on any Java VM, or interpret and run applets in any Web browser with a Java VM built in such as Netscape or Internet Explorer. Interpreting and running a Java program means invoking the Java VM byte code interpreter, which converts the Java byte codes to platformdependent machine codes so your computer can understand and run the program.
The Java interpreter is invoked at the command line on Unix and DOS shell operating systems as follows:
java ExampleProgram
At the command line, you should see:
I'm a Simple Program
1.14 Comments in Java
Code comments are placed in source files to describe what is happening in the code to someone who might be reading the file, to comment-out lines of code to isolate the source of a problem for debugging purposes, or to generate API documentation. To these ends, the Java language supports three kinds of comments: double slashes, C-style, and doc comments.
Double Slashes
Double slashes (//) are used in the C/C++ programming language, and tell the compiler to treat everything from the slashes to the end of the line as text.
//A Very Simple Example class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
17
C-Style Comments
Instead of double slashes, you can use C-style comments (/* */) to enclose one or more lines of code to be treated as text.
/* These are
C-style comments
*/ class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
Doc Comments
To generate documentation for your program, use the doc comments (/** */) to enclose lines of text for the javadoc tool to find. The javadoc tool locates the doc comments embedded in source files and uses those comments to generate API documentation.
/** This class displays a text string at
* the console.
*/ class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
With one simple class, there is no reason to generate API documentation. API documentation makes sense when you have an application made up of a number of complex classes that need documentation. The tool generates HTML files (Web pages) that describe the class structures and contain the text enclosed by doc comments.
1.15 Application Structure and Elements
An application is created from classes. A class is similar to a RECORD in the Pascal language or a struct in the C language in that it stores related data in fields, where the fields can be different types. So you could, for example, store a text string in one field, an integer in another field, and a floating point in a third field. The difference between a class and a RECORD or struct is that a class also defines the methods to work on the data.
For example, a very simple class might store a string of text and define one method to set the string and another method to get the string and print it to the console. Methods that work on the data are called accessor methods.
Every application needs one class with a main method. This class is the entry point for the program, and is the class name passed to the java interpreter command to run the application.
18
The code in the main method executes first when the program starts, and is the control point from which the controller class accessor methods are called to work on the data.
It has no fields or accessor methods, but because it is the only class in the program, it has a main method.
The public static void keywords mean the Java virtual machine (JVM) interpreter can call the program's main method to start the program (public) without creating an instance of the class
(static), and the program does not return data to the Java VM interpreter (void) when it ends.
Summary
In this chapter you learnt the history of Java and how the Java language has evolved. Java is not only a programming language it’s a platform. Java is important because of its cross-platform functionality. Also Java is a secure language. Java can be used for simple as well as complex applications. It can be used for embedded as well as mobile applications.
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Chapter 2 : Data types, variables and Arrays
2.1 Data Types
The Java programming language has two categories of data types: primitive and reference. A variable of primitive type contains a single value of the appropriate size and format for its type: a number, a character, or a boolean value.
Arrays, classes, and interfaces are reference types. The value of a reference type variable, in contrast to that of a primitive type, is a reference to (an address of) the value or set of values represented by the variable.
A reference is called a pointer, or a memory address in other languages. The Java programming language does not support the explicit use of addresses like other languages do. You use the variable's name instead.
Primitive Data Types
Keyword
byte short int long float double char boolean
Description
Byte-length integer
(integers)
Size/Format
Short integer
Integer
Long integer
8-bit two's complement
16-bit two's complement
32-bit two's complement
64-bit two's complement
(real numbers)
Single-precision floating point 32-bit IEEE 754
Double-precision floating point 64-bit IEEE 754
A single character
(other types)
A boolean value ( true
or false
)
16-bit Unicode character true or false
You can put a literal primitive value directly in your code.
int anInt = 4;
Range of primitive data types
Primitive Type Size Range of Values
byte 8 bit -2 7 to 2 7 -1 short int long char
16 bit -2
32 bit -2
64 bit -2
15
31
63
to 2
to 2
to 2
15
-1
31 -1
63 -1
16 bit
'\u0000'
to
'\uffff'
(0 to 2
16
-1 ) float 32 bit Max. positive value: (2-2 -23 )*2 127 . Min. positive value: 2 -149
20
double
2.2 Literals
64 bit Max. positive value: (2-2
-52
)*2
1023
. Min. positive value: 2
-1074
A specific primitive value in a line of code is called a literal. These will be translated into values by the compiler and inserted into the byte code.
Examples of Literal Values and Their Data Types
Literal Data Type
178 int
8864L long
37.266
double
37.266D
double
87.363F float
26.77e3
double char
' c
' true false boolean boolean
A series of digits with no decimal point is typed as an integer. You can specify a long integer by putting an 'L' or 'l' after the number. 'L' is preferred as it cannot be confused with the digit '1'. A series of digits with a decimal point is of type double. You can specify a float by putting an 'f' or
'F' after the number. A literal character value is any single Unicode character between single quote marks. The two boolean literals are simply true and false.
Constructing literal numeric values using octal and hexadecimal formats.
Octal literals begin with zero e.g. 013042 (and obviously only digits 0-7 are allowed).
Hexadecimal literals begin with zero and an 'x' e.g. 0x23e4A (digits allowed are 0-9 and a to f, the 'x' and the letters can be upper or lower case).
Construct a literal value of char type using Java's unicode escape format for a specified character code.
Use \u followed by four hexadecimal digits representing the 16 bit unicode character e.g.
char x='\u1234'
Java also supports certain escape codes for special characters such as '\n' for newline.
2.3 Java Character Encoding: UTF and Unicode
Java uses two closely related encoding systems UTF and Unicode. Java was designed from the ground up to deal with multibyte character sets and can deal with the vast numbers of characters that can be stored using the Unicode character set. Unicode characters are stored in two bytes
21
which allows for up to 65K worth of characters. This means it can deal with Japanese Chinese, and just about any other character set known.
Although Unicode can represent almost any character you would ever likely to use it is not an efficient coding method for programming. Most of the text data within a program uses standard
ASCII, most of which can easily be stored within one byte. For reasons of compactness Java uses a system called UTF-8 for string literals, identifiers and other text within programs. This can result in a considerable saving by comparison with using Unicode where every character requires
2 bytes.
2.4 EscapeSequences
\b
\t
\n
\f
\r
\"
\'
\\
OctalEscape
2.5 Rules for naming an identifier
/* \u0008: backspace BS */
/* \u0009: horizontal tab HT */
/* \u000a: linefeed LF */
/* \u000c: form feed FF */
/* \u000d: carriage return CR */
/* \u0022: double quote " */
/* \u0027: single quote ' */
/* \u005c: backslash \ */
/* \u0000 to \u00ff: from octal value */
When we learn to program we have to give names or identifiers to things we create such as files, classes, objects, variables and attributes (attributes could be numbers, words or even objects).
There are rules which govern what is allowable.
Identifiers must be chosen according to certain rules:
•
•
•
• they can contain letters, numbers, the underscore character ( _ ) or dollar character ($) they cannot start with a number they must not include spaces, other special characters or punctuation marks a Java keyword must not be used.
A convention that is sometimes adopted to make identifiers more readable is to use a capital letter to indicate the beginning of a new word, for example: cubeRoot, firstNumber, this is sometimes called camel hump notation.
Note that Java is case sensitive so A1 and a1 are different identifiers.
Naming Conventions
Class identifiers begin with a capital letter.
The filename for the class code is the same as the name of the class, e.g. the code for the class
JTRectangle is stored in the file JTRectangle.java.
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Object identifiers start with a lowercase letter.
Attributes identifiers start with a lowercase letter.
2.6 Java Language Keywords
true
, false
, and null
are not keywords but they are reserved words, so you cannot use them as names in your programs either.
abstract double boolean Else break extends byte case catch
Final finally
Float int strictfp
** interface super long switch native new package synchronized this throw char class
*
For goto
If
* private protected transient const public continue implements return throws try void default import do short instanceof static enum**** assert*** volatile while
* indicates a keyword that is not currently used
** indicates a keyword that was added for Java 2
*** new in J2SE 1.4
**** new in J2SE 5.0
2.7 Variables
A variabl e is an item of data named by an identifier. You must explicitly provide a name and a type for each variable you want to use in your program. The variable's name must be a legal
identifier --an unlimited series of Unicode characters that begins with a letter. You use the variable name to refer to the data that the variable contains. The variable's type determines what values it can hold and what operations can be performed on it. To give a variable a type and a name, you write a variable declaration, which generally looks like this:
type name
In addition to the name and type that you explicitly give a variable, a variable has scope.
Every variable must have a data type. A variable's data type determines the values that the variable can contain and the operations that can be performed on it. Integers can contain only integral values (both positive and negative). You can perform arithmetic operations, such as addition, on integer variables.
23
Variable names begin with a lowercase letter, and class names begin with an uppercase letter. If a variable name consists of more than one word, the words are joined together, and each word after the first begins with an uppercase letter, like this: isVisible
. The underscore character (
_
) is acceptable anywhere in a name, but by convention is used only to separate words in constants
(because constants are all caps by convention and thus cannot be case-delimited).
2.8 Declaring constants – Final variables
You can declare a variable in any scope to be final. The value of a final variable cannot change after it has been initialized. Such variables are similar to constants in other programming languages.
To declare a final variable, use the final keyword in the variable declaration before the type: final int aFinalVar = 0;
The previous statement declares a final variable and initializes it, all at once. Subsequent attempts to assign a value to aFinalVar
result in a compiler error. You may, if necessary, defer initialization of a final local variable. Simply declare the local variable and initialize it later, like this: final int blankfinal;
. . .
blankfinal = 0;
A final local variable that has been declared but not yet initialized is called a blank final. Again, once a final local variable has been initialized, it cannot be set, and any later attempts to assign a value to blankfinal
result in a compile-time error.
The following variable declaration defines a constant named
PI
, whose value is pi, the ratio of the circumference of a circle to its diameter (3.141592653589793) and cannot be changed: final double PI = 3.141592653589793;
By convention, the name of constant values are spelled in uppercase letters.
2.9 Arrays
An array is a structure that holds multiple values of the same type. The length of an array is established when the array is created (at runtime). After creation, an array is a fixed-length structure.
24
An array element is one of the values within an array and is accessed by its position within the array.
Declaring an Array
This line of code from the sample program declares an array variable: int[] anArray;
// declare an array of integers
Like declarations for variables of other types, an array declaration has two components: the array's type and the array's name. An array's type is written
type[]
, where
type
is the data type of the elements contained within the array, and
[]
indicates that this is an array. Remember that all of the elements within an array are of the same type. The sample program uses int[]
, so the array called anArray
will be used to hold integer data. Here are declarations for arrays that hold other types of data: float[] anArrayOfFloats; boolean[] anArrayOfBooleans;
Object[] anArrayOfObjects;
String[] anArrayOfStrings;
As with declarations for variables of other types, the declaration for an array variable does not allocate any memory to contain the array elements. The sample program must assign a value to anArray
before the name refers to an array.
Creating an Array
You create an array explicitly using Java's new
operator. The next statement in the sample program allocates an array with enough memory for ten integer elements and assigns the array to the variable anArray
declared earlier. anArray = new int[10];
// create an array of integers
In general, when creating an array, you use the new
operator, plus the data type of the array elements, plus the number of elements desired enclosed within square brackets ('[' and ']'). new elementType[arraySize]
25
If the new
statement were omitted from the sample program, the compiler would print an error like the following one and compilation would fail.
ArrayDemo.java:4: Variable anArray may not have been initialized.
Accessing an Array Element
Now that some memory has been allocated for the array, the program assigns values to the array elements:
anArray[2] = 10;
System.out.print(anArray[2] + " ");
This part of the code shows that to reference an array element, either to assign a value to it, or to access the value, you append square brackets to the array name. The value between the square brackets indicates (either with a variable or some other expression) the index of the element to access. Note that in Java, array indices begin at 0 and end at the array length minus 1.
Getting the Size of an Array
To get the size of an array, you write
arrayname.length
Be careful: Programmers new to the Java programming language are tempted to follow length with an empty set of parenthesis. This doesn't work because length
is not a method. length
is a property provided by the Java platform for all arrays.
Array Initializers
The Java programming language provides a shortcut syntax for creating and initializing an array.
Here's an example of this syntax: boolean[] answers = { true, false, true, true, false };
The length of the array is determined by the number of values provided between { and }.
Before initialization arrays are always set to contain default values wherever they are created.
2.10 Multidimensional Arrays
A multi-dimensional array of dimension n (i.e., an n-dimensional array or simply n-D array) is a collection of items which is accessed via n subscript expressions
26
The Java programming language does not really support multi-dimensional arrays. It does, however, support arrays of arrays. In Java, a two-dimensional array x
is really an array of onedimensional arrays: int[][] x = new int[3][5];
When you allocate memory for multidimensional array, you need only specify the memory for the first (leftmost) dimension. You can allocate remaining dimensions separately.
public class ArrayOfArraysDemo2 {
public static void main(String[] args) {
int[][] aMatrix = new int[3][];
//populate matrix
aMatrix[0]=new int[]{1}; aMatrix[1]=new int[]{2,3}; aMatrix[2]=new int[]{4,5,6};
}
}
2.11 Copying Arrays
Use
System
's arraycopy
method to efficiently copy data from one array into another. The arraycopy
method requires five arguments: public static void arraycopy(Object source, int srcIndex, Object
dest,int destIndex, int length);
The two
Object
arguments indicate the array to copy from and the array to copy to. The three integer arguments indicate the starting location in each the source and the destination array, and the number of elements to copy. This diagram illustrates how the copy takes place:
The following program, ArrayCopyDemo, uses arraycopy
to copy some elements from the copyFrom
array to the copyTo
array. public class ArrayCopyDemo {
public static void main(String[] args) {
char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',
27
'i', 'n', 'a', 't', 'e', 'd' };
char[] copyTo = new char[7];
System.arraycopy(copyFrom, 2, copyTo, 0, 7);
System.out.println(new String(copyTo));
}
}
The arraycopy
method call in this example program begins the copy at element number 2 in the source array. Recall that array indices start at 0, so that the copy begins at the array element 'c'.
The arraycopy
method call puts the copied elements into the destination array beginning at the first element (element 0) in the destination array copyTo
. The copy copies 7 elements: 'c', 'a', 'f',
'f', 'e', 'i', and 'n'. Effectively, the arraycopy
method takes the "caffein" out of "decaffeinated", like this:
Note that the destination array must be allocated before you call arraycopy
and must be large enough to contain the data being copied.
Summary
When you declare a variable, you explicitly set the variable's name and data type. The Java programming language has two categories of data types: primitive and reference. A variable of primitive type contains a value. The table in the Data Types section shows all of the primitive data types along with their sizes and formats. Arrays, classes, and interfaces are reference types.
You can provide an initial value for a variable within its declaration by using the assignment operator (=). You can declare a variable as final. The value of a final variable cannot change after it's been initialized.
An array is a fixed-length data structure that can contain multiple objects of the same type. An array can contain any type of object, including arrays. To declare an array, you use the type of object that the array can contain and brackets.
28
The length of the array must be specified when it is created.You can use the new
operator to create an array, or you can use an array initializer. Once created, the size of the array cannot change. To get the length of the array, you use the length
attribute.
An element within an array can be accessed by its index. Indices begin at 0 and end at the length of the array minus 1.
To copy an array, use the arraycopy
method in the
System
class.
Questions
1. class MCZ11 { public static void main (String[] args) { char a = '\c'; // 1 char b = '\r'; // 2 char c = '\"'; // 3 char d = '\b'; // 4 char e = '\''; // 5
}
}
A compile-time error is generated at which line? a. 1 b. 2 c. 3 d. 4 e. 5 f. None of the above
29
2. class GRC4 {public static void main(String[] args) {}} // 1
class GRC5 {public static void main(String []args) {}} // 2
class GRC6 {public static void main(String args[]) {}} // 3
What is the result of attempting to compile and run the above programs? a. Compile-time error at line 1.
b. Compile-time error at line 2.
c. Compile-time error at line 3.
d. An attempt to run GRC4 from the command line fails.
e. An attempt to run GRC5 from the command line fails.
f. An attempt to run GRC6 from the command line fails.
g. None of the above
3. Which of these words belongs to the set of Java keywords? a. qualified b. record c. repeat d. restricted e. label f. to g. type h. until
4. class Identifiers { int i1; // 1 int _i2; // 2 int i_3; // 3 int #i4; // 4 int $i5; // 5 int %i6; // 6 int i$7; // 7 int 8i; // 8
}
Compile-time errors are generated at which lines? a. 1 b. 2 c. 3 d. 4 e. 5 f. 6 g. 7 h. 8 i. value j. virtual k. xor l. None of the above
30
5. Which of the following represent the full range of type char? a. '\u0000' to '\u7fff' b. '\u0000' to '\uffff' c. 0 to 32767 d. 0 to 65535 e. -32768 to 32767 f. -65536 to 65535
31
6. class MWC101 {
public static void main(String[] args) {
int[] a1 = new int[]; // 1
int a2[] = new int[5]; // 2
int[] a3 = new int[]{1,2}; // 3
int []a4 = {1,2}; // 4
int[] a5 = new int[5]{1,2,3,4,5}; // 5
}
}
Compile-time errors are generated at which lines? a. 1 b. 2 c. 3 d. 4
7. class MWC201 { public static void main(String[] args) { int[][] a1 = {{1,2,3},{4,5,6},{7,8,9,10}};
System.out.print(a1[0][2]+","+a1[1][0]+","+a1[2][1]);
}
}
What is the result of attempting to compile and run the program? a. Prints: 3,4,8 b. Prints: 7,2,6 c. Compile-time error d. Run-time error e. None of the above e. 5
32
Chapter 3 : Operators
3.1 Operands
An operand can be:
• a numeric variable - integer, floating point or character
•
• any primitive type variable - numeric and boolean reference variable to an object
• a literal - numeric value, boolean value, or string.
• an array element, "a[2]"
• char
primitive, which in numeric operations is treated as an unsigned two byte integer
3.2 Operator
An operator performs a function on one, two, or three operands. An operator that requires one operand is called a unary operator. For example,
++
is a unary operator that increments the value of its operand by 1. An operator that requires two operands is a binary operator. For example,
=
is a binary operator that assigns the value from its right-hand operand to its left-hand operand. And finally, a ternary operator is one that requires three operands. The Java programming language has one ternary operator,
?:
, which is a short-hand if
else
statement.
The unary operators support either prefix or postfix notation. Prefix notation means that the operator appears before its operand: operator op //prefix notation
Postfix notation means that the operator appears after its operand: op operator //postfix notation
All of the binary operators use infix notation, which means that the operator appears between its operands: op1 operator op2 //infix notation
The ternary operator is also infix; each component of the operator appears between operands: op1 ? op2 : op3 //infix notation
In addition to performing the operation, an operator returns a value. The return value and its type depend on the operator and the type of its operands. For example, the arithmetic operators, which perform basic arithmetic operations such as addition and subtraction, return numbers-the result of the arithmetic operation. The data type returned by an arithmetic operator depends on the type of its operands: If you add two integers, you get an integer back. An operation is said to
evaluate to its result.
33
We divide the operators into these categories:
•
•
•
•
•
•
•
Arithmetic Operators
Increment and decrement operators
Relational Operators
Bitwise Operators
Logical Operators
Assignment Operators
Conditional or ternary operator
3.2.1 Arithmetic Operators
The Java programming language supports various arithmetic operators for all floating-point and integer numbers. These operators are
+
(addition),
-
(subtraction),
*
(multiplication),
/
(division), and
%
(modulo).
The following table summarizes the binary arithmetic operations in the Java programming language.
Operator
+
-
*
/
%
Use
op1 + op2 op1 – op2 op1 * op2 op1 / op2 op1 % op2
Description
Adds op1
and op2
Subtracts op2
from op1
Multiplies op1
by op2
Divides op1
by op2
If floating point arithmetic and op2 = 0.0, then infinity returned if op1 is not zero otherwise NaN if op1 is zero.
ArthmeticException thrown if op1 & op2 are integer types and op2 is zero.
Computes the remainder of dividing op1
by op2
If floating point arithmetic and op2 = 0.0 or infinity, then
NaN returned
ArthmeticException thrown if op1 & op2 are integer types and op2 is zero.
public class ArithmeticDemo { public static void main(String[] args) {
//a few numbers
int i = 37;
int j = 42;
double x = 27.475;
double y = 7.22;
System.out.println("Variable values...");
System.out.println(" i = " + i);
System.out.println(" j = " + j);
System.out.println(" x = " + x);
System.out.println(" y = " + y);
//adding numbers
34
System.out.println("Adding...");
System.out.println(" i + j = " + (i + j));
System.out.println(" x + y = " + (x + y));
//subtracting numbers
System.out.println("Subtracting...");
System.out.println(" i - j = " + (i - j));
System.out.println(" x - y = " + (x - y));
//multiplying numbers
System.out.println("Multiplying...");
System.out.println(" i * j = " + (i * j));
System.out.println(" x * y = " + (x * y));
//dividing numbers
System.out.println("Dividing...");
System.out.println(" i / j = " + (i / j));
System.out.println(" x / y = " + (x / y));
//computing the remainder resulting from dividing numbers
System.out.println("Computing the remainder...");
System.out.println(" i % j = " + (i % j));
System.out.println(" x % y = " + (x % y));
//mixing types
System.out.println("Mixing types...");
System.out.println(" j + y = " + (j + y));
System.out.println(" i * x = " + (i * x));
}
}
Note that when an integer and a floating-point number are used as operands to a single arithmetic operation, the result is floating point. The integer is implicitly converted to a floating-point number before the operation takes place. The following table summarizes the data type returned by the arithmetic operators, based on the data type of the operands. The necessary conversions take place before the operation is performed.
Data Type of
Result
long int double float
Data Type of Operands
Neither operand is a float
or a double
(integer arithmetic); at least one operand is a long
.
Neither operand is a float
or a double
(integer arithmetic); neither operand is a long
.
At least one operand is a double
.
At least one operand is a float
; neither operand is a double
.
In addition to the binary forms of
+
and
-
, each of these operators has unary versions that perform the following operations:
Operator Use Description
35
+
-
+op
-op
Promotes op
to int
if it's a byte
, short
, or char
Arithmetically negates op
3.2.2 Increment and Decrement Operators
Increment operator (
++)
increments its operand by 1, and decrement operator (
--)d ecrements its operand by 1. Either
++
or
--
can appear before (prefix) or after (postfix) its operand. The prefix version,
++op
/
--op
, evaluates to the value of the operand after the increment/decrement operation. The postfix version, op++
/ op--
, evaluates the value of the operand before the increment/decrement operation.
Operator Use
++ op++
++
--
--
++op op--
--op
Description
Increments op
by 1; evaluates to the value of op before it was incremented
Increments op
by 1; evaluates to the value of op after it was incremented
Decrements op
by 1; evaluates to the value of op before it was decremented
Decrements op
by 1; evaluates to the value of op after it was decremented
3.2.3 Relational Operators
A relational operator compares two values and determines the relationship between them. For example,
!=
returns true
if the two operands are unequal. This table summarizes the relational operators:
Operator
>
>=
<
<=
==
!=
Use Returns
true
if
op1 > op2 op1
is greater than op2 op1 >= op2 op1
is greater than or equal to op2 op1 < op2 op1
is less than op2 op1 <= op2 op1
is less than or equal to op2 op1 == op2 op1
and op2
are equal op1 != op2 op1
and op2
are not equal public class RelationalDemo {
public static void main(String[] args) {
//a few numbers
int i = 37;
int j = 42;
int k = 42;
System.out.println("Variable values...");
System.out.println(" i = " + i);
System.out.println(" j = " + j);
36
System.out.println(" k = " + k);
//greater than
System.out.println("Greater than...");
System.out.println(" i > j = " + (i > j)); //false
System.out.println(" j > i = " + (j > i)); //true
System.out.println(" k > j = " + (k > j));
//false, they are equal
//greater than or equal to
System.out.println("Greater than or equal to...");
System.out.println(" i >= j = " + (i >= j)); //false
System.out.println(" j >= i = " + (j >= i)); //true
System.out.println(" k >= j = " + (k >= j)); //true
//less than
System.out.println("Less than...");
System.out.println(" i < j = " + (i < j)); //true
System.out.println(" j < i = " + (j < i)); //false
System.out.println(" k < j = " + (k < j)); //false
//less than or equal to
System.out.println("Less than or equal to...");
System.out.println(" i <= j = " + (i <= j)); //true
System.out.println(" j <= i = " + (j <= i)); //false
System.out.println(" k <= j = " + (k <= j)); //true
//equal to
System.out.println("Equal to...");
System.out.println(" i == j = " + (i == j)); //false
System.out.println(" k == j = " + (k == j)); //true
//not equal to
System.out.println("Not equal to...");
System.out.println(" i != j = " + (i != j)); //true
System.out.println(" k != j = " + (k != j)); //false
}
}
Relational operators often are used with conditional operators to construct more complex decision-making expressions. The Java programming language supports six conditional operators-five binary and one unary--as shown in the following table.
Operator
&&
||
!
&
Use
op1 && op2 op1 || op2
! op op1 & op2
Returns
true
if
op1
and op2
are both true
, conditionally evaluates op2 either op1
or op2
is true
, conditionally evaluates op2 op
is false op1
and op2
are both true
, always evaluates op1
and op2
37
|
^ op1 | op2 op1 ^ op2 either op1
or op2
is true
, always evaluates op1
and op2 if op1 and op2 are different--that is if one or the other of the operands is true but not both
3.2.4 Bitwise Operators
A shift operator performs bit manipulation on data by shifting the bits of its first operand right or left. This table summarizes the shift operators available in the Java programming language.
Operator
>>
<<
>>>
Use
op1 >> op2 op1 << op2 op1 >>> op2
Operation
shift bits of op1
right by distance op2 shift bits of op1
left by distance op2 shift bits of op1
right by distance op2
(unsigned)
Each operator shifts the bits of the left-hand operand over by the number of positions indicated by the right-hand operand. The shift occurs in the direction indicated by the operator itself. For example, the following statement shifts the bits of the integer 13 to the right by one position:
13 >> 1;
The binary representation of the number 13 is 1101. The result of the shift operation is 1101 shifted to the right by one position-110, or 6 in decimal. The left-hand bits are filled with 0s as needed.
The following table shows the four operators the Java programming language provides to perform bitwise functions on their operands:
Operator
&
|
^
~
Use
op1 & op2 op1 | op2 op1 ^ op2
~op2
Operation
bitwise and bitwise or bitwise xor bitwise complement
When its operands are numbers, the & operation performs the bitwise AND function on each parallel pair of bits in each operand. The AND function sets the resulting bit to 1 if the corresponding bit in both operands is 1, as shown in the following table.
op1 op2
Result
0 0 0
0 1 0
1 0 0
1 1 1
38
Suppose that you were to AND the values 13 and 12, like this: 13 & 12. The result of this operation is 12 because the binary representation of 12 is 1100, and the binary representation of
13 is 1101.
1101 //13
& 1100 //12
------
1100 //12
If both operand bits are 1, the
AND
function sets the resulting bit to 1; otherwise, the resulting bit is 0. So, when you line up the two operands and perform the
AND
function, you can see that the two high-order bits (the two bits farthest to the left of each number) of each operand are 1. Thus, the resulting bit in the result is also 1. The low-order bits evaluate to 0 because either one or both bits in the operands are 0. When both of its operands are numbers, the
|
operator performs the inclusive or operation, and
^
performs the exclusive or (
XOR
) operation. Inclusive or means that if either of the two bits is 1, the result is 1. The following table shows the results of inclusive or operations:
op1 op2
Result
0 0 0
0 1 1
1 0 1
1 1 1
Exclusive or means that if the two operand bits are different the result is 1, otherwise the result is
0. The following table shows the results of an exclusive or operation.
op1 op2
Result
0 0 0
0 1 1
1 0 1
1 1 0
And finally, the complement operator inverts the value of each bit of the operand: if the operand bit is 1 the result is 0 and if the operand bit is 0 the result is 1.
Among other things, bitwise manipulations are useful for managing sets of boolean flags.
static final int VISIBLE = 1; static final int DRAGGABLE = 2; static final int SELECTABLE = 4; static final int EDITABLE = 8; int flags = 0;
To set the "visible" flag when something became visible you would use this statement: flags = flags | VISIBLE;
39
To test for visibility, you could then write: if ((flags & VISIBLE) == VISIBLE) {
...
}
A program example is given below: public class BitwiseDemo {
static final int VISIBLE = 1;
static final int DRAGGABLE = 2;
static final int SELECTABLE = 4;
static final int EDITABLE = 8;
public static void main(String[] args)
{
int flags = 0;
flags = flags | VISIBLE;
flags = flags | DRAGGABLE;
if ((flags & VISIBLE) == VISIBLE) {
if ((flags & DRAGGABLE) == DRAGGABLE) {
System.out.println("Flags are Visible and
Draggable.");
}
}
flags = flags | EDITABLE;
if ((flags & EDITABLE) == EDITABLE) {
System.out.println("Flags are now also Editable.");
}
}
}
3.2.5 Logical Operators
The relational operators you've learned so far (
<
,
<=
,
>
,
>=
,
!=
,
==
) are sufficient when you only need to check one condition. However what if a particular action is to be taken only if several conditions are true? You can use a sequence of if
statements to test the conditions, as follows: if (x == 2) {
if (y != 2) {
System.out.println("Both conditions are true.");
}
}
40
This, however, is hard to write and harder to read. It only gets worse as you add more conditions.
Fortunately, Java provides an easy way to handle multiple conditions: the logic operators. There are three logic operators,
&&
,
||
and
!
.
&&
is logical and.
&&
combines two boolean values and returns a boolean which is true if and only if both of its operands are true. For instance boolean b; b = 3 > 2 && 5 < 7; // b is true b = 2 > 3 && 5 < 7; // b is now false
||
is logical or.
||
combines two boolean variables or expressions and returns a result that is true if either or both of its operands are true. For instance boolean b; b = 3 > 2 || 5 < 7; // b is true b = 2 > 3 || 5 < 7; // b is still true b = 2 > 3 || 5 > 7; // now b is false
The last logic operator is
!
which means not. It reverses the value of a boolean expression. Thus if b
is true
!b
is false. If b
is false
!b
is true. boolean b; b = !(3 > 2); // b is false b = !(2 > 3); // b is true
These operators allow you to test multiple conditions more easily. For instance the previous example can now be written as if (x == 2 && y != 2) {
System.out.println("Both conditions are true.");
}
The Order of Evaluation of Logic Operators
When Java sees a && operator or a ||, the expression on the left side of the operator is evaluated first. For example, consider the following: boolean b, c, d; b = !(3 > 2); // b is false c = !(2 > 3); // c is true d = b && c; // d is false
When Java evaluates the expression d = b && c;, it first checks whether b is true. Here b is false, so b && c must be false regardless of whether c is or is not true, so Java doesn't bother checking the value of c.
41
On the other hand when faced with an || Java short circuits the evaluation as soon as it encounters a true value since the resulting expression must be true. This short circuit evaluation is less important in Java than in C because in Java the operands of && and || must be booleans which are unlikely to have side effects that depend on whether or not they are evaluated. Still it's possible to force them. For instance consider this code.
boolean b = (n == 0) || (m/n > 2);
Even if n is zero this line will never cause a division by zero, because the left hand side is always evaluated first. If n is zero then the left hand side is true and there's no need to evaluate the right hand side. Mathematically this makes sense because m/0 is in some sense infinite which is greater than two.
This isn't a perfect solution though because m may be 0 or it may be negative. If m is negative and n is zero then m/n is negative infinity which is less than two. And if m is also zero, then m/n is very undefined.
Therefore if there's a real chance your program will have a divide by zero error think carefully about what it means and how you should respond to it. If, upon reflection, you decide that what you really want to know is whether m/n is finite and greater than zero you should use a line like this boolean b = (n != 0) && (m/n > 0);
The short circuit effect with logical operators
The logical operators (&& and ||) have a slightly peculiar effect in that they perform "shortcircuited" logical AND and logical OR operations. The Java approach makes sense if you consider that for an AND, if the first operand is false it doesn't matter what the second operand evaluates to, the overall result will be false. Also for a logical OR, if the first operand has turned out true, the overall calculation will show up as true because only one evaluation must return true to return an overall true. This can have an effect with those clever compressed calculations that depend on side effects. Take the following example.
public class MyClass1{ public static void main(String argv[]){ int Output=10;
boolean b1 = false;
if((b1==true) && ((Output+=10)==20))
{
System.out.println("We are equal "+Output);
} else
{
System.out.println("Not equal! "+Output);
}
}
}
42
The output will be "Not equal 10". This illustrates that the Output +=10 calculation was never performed because processing stopped after the first operand was evaluated to be false. If you change the value of b1 to true processing occurs as you would expect and the output is "We are equal 20";.
This may be handy sometimes when you really don't want to process the other operations if any of them return false, but it can be an unexpected side effect if you are not completely familiar with it.
Avoiding Short Circuits
If you want all of your boolean expressions evaluated regardless of the truth value of each, then you can use & and | instead of && and ||. However make sure you use these only on boolean expressions. Unlike && and ||, & and | also have a meaning for numeric types which is completely different from their meaning for booleans.
3.2.6 Assignment Operators
You use the basic assignment operator,
=
, to assign one value to another.
The Java programming language also provides several shortcut assignment operators that allow you to perform an arithmetic, shift, or bitwise operation and an assignment operation all with one operator. Suppose you wanted to add a number to a variable and assign the result back into the variable, like this: i = i + 2;
You can shorten this statement using the shortcut operator
+=
, like this: i += 2;
The two previous lines of code are equivalent. x operation= y is equivalent to x = x operation y x and y must be numeric or char types except for "=", which allows x and y also to be object references.
In this case, x must be of the same type of class or interface as y. If mixed floating-point and integer types, the rules for mixed types in expressions apply.
The following table lists the shortcut assignment operators and their lengthy equivalents:
Operator
+=
-=
*=
/=
Use
op1 += op2 op1 -= op2 op1 *= op2 op1 /= op2
Equivalent to
op1 = op1 + op2 op1 = op1 - op2 op1 = op1 * op2 op1 = op1 / op2
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%=
&=
|=
^=
<<=
>>=
>>>= op1 %= op2 op1 &= op2 op1 |= op2 op1 ^= op2 op1 = op1 % op2 op1 = op1 & op2 op1 = op1 | op2 op1 = op1 ^ op2 op1 <<= op2 op1 = op1 << op2 op1 >>= op2 op1 = op1 >> op2 op1 >>>= op2 op1 = op1 >>> op2
Other Operators
Operator
?:
[]
.
( params )
( type ) new instanceof
Description
Shortcut if-else
statement. Conditional or ternary operator
Used to declare arrays, create arrays, and access array elements
Used to form qualified names
Delimits a comma-separated list of parameters
Casts (converts) a value to the specified type
Creates a new object or a new array
Determines whether its first operand is an instance of its second operand
3.2.7 Ternary or Conditional operator
The
?:
operator is a conditional operator that is short-hand for an if
else
statement: op1 ? op2 : op3
The
?:
operator returns op2
if op1
is true or returns op3
if op1
is false.
The value of a variable often depends on whether a particular boolean expression is or is not true and on nothing else. For instance one common operation is setting the value of a variable to the maximum of two quantities. In Java you might write if (a > b) {
max = a;
} else {
max = b;
}
Setting a single variable to one of two states based on a single condition is such a common use of if-else
that a shortcut has been devised for it, the conditional operator, ?:. Using the conditional operator you can rewrite the above example in a single line like this: max = (a > b) ? a : b;
(a > b) ? a : b;
is an expression which returns one of two values, a
or b
. The condition,
(a
> b)
, is tested. If it is true the first value, a
, is returned. If it is false, the second value, b
, is
44
returned. Whichever value is returned is dependent on the conditional test, a > b
. The condition can be any expression which returns a boolean value.
The conditional operator only works for assigning a value to a variable, using a value in a method invocation, or in some other way that indicates the type of its second and third arguments. For example, consider the following if (name.equals("Rumplestiltskin")) {
System.out.println("Give back child");
} else {
System.out.println("Laugh");
}
This may not be written like this: name.equals("Rumplestiltskin")
? System.out.println("Give back child")
: System.out.println("Laugh");
First of all, both the second and third arguments are void. Secondly, no assignment is present to indicate the type that is expected for the second and third arguments (though you know void must be wrong).
The first argument to the conditional operator must have or return boolean type and the second and third arguments must return values compatible with the value the entire expression can be expected to return. You can never use a void method as an argument to the ? : operator.
3.2.8 The [ ] Operator
You use square brackets to declare arrays, to create arrays, and to access a particular element in an array. Here's an example of an array declaration: float[] arrayOfFloats = new float[10];
The previous code declares an array that can hold ten floating point numbers. Here's how you would access the 7th item in that array: arrayOfFloats[6];
Note that array indices begin at 0.
3.2.9 The . Operator
The dot (.) operator accesses instance members of an object or class members of a class.
3.2.10 The () Operator
45
When declaring or calling a method, you list the method's arguments between
(
and
)
. You can specify an empty argument list by using
()
with nothing between them.
3.2.11 The (type) Operator
Casts (or "converts") a value to the specified type.
3.2.12 The new Operator
You use the new operator to create a new object or a new array. Here's an example of creating a new
Integer
object from the
Integer
class in the java.lang
package:
Integer anInteger = new Integer(10);
3.2.13 The instanceof Operator
The instanceof
operator tests whether its first operand is an instance of its second. op1 instanceof op2 op1
must be the name of an object and op2
must be the name of a class. An object is considered to be an instance of a class if that object directly or indirectly descends from that class.
3.3 Expressions
An expression produces a result and returns a value. Examples include:
• i = 2
: the assignment puts 2 into the i variable and returns the value 2
• k++
: returns k, then k is incremented by 1
• x < y
: logical "less than" comparison, returns a Boolean true or false value
• i | j
: returns the value of a bitwise OR operation on bits in the two variables.
Expressions involve at least one operator. A single operator can have 1, 2 or 3 operands.
3.4 Statements
A statement is essentially any complete sentence that causes some action to occur. It can encompass multiple operators and operands, as well as multiple sub-statements. For example, the statement int x = 1; declares a variable x and then assigns the value 1 to it. This statement x = 5.3 *(4.1 / Math.cos (0.2*y));
46
consists of several expressions - multiplication, division, a method call to a math function - but is still considered a single statement.
3.5 Operator Precedence
Highest Precedence
()
++expr --expr +expr -expr ~ !
* / %
+ -
<< >> >>>
< > <= >= instanceof
== !=
&
|
^
&&
||
? :
= += -= *= /= %= &= ^= |= <<= >>= >>>=
Lowest Precedence
Overriding Operator Precedence
Parenthesis is used to override operator precedence.
Sometimes the default order of evaluation isn't what you want. For instance, the formula to change a Fahrenheit temperature to a Celsius temperature is C = (5/9) (F - 32) where C is degrees
Celsius and F is degrees Fahrenheit. You must subtract 32 from the Fahrenheit temperature before you multiply by 5/9, not after. You can use parentheses to adjust the order much as they are used in the above formula. The next program prints a table showing the conversions from
Fahrenheit and Celsius between zero and three hundred degrees Fahrenheit every twenty degrees.
// Print a Fahrenheit to Celsius table class FahrToCelsius {
public static void main (String args[]) {
// lower limit of temperature table
double lower = 0.0;
// upper limit of temperature table
double upper = 300.0;
// step size
47
double step = 20.0;
double fahr = lower;
while (fahr <= upper) {
double celsius = (5.0 / 9.0) * (fahr-32.0);
System.out.println(fahr + " " + celsius);
fahr = fahr + step;
}
}
}
Here's the output:
0 -17.7778
20 -6.66667
40 4.44444
60 15.5556
80 26.6667
100 37.7778
120 48.8889
140 60
160 71.1111
180 82.2222
200 93.3333
220 104.444
240 115.556
260 126.667
280 137.778
300 148.889
Everything inside the parentheses will be calculated before anything outside of the parentheses is calculated.
Operator Associativity
The following operators have Right to Left associativity. All other operators (see precedence table above) are evaluated left to right.
48
=
*=
/=
%=
+=
-=
<<=
>>=
>>>=
&=
^=
|=
?: new
(type cast)
++x
--x
+x
-x
~
3.6 Type Conversion and Casting
Converting one type of data into another must follow the rules of casting. If a conversion results in the loss of precision, as in an int value converted to a short, then the compiler will issue an error message unless an explicit cast is made.
To convert type AA data into type BB data, put the type BB name in parentheses in front of the type AA data:
AA a = aData;
BB b = (BB)a; // cast type AA to type BB
For example, to convert integer data to floating point:
int i=0;
float f;
f=(float)i; // Cast int as float
Expressions can promote to a wider type without an explicit cast:
int i=1;
long j=3L; // Literals are int types so require L suffix
j=i; // OK
However, you can not assign a value to a more narrow type without an explicit cast:
i=j; // Error in assigning long to int
i=(int)j; // OK
So a data type with lower precision (fewer bits) can be converted to a type of higher precision without explicit casting. To convert a higher precision type to a lower precision, however, an explicit cast is required or the compiler will flag an error.
Note that when you cast a value of a wider type down to a more narrow type, such as an int value to a byte variable, the upper bytes will be truncated. That is, the lowest order byte in the int value will be copied to the byte value.
49
Primitive Type Conversion Table
Below is a table that indicates to which of the other primitive types you can cast a given primitive data type. The symbol C indicates that an explicit cast is required since the precision is decreasing. The symbol A indicates that the precision is increasing so an automatic cast occurs without the need for an explicit cast. N indicates that the conversion is not allowed.
int long float double char byte short boolean
C
C
A
A
A
N
int
-
C
long
A
-
A
A
A
C
C
N
float double char
A*
A*
A
A*
C
C
-
C
A
A
A
N
A
-
A
A
A
N
-
C
C
C
C
N
byte
C
C
C
-
C
C
C
N
short boolean
C
C
N
N
C
C
C
A
-
N
N
N
N
N
N
-
The * asterisk indicates that the least significant digits may be lost in the conversion even though the target type allows for bigger numbers. For example, a large value in an int type value that uses all 32 bits will lose some of the lower bits when converted to float since the exponent uses 8 bits of the 32 provided for float values.
Mixed Types in an Expression
If an expression holds a mix of types, the lower precision or narrower value operand is converted to a higher precision or wider type. This result then must be cast if it goes to a lower precision type:
float x,y=3;
int j,i=3;
x= i*y; // OK since i will be promoted to float
j= i*y; // Error since result is a float value
j= (int)(i*y) // OK
The process of converting a value to a wider or higher precision integer or floating point type is called "numeric promotion". The Java VM specification states the following rules for promotion in an expression of two operands, as in x+i:
•
•
•
•
If either operand is of type double
, the other is converted to double.
Otherwise, if either operand is of type float
, the other is converted to float
.
Otherwise, if either operand is of type long
, the other is converted to long
.
Otherwise, both operands are converted to type int
.
The program below uses both ints and doubles, for example. class IntAndDouble {
50
public static void main (String args[]) {
int i = 10;
double x = 2.5;
double k;
System.out.println("i is " + i);
System.out.println("x is " + x);
k = i + x;
System.out.println("i + x is " + k);
k = i * x;
System.out.println("i * x is " + k);
k = i - x;
System.out.println("i - x is " + k);
k = x - i;
System.out.println("x - i is " + k);
k = i / x;
System.out.println("i / x is " + k);
k = x / i;
System.out.println("x / i is " + k);
}
}
This program produces the following output: i is 10 x is 2.5
i + x is 12.5
i * x is 25 i - x is 7.5
x - i is -7.5
i / x is 4 x / i is 0.25
Order can make a difference when data types are mixed. For example,
1 / 2 * 3.5 = 0.0
3.5 * 1 / 2 = 1.75
3.5 / 2 = 1.75
You cannot assume that the usual mathematical laws of commutativity apply when mixing data types, especially integer and floating point types.
1.0 / 2 * 3.5 = 1.75
3.5 * 1.0 / 2 = 1.75
1 / 2.0 * 3.5 = 1.75
3.5 * 1.0 / 2.0 = 1.75
51
3.7 Automatic Type Promotions
An int divided by an int is an int, and a double divided by a double is a double, but what about an int divided by a double or a double divided by an int? When doing arithmetic on unlike types
Java tends to widen the types involved so as to avoid losing information. After all 3 * 54.2E18 will be a perfectly valid double but much too big for any int.
The basic rule is that if either of the variables in a binary operation (addition, multiplication, subtraction, addition, remainder) are doubles then Java treats both values as doubles. If neither value is a double but one is a float, then Java treats both values as floats. If neither is a float or a double but one is a long, then Java treats both values as longs. Finally if there are no doubles, floats or longs, then Java treats both values as an int, even if there aren't any ints in the equation.
Therefore the result will be a double, float, long or int depending on the types of the arguments.
In an assignment statement, i.e. if there's an equals sign, Java compares the type of the left hand side to the final type of the right hand side. It won't change the type of the left hand side, but it will check to make sure that the value it has ( double
, float
, int
or long
) on the right hand side can fit in the type on the left hand side. Anything can fit in a double
. Anything except a double can fit in a float
. Any integral type can fit in a long
, but a float
or a double
can't, and ints, shorts, and bytes can fit inside ints. If the right hand side can fit inside the left hand side, the assignment takes place with no further ado.
Assigning long
values to int
variables or double
values to float
variables can be equally troublesome. In fact it's so troublesome the compiler won't let you do it unless you tell it you really mean it with a cast. When it's necessary to force a value into a particular type, use a cast.
To cast a variable or a literal or an expression to a different data type just precede it with the type in parentheses. For instance: int i = (int) (9.0/4.0);
A cast lets the compiler know that you're serious about the conversion you plan to make.
When a value is cast down before assignment, series of operations takes place to chop the right hand side down to size. For a conversion between a floating point number and an int or a long, the fractional part of the floating point number is truncated (rounded toward zero). This produces an integer. If the integer is small enough to fit in the left hand side, the assignment is completed.
On the other hand if the number is too large, then the integer is set to the largest possible value of its type. If the floating point number is too small the integer is set to the smallest possible value of its type.
This can be a nasty bug in your code. It can also be hard to find since everything may work perfectly 99 times out of a hundred and only on rare occasions will the rounding become a problem. However when it does there will be no warning or error message. You need to be very careful when assigning floating point values to integer types.
52
Summary
This chapter discussed the operators that you can use in Java to manipulate the values of variables. It included all the standard arithmetic operators, the increment and decrement operators, the relational operators, the bit-wise operators, and the shift operators. Finally it discussed the concept of operator precedence that defined the rules by which the order of operators is evaluated.
Questions
1. class GFM11{ public static void main (String[] args) {
int x,y,z;
System.out.println(x+y+z);
}
}
What is the result of attempting to compile and run the program? a. Prints nothing.
b. Prints an undefined value.
c. Prints: null d. Prints: 0 e. Run-time error f. Compile-time error g. None of the above
2. class EBH201 { public static void main (String[] args) { int a = 1 || 2 ^ 3 && 5; int b = ((1 || 2) ^ 3) && 5; int c = 1 || (2 ^ (3 && 5));
System.out.print(a + "," + b + "," + c);
}
}
What is the result of attempting to compile and run the program? a. Prints: 0,0,0 b. Prints: 0,0,3 c. Prints: 0,3,0 e. Prints: 3,0,0 f. Prints: 3,0,3 g. Prints: 3,3,0 d. Prints: 0,3,3 h. Prints: 3,3,3
3. class MCZ24 { public static void main (String[] args) {
char a = 061; // 1
char b = '\61'; // 2
char c = '\061'; // 3
char d = 0x0031; // 4
char e = '\u0031'; // 5
System.out.print(""+a+b+c+d+e);
} i. Run-time error j. Compile-time error k. None of the above
53
}
A compile-time error is generated at which line? a. 1 b. 2 c. 3 d. 4 e. 5 f. None of the above
54
4. class EBH012 { public static void main (String[] args) {
byte x = 3, y = 5;
System.out.print((-x == ~x + 1)+","+(-y == ~y + 1));
}
}
What is the result of attempting to compile and run the program? a. Prints: false,false b. Prints: false,true c. Prints: true,false d. Prints: true,true e. Run-time error f. Compile-time error g. None of the above
5. class EBH007{ public static void main (String[] s) { byte b = 5; System.out.println(b<<33);
}
}
What is the result of attempting to compile and run the program? a. Prints: -1 b. Prints: 0 c. Prints: 1 d. Prints: 5 e. Prints: 10 f. Run-time error g. Compile-time error h. None of the above
6. class EBH106 { public static void main(String args[]) { int a = 1; a += ++a + a++; System.out.print(a);
}
}
What is the result of attempting to compile and run the above program? a. Prints: 3 b. Prints: 4 c. Prints: 5 d. Prints: 6 e. Prints: 7 f. Run-time error g. Compile-time error h. None of the above
7. class EBH014 { public static void main (String[] args) { byte x = 3, y = 5;
System.out.print((y % x) + ",");
55
}
}
System.out.print(y == ((y/x)*x + (y%x)));
What is the result of attempting to compile and run the program? a. Prints: 1,true b. Prints: 2,true c. Prints: 1,false d. Prints: 2,false e. Run-time error f. Compile-time error g. None of the above
8. class EBH023 { static String m1(boolean b){return b?"T":"F";} public static void main(String [] args) { boolean b1 = false?false:true?false:true?false:true; boolean b2 = false?false:(true?false:(true?false:true)); boolean b3 = ((false?false:true)?false:true)?false:true;
System.out.println(m1(b1) + m1(b2) + m1(b3));
}
}
What is the result of attempting to compile and run the program? a. Prints: FFF b. Prints: FFT c. Prints: FTF d. Prints: FTT e. Prints: TFF f. Prints: TFT g. Prints: TTF h. Prints: TTT i. Run-time error j. Compile-time error k. None of the above
56
Chapter 4 : Control flow statements
Without control flow statements, the interpreter executes these statements in the order they appear in the file from left to right, top to bottom. You can use control flow statements in your programs to conditionally execute statements, to repeatedly execute a block of statements, and to otherwise change the normal, sequential flow of control.
Statement Type
looping
Keyword
while, do-while , for decision making if-else, switch-case branching break, continue, label:, return exception handling try-catch-finally, throws,throw
In the sections that follow, you will see the following notation to describe the general form of a control flow statement: control flow statement details {
statement(s)
}
Technically, the braces, { and }, are not required if the block contains only one statement.
However, we recommend that you always use { and }, because the code is easier to read and it helps to prevent errors when modifying code.
4.1 The while and do-while Statements
You use a while statement to continually execute a block of statements while a condition remains true. The general syntax of the while
statement is: while (expression) {
statement
}
First, the while statement evaluates expression, which must return a boolean value. If the expression returns true, then the while statement executes the statement(s) associated with it.
The while statement continues testing the expression and executing its block until the expression returns false.
public class WhileDemo {
public static void main(String[] args) { int x=10;
while (x != 0) {
System.out.println(x--);
}
}
57
}
This program prints numbers from 10 to 1 in the descending order
The Java programming language provides another statement that is similar to the while statement--the do-while statement. The general syntax of the do-while is: do {
statement(s)
} while (expression);
Instead of evaluating the expression at the top of the loop, do
while
evaluates the expression at the bottom. Thus the statements associated with a do
while
are executed at least once.
public class DoWhileDemo {
public static void main(String[] args) { int x=10;
do {
System.out.println(x--);
} while(x != 0);
}
}
This program prints numbers from 10 to 0 in the descending order
4.2 The for Statement
The for statement provides a compact way to iterate over a range of values. The general form of the for
statement can be expressed like this: for (initialization; termination; increment) {
statement
}
The
initialization
is an expression that initializes the loop-it's executed once at the beginning of the loop. The
termination
expression determines when to terminate the loop. This expression is evaluated at the top of each iteration of the loop. When the expression evaluates to false
, the loop terminates. Finally,
increment
is an expression that gets invoked after each iteration through the loop. All these components are optional. In fact, to write an infinite loop, you omit all three expressions: for ( ; ; ) { // infinite loop
...
}
The for loop in the sample program iterates over each element of anArray, assigning values to its elements. The for loop uses anArray.length to determine when to terminate the loop.
58
Here's a simple program, called ArrayDemo, that creates the array, puts some values in it, and displays the values. public class ArrayDemo {
public static void main(String[] args) {
int[] anArray; // declare an array of integers
anArray = new int[10]; // create an array of integers
// assign a value to each array element and print
for (int i = 0; i < anArray.length; i++) {
anArray[i] = i;
System.out.print(anArray[i] + " ");
}
System.out.println();
}
}
Often for
loops are used to iterate over the elements in an array, or the characters in a string.
public class ForDemo {
public static void main(String[] args) {
int[] arrayOfInts = { 32, 87, 3, 589, 12, 1076,
2000, 8, 622, 127 };
for (int i = 0; i < arrayOfInts.length; i++) {
System.out.print(arrayOfInts[i] + " ");
}
System.out.println();
}
}
Note that you can declare a local variable within the initialization expression of a for
loop. The scope of this variable extends from its declaration to the end of the block governed by the for statement so it can be used in the termination and increment expressions as well. If the variable that controls a for
loop is not needed outside of the loop, it's best to declare the variable in the initialization expression. The names i
, j
, and k
are often used to control for
loops; declaring them within the for
loop initialization expression limits their life-span and reduces errors.
The expression x[i]
selects the ith one-dimensional array; the expression x[i][j]
selects the jth element from that array.
public class ArrayofArraysDemo1 { public static void main(String args[]) { int [] [] x = new int [3][5]; int k=0; for(int i = 0 ; i < 3 ; i++) { for (int j = 0; j < 5 ; j++) { x[i][j] = k++;
}
} for(int i = 0 ; i < x.length ; i++) {
59
for (int j = 0; j < x[i].length ; j++) {
System.out.print(x[i][j] + ‘\t’);
}
System.out.println();
}
}
}
Enhanced for loop (added with Java 5.0)
for (type value : container) statement
This for
statement was created especially for collections and arrays. An example to print all elements in the array. public class ForEachDemo {
public static void main(String[] args) {
int[] arrayOfInts = { 32, 87, 3, 589, 12,
1076, 2000, 8, 622, 127
};
for (int element : arrayOfInts) {
System.out.print(element + " ");
}
System.out.println();
}
}
You can read the for
statement in the preceding snippet like this: For each
int element
in arrayOfInts
...
4.3 The if/else Statements
All but the most trivial computer programs need to make decisions. They test a condition and operate differently based on the outcome of the test. This is quite common in real life. For instance you stick your hand out the window to test if it's raining. If it is raining then you take an umbrella with you. If it isn't raining then you don't.
All programming languages have some form of an if
statement that tests conditions.
This is the simplest version of the if
statement: The block governed by the if
is executed if a condition is true. Generally, the simple form of if
can be written like this: if (expression) {
statement(s)
}
60
The arguments to a conditional statement like if
must be a boolean value, that is something that evaluates to true or false. Integers are not permissible.
class HelloProgram { public static void main(String args[]) { int x=10; if(x%10 == 0) {
System.out.println(“Number is divisible by 10”);
}
}
}
In Java numerical greater than and lesser than tests are done with the
>
and
<
operators respectively. You can test whether a number is less than or equal to or greater than or equal to another number with the
<=
and
>=
operators.
It's not uncommon for even experienced programmers to write
==
when they mean
=
or vice versa. Fortunately in Java, you are not allowed to use
==
and
=
in the same places. Therefore the compiler can catch your mistake and make you fix it before you can run the program.
However there is one way you can still get into trouble:
boolean b = true;
if (b = false) {
System.out.println("b is false");
}
To avoid this, some programmers get in the habit of writing condition tests like this: boolean b = true;
if (false = b) {
System.out.println("b is false");
}
Since you can't assign to a literal, this causes a compiler error if you misuse the
=
sign when you mean to write
==
.
Using the else statement class HelloProgram { public static void main(String args[]) { int x=10; if(x%10 == 0) {
System.out.println(“Number is divisible by 10”);
} else {
System.out.println(“Number is not divisible by 10”);
}
}
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}
Nested if/else statements
Nested if..else statement is writing an if..else statement into an if block or an else block. There can be any level of nested if..else statement. Some forms are given below
1.
if
(condition){
// code to run
if
(condition){
// code to run
}
else
{
// other code to run
}
}
2. if
(condition){
// code to run
if
(condition){
// code to run
}
}
else
{
// other code to run
}
3. boolean
flag =
true
;
if
(flag)
{
int
flag2 = 1;
if
(flag2 < 0)
{
// Do something!
}
else
{
// Do something!
}
}
else
{
// Do something!
}
Example: if (i == j) {
if (j == k)
System.out.println("i equals k");
} else {
System.out.println("i doesn't equal j");
}
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if..else if ladder
The else
block is executed if the if
part is false. Another form of the else
statement, else if
, executes a statement based on another expression. An if statement can have any number of companion else if
statements but only one else
.
public class IfElseDemo {
public static void main(String[] args) {
int testscore = 76;
char grade;
if (testscore >= 90) {
grade = 'A';
} else if (testscore >= 80) {
grade = 'B';
} else if (testscore >= 70) {
grade = 'C';
} else if (testscore >= 60) {
grade = 'D';
} else {
grade = 'F';
}
System.out.println("Grade = " + grade);
}
}
4.4 The switch Statement
Use the switch statement to conditionally perform statements based on an integer expression.
public class SwitchDemo {
public static void main(String[] args) {
int month = 8;
switch (month) {
case 1: System.out.println("January"); break;
case 2: System.out.println("February"); break;
case 3: System.out.println("March"); break;
case 4: System.out.println("April"); break;
case 5: System.out.println("May"); break;
case 6: System.out.println("June"); break;
case 7: System.out.println("July"); break;
case 8: System.out.println("August"); break;
case 9: System.out.println("September"); break;
case 10: System.out.println("October"); break;
case 11: System.out.println("November"); break;
case 12: System.out.println("December"); break;
}
}
}
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The switch
statement evaluates its expression, in this case the value of month
, and executes the appropriate case statement.
int month = 8; if (month == 1) {
System.out.println("January");
} else if (month == 2) {
System.out.println("February");
}
. . . // and so on
Deciding whether to use an if
statement or a switch
statement is a judgment call. You can decide which to use, based on readability and other factors. An if
statement can be used to make decisions based on ranges of values or conditions, whereas a switch
statement can make decisions based only on a single integer value. Also, the value provided to each case
statement must be unique.
Another point of interest in the switch
statement is the break statement after each case
. Each break
statement terminates the enclosing switch
statement, and the flow of control continues with the first statement following the switch
block. The break
statements are necessary because without them, the case
statements fall through. That is, without an explicit break
, control will flow sequentially through subsequent case
statements.
public class SwitchDemo2 {
public static void main(String[] args) {
int month = 2;
int year = 2000;
int numDays = 0;
switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
numDays = 31;
break;
case 4:
case 6:
case 9:
case 11:
numDays = 30;
break;
case 2:
if (((year % 4 == 0) && !(year % 100 == 0))
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|| (year % 400 == 0) )
numDays = 29;
else
numDays = 28;
break;
}
System.out.println("Number of Days = " + numDays);
}
}
Technically, the final break
is not required because flow would fall out of the switch
statement anyway. However, we recommend using a break
for the last case
statement just in case you need to add more case
statements at a later date. This makes modifying the code easier and less error-prone.
Finally, you can use the default statement at the end of the switch
to handle all values that aren't explicitly handled by one of the case
statements.
The variable or expression in the switch statement can be of only int or byte or short or char data types.
4.5 Branching Statements
The Java programming language supports three branching statements:
•
The break statement
•
The continue statement
•
The return statement
label is an identifier placed before a statement. The label is followed by a colon (
:
):
statementName: someJavaStatement;
4.5.1 The break Statement
The break statement has two forms: unlabeled and labeled. An unlabeled break
terminates the enclosing switch
statement, and flow of control transfers to the statement immediately following the switch
. You can also use the unlabeled form of the break
statement to terminate a for
, while
, or do-while
loop.
public static void main(String[] args) {
int[] arrayOfInts = { 32, 87, 3, 589, 12, 1076,
2000, 8, 622, 127 };
int searchfor = 12;
int i = 0;
boolean foundIt = false;
for ( ; i < arrayOfInts.length; i++) {
if (arrayOfInts[i] == searchfor) {
foundIt = true;
65
break;
}
}
if (foundIt) {
System.out.println("Found "+ searchfor +" at index "+ i);
}
else {
System.out.println(searchfor + "not in the array");
}
}
}
The break
statement terminates the labeled statement; it does not transfer the flow of control to the label. The flow of control transfers to the statement immediately following the labeled
(terminated) statement.
public static void main(String[] args) {
int[][] arrayOfInts = { { 32, 87, 3, 589 },
{ 12, 1076, 2000, 8 },
{ 622, 127, 77, 955 }
};
int searchfor = 12;
int i = 0;
int j = 0;
boolean foundIt = false;
search:
for ( ; i < arrayOfInts.length; i++) {
for (j = 0; j < arrayOfInts[i].length; j++) {
if (arrayOfInts[i][j] == searchfor) {
foundIt = true;
break search;
}
}
}
if (foundIt) {
System.out.println("Found "+searchfor+" at "+i+", " + j);
}
else {
System.out.println(searchfor + "not in the array");
}
}
}
4.5.2 The continue Statement
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You use the continue statement to skip the current iteration of a for
, while
, or do-while
loop.
The unlabeled form skips to the end of the innermost loop's body and evaluates the boolean expression that controls the loop, basically skipping the remainder of this iteration of the loop.
The labeled form of the continue
statement skips the current iteration of an outer loop marked with the given label.
A continue
statement returns to the beginning of the innermost enclosing loop without completing the rest of the statements in the body of the loop. If you're in a for
loop, the counter is incremented. For example this code fragment skips even elements of an array for (int i = 0; i < m.length; i++) {
if (m[i] % 2 == 0) continue;
// process odd elements...
}
The continue
statement is rarely used in practice, perhaps because most of the instances where it's useful have simpler implementations. For instance, the above fragment could equally well have been written as for (int i = 0; i < m.length; i++) {
if (m[i] % 2 != 0) {
// process odd elements...
}
}
continue with label
public class ContinueWithLabelDemo {
public static void main(String[] args) {
String searchMe = "Look for a substring in me";
String substring = "sub";
boolean foundIt = false;
int max = searchMe.length() - substring.length();
test:
for (int i = 0; i <= max; i++) {
int n = substring.length();
int j = i;
int k = 0;
while (n-- != 0) {
if (searchMe.charAt(j++)
!= substring.charAt(k++)) {
continue test;
}
}
foundIt = true;
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break test;
}
System.out.println(foundIt ? "Found it" :
"Didn't find it");
}
}
Here is the output from this program:
Found it
4.5.3 The return Statement
You use return
to exit from the current method. The flow of control returns to the statement that follows the original method call. The return
statement has two forms: one that returns a value and one that doesn't. To return a value, simply put the value (or an expression that calculates the value) after the return
keyword: return ++count;
The data type of the value returned by return
must match the type of the method's declared return value. When a method is declared void
, use the form of return
that doesn't return a value: return;
4.6 Exception Handling Statements
The Java programming language provides a mechanism known as exceptions to help programs report and handle errors. When an error occurs, the program throws an exception. It means that the normal flow of the program is interrupted and that the runtime environment attempts to find an exception handler--a block of code that can handle a particular type of error. The exception handler can attempt to recover from the error or, if it determines that the error is unrecoverable, provide a gentle exit from the program.
Three statements play a part in handling exceptions:
•
•
•
The try statement identifies a block of statements within which an exception might be thrown.
The catch statement must be associated with a try
statement and identifies a block of statements that can handle a particular type of exception. The statements are executed if an exception of a particular type occurs within the try
block.
The finally statement must be associated with a try
statement and identifies a block of statements that are executed regardless of whether or not an error occurs within the try block.
Here's the general form of these statements:
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try {
statement(s)
} catch (exceptiontype name) {
statement(s)
} finally {
statement(s)
}
Summary
For controlling the flow of a program, the Java programming language has three loop constructs, a flexible if
else
statement, a switch
statement, exception-handling statements, and branching statements.
Use the while statement to loop over a block of statements while a boolean expression remains true. Use the do-while statement to loop over a block of statements while a boolean expression remains true. The expression is evaluated at the bottom of the loop, so the statements within the do-while block execute at least once. The for
statement loops over a block of statements and includes an initialization expression, a termination condition expression, and an increment expression
The Java programming language has two decision-making statements: if
else
and switch
. The more general-purpose statement is if
; use switch
to make multiple-choice decisions based on a single integer value.
Some branching statements change the flow of control in a program to a labeled statement. You label a statement by placing a legal identifier (the label) followed by a colon (
:
) before the statemen. Use the unlabeled form of the break
statement to terminate the innermost switch
, for
, while
, or do
while
statement. Use the labeled form of the break
statement to terminate an outer switch
, for
, while
, or do
while
statement with the given label. A continue
statement terminates the current iteration of the innermost loop and evaluates the boolean expression that controls the loop. The labeled form of the continue
statement skips the current iteration of the loop with the given label. Use return
to terminate the current method. You can return a value to the method's caller, by using the form of return
that takes a value.
Questions
1. class Black { public static void main(String args[]) {
int[] i = {1,2,3,4,5}; // 1
long[] l = new long[5]; // 2
for (int j = 0; j < l.length(); j++) { // 3
l[j] = i[j]; // 4
}
}
}
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A compile-time error is generated at which line? a. 1 b. 2 c. 3 d. 4 e. None of the above
70
2. class MWC206 { public static void main (String[] args) { int[][] a1 = {{1,2,3},{4,5,6},{7,8,9}}; for (int i = 0; i < 3; i++) { for (int j = 0; j < 3; j++) {
System.out.print(a1[j][i]);
}
}
}
}
What is the result of attempting to compile and run the program? a. Prints: 123456789 b. Prints: 147258369 c. Prints: 321654987 d. Prints: 369258147 e. Run-time error f. Compile-time error g. None of the above
3. class JMM102 { public static void main(String args[]) { for (int i = 0; i<5 ;i++) { switch(i) {
case 0: System.out.print("v ");break;
case 1: System.out.print("w ");
case 2: System.out.print("x ");break;
case 3: System.out.print("y ");
case 4: System.out.print("z ");break;
default: System.out.print("d ");
}
}
}
}
What is the result of attempting to compile and run the program? a. Prints: v w x y z b. Prints: v w x y z d c. Prints: v w x x y z z d. Prints: v w w x y y z d e. Prints: d d d d d d f. Run-time error g. Compile-time error h. None of the above
4. class JMM105 { public static void main(String args[]) { int x = 6; int success = 0; do {
switch(x) {
case 0: System.out.print("0"); x += 5; break;
case 1: System.out.print("1"); x += 3; break;
case 2: System.out.print("2"); x += 1; break;
case 3: System.out.print("3"); success++; break;
case 4: System.out.print("4"); x -= 1; break;
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}
case 5: System.out.print("5"); x -= 4; break;
case 6: System.out.print("6"); x -= 5; break;
}
} while ((x != 3) || (success < 2));
}
What is the result of attempting to compile and run the program? a. Prints: 60514233 b. Prints: 6152433 c. Prints: 61433 d. Prints: 6143 e. Run-time error f. Compile-time error
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Chapter 5 : Class Fundamentals and OOP
5.1 What Is an Object?
The real-world objects share two characteristics: They all have state and behavior. Software objects are modeled after real-world objects in that they too have state and behavior. A software object maintains its state in one or more variables. A software object implements its behavior with methods. A method is a function (subroutine) associated with an object.
You can represent real-world objects by using software objects.
The following illustration is a common visual representation of a software object:
Everything that the software object knows (state) and can do (behavior) is expressed by the variables and the methods within that object.
A particular object is called an instance.
The object diagrams show that the object's variables make up the center, or nucleus, of the object. Methods surround and hide the object's nucleus from other objects in the program.
Packaging an object's variables within the protective custody of its methods is called
encapsulation This conceptual picture of an object-a nucleus of variables packaged within a protective membrane of methods-is an ideal representation of an object and is the ideal that designers of object-oriented systems strive for. The access level determines which other objects and classes can access that variable or method.
Encapsulating related variables and methods into a neat software bundle is a simple yet powerful idea that provides two primary benefits to software developers:
•
Modularity: The source code for an object can be written and maintained independently
of the source code for other objects. Also, an object can be easily passed around in the system.
•
Information hiding: An object has a public interface that other objects can use to
communicate with it. The object can maintain private information and methods that can be changed at any time without affecting the other objects that depend on it.
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5.2 What Is a Class?
A class is a blueprint, or prototype, that defines the variables and the methods common to all objects of a certain kind.
Object is an instance of the class.
The role of classes in Java
Classes are the heart of Java, all Java code occurs within a class. There is no concept of free standing code and even the most simple application involves the creation of a class.
5.3 What Is a Message?
A single object alone is generally not very useful. Instead, an object usually appears as a component of a larger program or application that contains many other objects. Through the interaction of these objects, programmers achieve higher-order functionality and more complex behavior. Your bicycle hanging from a hook in the garage is just a bunch of metal and rubber; by itself, the bicycle is incapable of any activity. The bicycle is useful only when another object
(you) interacts with it (pedal).
Software objects interact and communicate with each other by sending messages to each other.
When object A wants object B to perform one of B's methods, object A sends a message to object B (see the following figure).
Sometimes, the receiving object needs more information so that it knows exactly what to do; for example, when you want to change gears on your bicycle, you have to indicate which gear you want. This information is passed along with the message as parameters.
The next figure shows the three parts of a message:
74
•
•
•
The object to which the message is addressed (
YourBicycle
)
The name of the method to perform ( changeGears
)
Any parameters needed by the method ( lowerGear
)
These three components are enough information for the receiving object to perform the desired method. No other information or context is required.
Messages provide two important benefits.
An object's behavior is expressed through its methods, so (aside from direct variable access) message passing supports all possible interactions between objects.
Objects don't need to be in the same process or even on the same machine to send and receive messages back and forth to each other.
5.4 Features of Object Oriented Programming
There are four features of OOP.
Encapsulation
Inheritance
Polymorphism
Abstraction
5.4.1 Encapsulation
Encapsulation is the principal of keeping the internal details of a classes state and behaviours hidden from the other classes that use it. This allows you to change those details where necessary, without breaking compatibility. The interconnectness between pieces of code is called 'coupling', and minimising it makes for more reusable and maintainable classes. The expectated behaviour of a class or method is referred to as its 'contract'. You do not want to
75
expose any more than the minimum required to support the contract, or you can end up with tangled interdependent code with poor maintainabilty, and in all likelyhood, poor quality.
Encapsulation also aids polymorphism and inheritance - if the internal details of a class are exposed and used, it is very hard to substitute a different implementation of the same contract.
To achieve good encapsulation, make everything have the tightest possible access control so if something should not be used in a given context, limit the access to prohibit it.
Encapsulation involves hiding data of a class and allowing access only through a public interface.
5.4.2 Inheritance
Generally speaking, objects are defined in terms of classes. You know a lot about an object by knowing its class. Even if you don't know what a penny-farthing is, if we tell you it was a bicycle, you would know that it had two wheels, handle bars, and pedals.
Object-oriented systems take this a step further and allow classes to be defined in terms of other classes. For example, mountain bikes, road bikes, and tandems are all kinds of bicycles. In object-oriented terminology, mountain bikes, road bikes, and tandems are all subclasses of the bicycle class. Similarly, the bicycle class is the superclass of mountain bikes, road bikes, and tandems. This relationship is shown in the following figure.
Each subclass inherits state and behavior from the superclass. Mountain bikes, road bikes, and tandems share some states: cadence, speed, and the like. Also, each subclass inherits methods from the superclass. Mountain bikes, road bikes, and tandems share some behaviors: braking and changing pedaling speed, for example.
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However, subclasses are not limited to the state and behaviors provided to them by their superclass. Subclasses can add variables and methods to the ones they inherit from the superclass.
Subclasses can also override inherited methods and provide specialized implementations for those methods.
The inheritance tree, or class hierarchy, can be as deep as needed. Methods and variables are inherited down through the levels. In general, the farther down in the hierarchy a class appears, the more specialized its behavior.
Note: Class hierarchies should reflect what the classes are. If we implemented a tricycle class, it
might be convenient to make it a subclass of the bicycle class — after all, both tricycles and bicycles have a current speed and cadence — but because a tricycle is not a bicycle, it's unwise to publicly tie the two classes together. It could confuse users, make the tricycle class have methods (such as "change gears") that it doesn't need, and make updating or improving the tricycle class difficult.
The
Object
class is at the top of class hierarchy, and each class is its descendant (directly or indirectly). A variable of type
Object
can hold a reference to any object, such as an instance of a class or an array.
Object
provides behaviors that are required of all objects running in the Java
Virtual Machine.
Inheritance offers the following benefits:
Subclasses provide specialized behaviors from the basis of common elements provided by the superclass. Through the use of inheritance, programmers can reuse the code in the superclass many times.
Programmers can implement superclasses called abstract classes that define "generic" behaviors. The abstract superclass defines and may partially implement the behavior, but much of the class is undefined and unimplemented. Other programmers fill in the details with specialized subclasses.
5.4.3 Polymorphism
The word polymorphism comes from the Greek for "many forms." It allows one interface to be used for a general class of action. The specific action is determined by the exact nature of the situation. Consider a stack. You might have a program that requires three types of stacks. One stack is used for integer values, one for floating point values and one for characters. The algorithm that implements each stack is same, even though the data being stored is different. In an non-object-oriented-programming language, you would require three different sets of stack routines, with each set using different names. However, because of polymorphism, in Java you can specify a general set of stack routines that all share same names.
The concept of polymorphism is expressed by the phrase “one interface, multiple methods”. This means that it is possible to design a generic interface to a group of related activities. This helps
77
reduce complexity by allowing the same interface to be used to specify a general class of action.
It is the compiler’s job to select the specific action (i.e method) as it applies to each situation.
You do not need to make the selection manually. You need only remember to utilize the general interface.
Polymorphism in Java is implemented with Method Overloading and Overriding.
5.4.4 Abstraction
Humans manage complexity through abstraction. For example, people do not think of a car as a set of tens of thousands of individual parts. They think of it as a well defined object with its own unique behavior. This abstraction allows people to use a car to drive to the grocery store without being overwhelmed by the complexity of thje parts that form the car. They can ignore the details of how the engine, transmission, and braking systems work. Instead they are free to utilize the object as a whole.
Abstraction is hiding the irrelevant details and knowing only the relevant ones.
5.5 Defining Classes
The general syntax for defining a class in Java is shown below. class MyClassName{
. . .
} //End of class definition.
This syntax defines a class and creates a new type named MyClassName.
5.6 Creating Objects
Declaring a Variable to Refer to an Object
The declared type matches the class of the object:
MyClass myObject = new MyClass();
The declared type is a parent class of the object's class:
MyParent myObject = new MyClass();
The declared type is an interface which the object's class implements:
MyInterface myObject = new MyClass();
You can also declare a variable on its own line, such as:
78
MyClass myObject;
When you use this approach, the value of myObject
will be automatically set to null
until an object is actually created and assigned to it. Remember, variable declaration alone does not actually create an object.
A variable in this state, which currently references no object, is said to hold a null reference.
Instantiating a Class
The new
operator instantiates a class by allocating memory for a new object.
Note: The phrase "instantiating a class" means the same thing as "creating an object"; you can
think of the two as being synonymous. When you create an object, you are creating an instance of a class, therefore "instantiating" a class.
The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate. The constructor initializes the new object.
The new operator returns a reference to the object it created. Often, this reference is assigned to a variable of the appropriate type. If the reference is not assigned to a variable, the object is unreachable after the statement in which the new operator appears finishes executing.
5.7 Defining Methods in a class
Classes usually consists of two things: instance variables and methods. The general form of a method is as follows:
type name (parameter-list) {
// body of method
}
The type specifies the return type of the method. This can be any valid type, including class types that you create. If the method does not return a value, its return type must be void. The name of the method is specified by name. This can be any legal identifier other than those already used by other items within the current scope. The parameter-list is a sequence of type and identifier pairs separated by commas. Parameters are essentially variables that receive the value of the arguments passed to the method when it is called. If the method has no parameters, then the parameter list will be empty.
Element
accessLevel
static
Method Declaration Elements
Function
(Optional) Access level for the method
(Optional) Declares a class method
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abstract final
(Optional) Indicates that the method is not implemented
(Optional) Indicates that the method cannot be overridden native (Optional) Indicates that the method is implemented in another language synchronized
returnType methodName
( paramList )
(Optional) The method requires a monitor to run
The method's return type and name
The list of arguments to the method throws exceptions (Optional) The exceptions thrown by the method
Each element of a method declaration can be further defined and is discussed as indicated in the following list:
accessLevel
As with member variables, you control which other classes have access to a method using one of four access levels: public, protected, package, and private static
As with member variables, static
declares this method as a class method rather than an instance method. abstract
An abstract method has no implementation and must be a member of an abstract class. final native
A final method cannot be overridden by subclasses
If you have a significant library of functions written in another language, such as C, you may wish to preserve that investment and to use those functions from a program written in the Java programming language. Methods implemented in another language are called native methods and are declared as such using the native
keyword synchronized
Concurrently running threads often invoke methods that operate on the same data. Mark these methods with the synchronized
keyword to ensure that the threads access information in a thread-safe manner.
returnType
A method must declare the data type of the value that it returns. If your method does not return a value, use the keyword void
for the return type.
methodName
A method name can be any legal identifier. You need to consider code conventions, name overloading, and method overriding when naming a method.
( paramlist )
You pass information into a method through its arguments.
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throws exceptionList
If your method throws any checked exceptions, your method declaration must indicate the type of those exceptions.
Two of these components comprise the method signature: the method's name and the parameter list.
Methods that have return type other than void return a value to the calling routine using the following form of return statement: return value;
Here, value is the value returned.
Let’s add a method to a Box class. class Box { double width, height, depth; void volume() {
System.out.println(“Volume is :”+
(width * depth * height));
}
} class BoxDemo { public static void main(String args[]) {
Box mybox1 = new Box(); mybox1.width = 10; mybox1.height = 20; mybox1.depth = 30; mybox1.volume();
}
}
Naming a Method
Although a method name can be any legal identifier, code conventions restrict method names. In general, method names should be verbs and should be in mixed case, with the first letter in lowercase and the first letter of each internal word in uppercase. Here are some examples: toString compareTo isDefined setX getX
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The JavaBeans architecture naming conventions further describe how to name methods for setting and getting properties.
Passing information to a method
The declaration for a method declares the number and the type of the arguments for that method or constructor. For example, the following is a method that computes the monthly payments for a home loan, based on the amount of the loan, the interest rate, the length of the loan (the number of periods), and the future value of the loan: public double computePayment(double loanAmt, double rate,
double futureValue,
int numPeriods) {
double I, partial1, denominator, answer;
I = rate / 100.0;
partial1 = Math.pow((1 + I), (0.0 - numPeriods));
denominator = (1 - partial1) / I;
answer = ((-1 * loanAmt) / denominator)
- ((futureValue * partial1) / denominator);
return answer;
}
This method takes four arguments: the loan amount, the interest rate, the future value and the number of periods. The first three are double-precision floating point numbers, and the fourth is an integer.
As with this method, the set of arguments to any method or constructor is a comma-separated list of variable declarations, where each variable declaration is a type/name pair. As you can see from the body of the computePayment method, you simply use the argument name to refer to the argument's value.
Argument Types
You can pass an argument of any data type into a method. This includes primitive data types, such as doubles, floats, and integers, as you saw in the computePayment
method, and reference data types, such as classes and arrays. Here's an example of a factory method that accepts an array as an argument. In this example, the method creates a new
Polygon
object and initializes it from a list of
Point s (assume that
Point
is a class that represents an x, y coordinate): public Polygon polygonFrom(Point[] listOfPoints) {
...
}
The Java programming language doesn't let you pass methods into methods. But you can pass an object into a method and then invoke the object's methods.
Argument Names
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When you declare an argument to a method, you provide a name for that argument. This name is used within the method body to refer to the data.
The name of an argument must be unique in its scope. It cannot be the same as the name of another argument for the same method or constructor, the name of a local variable within the method or constructor, or the name of any parameter to a catch clause within the same method or constructor.
An argument can have the same name as one of the class's member variables. If this is the case, the argument is said to shadow the member variable. Shadowing member variables can make your code difficult to read and is conventionally used only within methods that set a particular member variable. For example, consider the following Circle class and its setOrigin method: public class Circle {
private int x, y, radius;
public void setOrigin(int x, int y) {
...
}
}
The
Circle
class has three member variables: x
, y
, and radius
. The setOrigin
method accepts two arguments, each of which has the same name as one of the member variables. Each method argument shadows the member variable that shares its name. So using the simple names x
or y within the body of the method refers to the argument, not to the member variable. To access the member variable, you must use a qualified name.
Returning a Value from a Method
You declare a method's return type in its method declaration. Within the body of the method, you use the return
statement to return the value. Any method declared void
doesn't return a value and cannot contain a return
statement. Any method that is not declared void
must contain a return
statement.
Let's look at the isEmpty method in the Stack class: public boolean isEmpty() {
if (items.size() == 0) {
return true;
} else {
return false;
}
}
The data type of the return value must match the method's declared return type; you can't return an integer value from a method declared to return a boolean. The declared return type for the isEmpty
method is boolean
, and the implementation of the method returns the boolean value true
or false
, depending on the outcome of a test.
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The isEmpty method returns a primitive type. A method can return a reference type. For example, Stack declares the pop method that returns the Object reference type: public Object pop() {
if (top == 0) {
throw new EmptyStackException();
}
Object obj = items[--top];
items[top]=null;
return obj;
}
When a method uses a class name as its return type, such as pop
does, the class of the type of the returned object must be either a subclass of or the exact class of the return type. Suppose that you have a class hierarchy in which
ImaginaryNumber
is a subclass of java.lang.Number
, which is in turn a subclass of
Object
, as illustrated in the following figure.
Now suppose that you have a method declared to return a
Number
: public Number returnANumber() {
...
}
The returnANumber
method can return an
ImaginaryNumber
but not an
Object
.
ImaginaryNumber
is a
Number
because it's a subclass of
Number
. However, an
Object
is not necessarily a
Number
— it could be a
String
or another type.
You also can use interface names as return types. In this case, the object returned must implement the specified interface.
5.8 Declaring Variables in a Class
Stack
uses the following line of code to define its variables: class Stack { int top;
}
This code declares member variable and not other types of variable, such as local variable, because the declaration appears within the class body but outside any methods.
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Variable Declaration Elements
Element
accessLeve l
static
type name
Function
(Optional) Access level for the variable volatile
(Optional) Declares a class variable final
(Optional) Indicates that the variable is a constant transient (Optional) Indicates that the variable is transient
(Optional) Indicates that the variable is volatile
The type and name of the variable
Each component of a member variable declaration is further defined and discussed in later sections of this chapter, as follows:
accessLevel
Lets you control what other classes have access to a member variable by specifying one of four access levels: public, protected, package, and private. static
Declares this is a class variable rather than an instance variable. final
Indicates that the value of this member cannot change. transient
Marks member variables that should not be serialized. volatile
Prevents the compiler from performing certain optimizations on a member
type
Like other variables, a member variable must have a type. You can use primitive type names such as int
, float
, or boolean
. Or you can use reference types, such as array, object, or interface names.
name
A member variable's name can be any legal identifier and, by convention, begins with a lowercase letter. A member variable cannot have the same name as any other member variable in the same class.
Automatic local variables
Automatic variables are method variables. They come into scope when the method code starts to execute and cease to exist once the method goes out of scope. As they are only visible within the
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method they are typically useful for temporary manipulation of data. If you want a value to persist between calls to a method then a variable needs to be created at class level.
Variable Initialization
Local variables and member variables can be initialized with an assignment statement when they're declared. Local variables have to explicitly initialized before their use. The data type of the variable must match the data type of the value assigned to it.
char aChar = 'S'; boolean aBoolean = true;
Parameters and exception-handler parameters cannot be initialized in this way. The value for a parameter is set by the caller.
Variable Scope
A variable's scope is the region of a program within which the variable can be referred to by its simple name. Secondarily, scope also determines when the system creates and destroys memory for the variable. Scope is distinct from visibility, which applies only to member variables and determines whether the variable can be used from outside of the class within which it is declared.
Visibility is set with an access modifier. The location of the variable declaration within your program establishes its scope and places it into one of these four categories:
•
•
•
•
member variable
local variable method parameter exception-handler parameter
A member variable is a member of a class or an object. It is declared within a class but outside of any method or constructor. A member variable's scope is the entire declaration of the class.
However, the declaration of a member needs to appear before it is used when the use is in a member initialization expression.
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You declare local variables within a block of code. In general, the scope of a local variable extends from its declaration to the end of the code block in which it was declared. The scope of each variable in that program extends from the declaration of the variable to the end of the main method --indicated by the first right curly bracket
}
in the program code.
Parameters are formal arguments to methods or constructors and are used to pass values into methods and constructors. The scope of a parameter is the entire method or constructor for which it is a parameter.
Exception-handler parameters are similar to parameters but are arguments to an exception handler rather than to a method or a constructor. The scope of an exception-handler parameter is the code block between
{
and
}
that follow a catch statement.
if (...) {
int i = 17;
...
}
System.out.println("The value of i = " + i); // error
The final line won't compile because the local variable i
is out of scope. The scope of i
is the block of code between the
{
and
}
. The i
variable does not exist anymore after the closing
}
.
Either the variable declaration needs to be moved outside of the if
statement block, or the println
method call needs to be moved into the if
statement block.
5.9 Instance and Class Members
When you declare a member variable such as aFloat in MyClass: class MyClass {
float aFloat;
} you declare an instance variable. Every time you create an instance of a class, the runtime system creates one copy of each the class's instance variables for the instance.
Instance variables are in constrast to class variables (which you declare using the static modifier). The runtime system allocates class variables once per class regardless of the number of instances created of that class. The system allocates memory for class variables the first time it encounters the class. All instances share the same copy of the class's class variables. You can access class variables through an instance or through the class itself.
Methods are similar: your classes can have instance methods and class methods. Instance methods operate on the current object's instance variables but also have access to the class variables. Class methods, on the other hand, cannot access the instance variables declared within the class (unless it creates a new object and accesses them through the object). Also, class methods can be invoked on the class, you don't need an instance to call a class method.
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By default, unless otherwise specified, a member declared within a class is an instance member.
The class defined below has one instance variable--an integer named x--and two instance methods--x() and setX()--that let other objects set and query the value of x: class AnIntegerNamedX {
int x;
public int x() {
return x;
}
public void setX(int newX) {
x = newX;
}
}
Every time you instantiate a new object from a class, you get a new copy of each of the class's instance variables. These copies are associated with the new object. So, every time you instantiate a new AnIntegerNamedX object from the class, you get a new copy of x
that is associated with the new AnIntegerNamedX object.
All instances of a class share the same implementation of an instance method; all instances of
AnIntegerNamedX share the same implementation of x() and setX(). Note that both methods, x() and setX(), refer to the object's instance variable x by name. "But", you ask, "if all instances of
AnIntegerNamedX share the same implementation of x() and setX() isn't this ambiguous?" The answer is no. Within an instance method, the name of an instance variable refers to the current object's instance variable (assuming that the instance variable isn't hidden by a method parameter).
Objects outside of AnIntegerNamedX that wish to access x must do so through a particular instance of AnIntegerNamedX. Suppose that this code snippet was in another object's method. It creates two different objects of type AnIntegerNamedX, sets their x values to different values, then displays them:
. . .
AnIntegerNamedX myX = new AnIntegerNamedX();
AnIntegerNamedX anotherX = new AnIntegerNamedX(); myX.setX(1); anotherX.x = 2;
System.out.println("myX.x = " + myX.x());
System.out.println("anotherX.x = " + anotherX.x());
. . .
Notice that the code used setX()
to set the x
value for myX
but just assigned a value to anotherX.x
directly. Either way, the code is manipulating two different copies of x
: the one contained in the myX
object and the one contained in the anotherX
object. The output produced by this code snippet is: myX.x = 1 anotherX.x = 2
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showing that each instance of the class AnIntegerNamedX has its own copy of the instance variable x
and each x
has a different value.
You can, when declaring a member variable, specify that the variable is a class rather than an instance variable. Similarly, you can specify that a method is a class method rather than an instance method. The system creates a single copy of a class variable the first time it encounters the class in which the variable is defined. All instances of that class share the same copy of the class variable. Class methods can only operate on class variables--they cannot access the instance variables defined in the class.
To specify that a member variable is a class variable, use the static keyword. For example, let's change the AnIntegerNamedX class such that its x variable is now a class variable: class AnIntegerNamedX {
static int x;
public int x() {
return x;
}
public void setX(int newX) {
x = newX;
}
}
Now the exact same code snippet from before that creates two instances of AnIntegerNamedX, sets their x
values, and then displays them produces this, different, output. myX.x = 2 anotherX.x = 2
The output is different because x
is now a class variable so there is only one copy of the variable and it is shared by all instances of AnIntegerNamedX including myX
and anotherX
. When you invoke setX()
on either instance, you change the value of x
for all instances of
AnIntegerNamedX.
You use class variables for items that you need only one copy of and which must be accessible by all objects inheriting from the class in which the variable is declared. For example, class variables are often used with final to define constants (this is more memory efficient as constants can't change so you really only need one copy).
Similarly, when declaring a method, you can specify that method to be a class method rather than an instance method. Class methods can only operate on class variables and cannot access the instance variables defined in the class.
To specify that a method is a class method, use the static keyword in the method declaration.
Let's change the AnIntegerNamedX class such that its member variable x is once again an instance variable, and its two methods are now class methods: class AnIntegerNamedX {
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private int x;
static public int x() {
return x;
}
static public void setX(int newX) {
x = newX;
}
}
When you try to compile this version of AnIntegerNamedX, you will get compiler errors:
AnIntegerNamedX.java:4: Can't make a static reference to nonstatic variable x in class AnIntegerNamedX.
return x;
^
AnIntegerNamedX.java:7: Can't make a static reference to nonstatic variable x in class AnIntegerNamedX.
x = newX;
^
2 errors
This is because class methods cannot access instance variables unless the method created an instance of AnIntegerNamedX first and accessed the variable through it.
Let's fix AnIntegerNamedX by making its x variable a class variable: class AnIntegerNamedX {
static private int x;
static public int x() {
return x;
}
static public void setX(int newX) {
x = newX;
}
}
Now the class will compile and the same code snippet from before that creates two instances of
AnIntegerNamedX, sets their x
values, and then prints the x
values produces this output: myX.x = 2 anotherX.x = 2
Again, changing x
through myX
also changes it for other instances of AnIntegerNamedX.
Another difference between instance members and class members is that class members are accessible from the class itself. You don't need to instantiate a class to access its class members.
Let's rewrite the code snippet from before to access x() and setX() directly from the
AnIntegerNamedX class:
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. . .
AnIntegerNamedX.setX(1);
System.out.println("AnIntegerNamedX.x = " + AnIntegerNamedX.x());
. . .
Notice that you no longer have to create myX
and anotherX
. You can set x
and retrieve x
directly from the AnIntegerNamedX class. You cannot do this with instance members, you can only invoke instance methods from an object and can only access instance variables from an object.
You can access class variables and methods either from an instance of the class or from the class itself.
Default Values of Variables
Type boolean byte char short
Default value
false
0
'\u0000'
0
int long float double
0
0l
0.0f
0.0d
Object
null
Array
based on Array type
Field variables (class members) are automatically initialized to default values. Local variables
(method or constructor variables) are not automatically initialized. Arrays, whether field or local variables, are automatically initialized to the default values of their declared type. class CheckInit {
// field variable
static int i;
// field array reference variable
static String[] s = new String[10];
static void myMethod(){
int j; // local variable
int[] a = new int[5]; // local variable array
// causes compile error if not explicitly initialized
j = 10;
}
System.out.println(" Local variable: " + j);
System.out.println(" Local array ref: " + a[3]);
public static void main(String[] args) {
System.out.println("Field variable i: " + i);
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}
System.out.println(" Field array ref: " + s[2]); myMethod();
}
Output of CheckInit:
Field variable i: 0 // default value of int
Field array ref: null // default value for String[]
Local variable: 10 // explicit value
Local array ref: 0 // default value of int[]
5.10 Static Initializer
A static initializer block resembles a method with no name, no arguments, and no return type. It doesn't need a name, because there is no need to refer to it from outside the class definition. The code in a static initializer block is executed by the virtual machine when the class is loaded.
Like a constructor, a static initializer block cannot contain a return statement. Therefore, it doesn't need to specify a return type
.
Because it is executed automatically when the class is loaded, parameters don't make any sense, so a static initializer block doesn't have an argument list.
static {
}
Static initializers are blocks of code within a class that are outside the scope of a method. The code in a static initializer block is executed only once, when the class is loaded. The code is executed in the same order as it is declared in the class. The following code illustrates the use of a static initializer block:
1. public class TestStaticInitializer {
2. static String str = "Default";
3.
4. public TestStaticInitializer() {
5. System.out.println("1::str = " + str);
6. }
7.
8. // Static initializer block
9. static {
10. System.out.println("2::str = " + str);
11. str = "Static";
12. }
13.
14. // Main routine to test the class
15. public static void main(String[] args) {
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16. TestStaticInitializer tsi = new TestStaticInitializer();
17. }
18. }
A static string is declared on Line 2 and set to "Default." Next, a static initializer block prints the string out and resets it to the value "Static" on Lines 8-12. The constructor on Lines 4-6 merely prints the variable out.
The output of this program is:
2:: str = Default
1:: str = Static
When the class is loaded (because it was constructed) on line 17, lines 9-12 are executed before the constructor is called. The code in static initializer blocks is always executed when the class is loaded and before any method in the class is executed.
Only static members can be referenced within static initializer. Static initializer blocks may be used for executing code that initializes the state of the object or for providing debugging functionality. However, overuse of this feature can lead to complex and unreadable code.
Multiple static initializer blocks
You may include any number of static initializer blocks in your class definition. The static initializer blocks will be executed in the order in which they appear in the code.
5.11 Variable Shadowing
A field declaration can be shadowed by a local variable declaration class TestShadowing {
static int x = 1; // field variable
public static void main(String[] args) {
int x = 0; // local variable
System.out.println("x = " + x);
System.out.println("TestShadowing.x = " + TestShadowing.x)
}
}
Output: x = 0
TestShadowing.x = 1 because the identifier x is used within a code block main() a search is made for a declaration of x within the body of main(). As one is found, int x = 0, the simple identifier name x is assumed to be within scope as a local variable.
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To access the field variable x, you must use its fully-qualified name TestShadowing.x
5.12 Pass by value and Pass by reference
Primitive data type operations deal only with value. For example, the following code shows that assigning primitive data variable i
to another named j
simply passes a copy of the value in i
into j
. That is, a data value passes and not a reference or pointer to a data location.
int i = 1;
int j = i; // Now j holds "1"
// that is, i's value passes to j
i = 2; // j still holds "1"
Similarly, in method arguments, primitive variables pass by value. That is, a copy is made and passed to the method. Changes to the passed value do not affect the variable in the calling code.
The following shows code on top that creates an instance of
AClass
and invokes the aMethod with an int
argument. The code on the bottom shows the
AClass
definition and the aMethod
.
...
int i = 2;
AClass a1 = new AClass ();
a1.aMethod (i);
int m = i; // m = i = 2 not 5
...
class AClass
{
int j = 1;
void aMethod (int k)
{
int i = 10 * k;
k = 5;
j = k * 10;
}
}
The value in variable i
in the code on the left will be passed to variable k in the aMethod argument list. When that method assigns a value of
5
to k
, it has no affect whatsoever on variable i
the code on the left.
Modify Reference Variables
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You can always assign a new object to a reference variable. For example, in the following code, we create two instances of the
AClass
, " a
" and " b
", and then assign new values to the " j
" variable for each object:
...
AClass a,b;
a = new AClass ();
b = new AClass ();
a.j = 4;
b.j = 40;
a = b; // a now references same
// object as does the b variable.
a.j = 400;
int x = b.j; // x = 400 since a and b now
// reference same object.
... public class AClass
{
public int j = 1;
void aMethod (int k)
{
int i = 10*k;
k = 5;
j = k * 10;
}
}
Then we assign the " b
" object to the " a
" reference with the a=b;
statement. So both the " a
" and
" b
" variables now reference the same object.
Note that assigning a new object to the " a
" variable orphans the original object that " a
" pointed to previously. If an object is no longer referenced by any other variables, then the garbage
collector will eventually reclaim the memory that it occupies.
References in Method Arguments
The argument list of a method can include object references. The methods and data of that object can be accessed and modified by the method.
However, the reference itself passes by value. That is, the JVM makes a copy of the internal memory pointer and the copy of the pointer goes into the argument value.
So if the reference variable in the method is set to a new object, that will have no affect on the reference variable in the calling method.
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For example, in the following code, a reference to a
BClass
object passes to the method of a
AClass
object. In that method, the argument variable is assigned to a new instance of the
BClass
. However, when the process returns to the calling code, the original reference remains unaffected.
...
int i = 2;
AClass a;
BClass b;
a = new AClass ();
b = new BClass ();
b.j = 5;
a.aMethod (b);
j = b.j;// j = 20 not 5 or 100
...
class AClass {
void aMethod (BClass bb) {
bb.j = 20;
bb= new BClass ();
bb.j = 100;
}
}
class BClass {
int j = 0;
}
Since reference variables only hold pointers, the test
if (a == b)... simply tests whether a and b point to the same object, not if the referenced objects have equal data values.
If you need to test whether two objects hold the same data values, many classes provide an
" equals()
" method such that
a.equals(b) returns true if the object b
data matches that in object a
.
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Let's look at a method called getRGBColor within a class called Pen. This method is attempting to return three values by setting the values of its arguments: public class Pen {
private int redValue, greenValue, blueValue;
...
//This method does not work as intended.
public void getRGBColor(int red, int green, int blue) {
red = redValue;
green = greenValue;
blue = blueValue;
}
}
This simply does not work. The red
, green
, and blue
variables exist only within the scope of the getRGBColor
method. When that method returns, those variables are gone and any changes to them lost.
Let's rewrite the getRGBColor method so that it does what was intended. First, we need a new type of object, RGBColor, that can hold the red, green, and blue values of a color in RGB space: public class RGBColor {
public int red, green, blue;
}
Now we can rewrite getRGBColor
so that it accepts an
RGBColor
object as an argument. The getRGBColor
method returns the current color of the pen by setting the red
, green
, and blue member variables of its
RGBColor
argument: public class Pen {
private int redValue, greenValue, blueValue;
...
public void getRGBColor(RGBColor aColor) {
aColor.red = redValue;
aColor.green = greenValue;
aColor.blue = blueValue;
}
}
The changes made to the
RGBColor
object within the getRGBColor
method persist after the method returns, because aColor
is a reference to an object that exists outside the scope of the method.
5.13 Access Control
Access or visibility rules determine whether a method or a data variable can be accessed by another method in another class or subclass.
Java provides for 4 access modifiers :
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1. public - access by any other class anywhere.
2. protected - accessible by the package classes and any subclasses that are in other packages .
3. Default - (also known as "package private") accessible to classes in the same package but not by classes in other packages, even if these are subclasses.
4. private - accessible only within the class. Even methods in subclasses in the same package do not have access.
These access rules allow one to control the degree of encapsulation of your classes. For example, you may distribute your class files to other users who can make subclasses of them. By making some of your data and methods private
, you can prevent the subclass code from interfering with the internal workings of your classes.
Also, if you distribute new versions of your classes at a later point, you can change or eliminate these private attributes and methods without worrying that these changes will affect the subclasses.
Access levels affect you in two ways. First, when you use classes that come from another source, such as the classes in the Java platform, access levels determine which members of those classes your classes can use. Second, when you write a class, you need to decide what access level every member variable and every method in your class should have. One way of thinking about access levels is in terms of the API: access levels directly affect the public API of a class and determine which members of the class can be used by other classes. You need to put as much effort into deciding the access level for a member as you put into making other decisions about your class's
API, such as naming methods.
Modifier public protected private no modifier
Used with
Classes
Interfaces
Constructors
Inner Classes
Methods
Field variables
Constructors
Inner Classes
Methods
Field variables
Constructors
Inner Classes
Methods
Field variables
Classes
Interfaces
Constructors
Inner Classes
Methods
Description
A Class or Interface may be accessed from outside its package.
Constructors, Inner Classes, Methods and Field variables may be accessed from wherever their class is accessed.
May be accessed by other classes in the same package or from any subclasses of the class in which they are declared.
May be accessed only from within the class in which they are declared.
May only be accessed from within the package in which they are declared.
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Field variables
Private
Private variables are only visible from within the same class as they are created in. This means they are NOT visible within sub classes. This allows a variable to be insulated from being modified by any methods except those in the current class. class Base{ private int iEnc=10; public void setEnc(int iEncVal){
if(iEncVal < 1000){
iEnc=iEncVal;
}
else {
System.out.println("Enc value must be less than 1000");
}//End if
} public static void main(String args[]) {
Base b=new Base(); b.iEnc = 20;
}
} class Enc{ public static void main(String argv[]){
Base b = new Base();
b.iEnc = 20; //compile-time error
b.setEnc(20);
}//End of main
}
If the constructors are declared private then the object of that class can be created only in that class. It cannot be created outside the class definition.
Public
The public modifier can be applied to a variable (field) or a class. It is the first modifier you are likely to come across in learning Java.
A public class has global scope, and an instance can be created from anywhere within or outside of a program. Only one non inner class in any file can be defined with the public keyword. If you define more than one non inner class in a file with the keyword public the compiler will generate an error.
Using the public modifier with a variable makes it available from anywhere. It is used as follows, public int myint =10;
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If you want to create a variable that can be modified from anywhere you can declare it as public.
You can then access it using the dot notation similar to that used when calling a method.
class Base {
public int iNoEnc=77;
} class NoEnc{ public static void main(String argv[]){
Base b = new Base();
b.iNoEnc=2;
System.out.println(b.iNoEnc);
}//End of main
}
One .java file can contain multiple classes but only one public class. The public classes should be stored in separate class files. The name of the class file should be same as that of the public class.
Protected
A protected variable is visible within a class, and in sub classes, the same package but not elsewhere. The qualification that it is visible from the same package can give more visibility than you might suspect. Any class in the same directory is considered to be in the default package, and thus protected classes will be visible. This means that a protected variable is more visible than a variable defined with no access modifier.
A variable defined with no access modifier is said to have default visibility. Default visibility means a variable can be seen within the class, and from elsewhere within the same package, but not from sub-classes that is not in the same package.
Access Modifiers and encapsulation
The visibility modifiers are part of the encapsulation mechanism for Java. Encapsulation allows separation of the interface from the implementation of methods.
The visibility modifiers are a key part of the encapsulation mechanism for java. Encapsulation allows separation of the interface from the implementation of methods. The benefit of this is that the details of the code inside a class can be changed without it affecting other objects that use it.
This is a key concept of the Object Oriented paradigm.
Encapsulation generally takes form of methods to retrieve and update the values of private class variables. These methods are known as a accessor and mutator methods. The accessor (or get) method retrieves the value and the mutator changes (or sets) the value. The naming convention for these methods are setFoo to change a variable and getFoo to obtain the contents of a variable.
An aside note: the use of get and set in the naming of these methods is more significant than just programmer convenience and is an important part of the Javabeans system.
Take the example where you had a variable used to store the age of a student.
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You might store it simply with a public integer variable int iAge; later when your application is delivered you find that some of your students have a recorded age of more than 200 years and some have an age of less than zero. You are asked to put in code to check for these error conditions. So wherever your programs change the age value, you write if statements that check for the range.
if(iAge > 70){
//do something
} if (iAge <3){ //do something
}
In the process of doing this you miss some code that used the iAge variable and you get called back because you have a 19 year old student who is on your records has being 190 years old.
The Object Oriented approach to this problem using encapsulation, is to create methods that access a private field containing the age value, with names like setAge and getAge. The setAge method might take an integer parameter and update the private value for Age and the getAge method would take no parameter but return the value from the private age field.
public void setAge(int iStudentAge){
iAge = iStudentAge;
} public int getAge(){
return iAge;
}
At first this seems a little pointless as the code seems to be a long way around something that could be done with simple variable manipulation. However when they come back to you with the requirement to do more and more validation on the iAge field you can do it all in these methods without affecting existing code that uses this information.
By this approach the implementation of code, (the actual lines of program code), can be changed whilst the way it looks to the outside world (the interface) remains the same.
5.14 Constructors
Classes have a special method called a constructor that is called implicitly when a class instance is created. The class constructor always has the same name as the class and no return type.
Note: If you do not write your own constructor, the compiler adds an empty constructor, which
calls the no-arguments constructor of its parent class. The empty constructor is called the default constructor. The default constructor initializes all non-initialized fields and variables to zero.
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If you create constructors of your own, Java does not supply the default zero parameter constructor class LessonTwoD {
String text;
//Constructor
LessonTwoD(){
text = "I'm a Simple Program";
}
//Accessor method
String getText(){
return text;
}
public static void main(String[] args){
LessonTwoD progInst = new LessonTwoD();
String retrievedText = progInst.getText();
System.out.println(retrievedText);
}
}
As soon as you create any constructors of your own you loose the default no parameter constructor. If you then try to create an instance of the class without passing any parameters (i.e. invoking the class with a zero parameter constructor), you will get an error. Thus as soon as you create any constructors for a class you need to create a zero parameter constructor.
The following example illustrates code that will not compile. When the compiler checks to create the instance of the Base class called c it inserts a call to the zero parameter constructor. Because
Base has an integer constructor the zero parameter constructor is not available and a compile time error occurs. This can be fixed by creating a "do nothing" zero parameter constructor in the class Base.
//Warning: will not compile.
class Base{
Base(int i){
System.out.println("single int constructor");
}
} public class Cons {
public static void main(String argv[]){
Base c = new Base();
}
}
//This will compile
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class Base{
Base(int i){
System.out.println("single int constructor");
}
Base(){}
} public class Cons {
public static void main(String argv[]){
Base c = new Base();
}
}
Constructors cannot be native, abstract, static, synchronized or final.
5.15 The this keyword
Within an instance method or a constructor, this
is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this
. The most common reason for doing so is that a member variable is hidden by an argument to the method or the constructor.
For example, the following constructor for the HSBColor class initializes the object's member variables according to the arguments passed into the constructor. Each argument to the constructor hides one of the object's member variables, so this constructor must refer to the object's member variables through this: public class HSBColor {
private int hue, saturation, brightness;
public HSBColor (int hue, int saturation,
int brightness) {
this.hue = hue;
this.saturation = saturation;
this.brightness = brightness;
}
} this keyword cannot be used in a static reference.
5.16 Overloading
The Java programming language supports name overloading for methods, which means that multiple methods in the same class can share the same name if they have different parameter lists. Suppose that you have a class that can draw various types of data (strings, integers, and so on) and that contains a method for drawing each data type. In other languages, you have to think of a new name for each method, for example, drawString, drawInteger, drawFloat, and so on. In the Java programming language, you can use the same name for all the drawing methods but pass a different type of argument to each method. Thus, the data drawing class might declare three methods named draw, each of which takes a different type of argument.
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public class DataArtist {
...
public void draw(String s) {
...
}
public void draw(int i) {
...
}
public void draw(float f) {
...
}
}
Overloaded methods are differentiated by the number and the type of the arguments passed into the method. In the code sample, draw(String s)
and draw(int i)
are distinct and unique methods because they require different argument types. You cannot declare more than one method with the same name and the same number and type of arguments, because the compiler cannot tell them apart. The compiler does not consider return type when differentiating methods, so you cannot declare two methods with the same signature even if they have a different return type.
Overloading is a one of the ways in which Java implements one of the key concepts of Object orientation, polymorphism. Polymorphism is a word constructed from Poly meaning "many" and
"morphism" implying meaning. Thus a overloading allows the same method name to have multiple meanings or uses. The method call is resolved during the compile-time hence it uses static or early binding.
Thus imagine you were designing the interface for a system to run mock Java certification exams. An answer may come in as an integer, a boolean or a text string. You could create a version of the method for each parameter type and give it a matching name thus.
markanswerboolean(boolean answer){
} markanswerint(int answer){
} markanswerString(String answer){
}
This would work but it means that future users of your classes have to be aware of more method names than is strictly necessary. It would be more useful if you could use a single method name and the compiler would resolve what actual code to call according to the type and number of parameters in the call.
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The following is not an example of overloading and will cause a compile time error indicating a duplicate method declaration.
void markanswer(String answer){
} void markanswer(String title){
}
The return type does not form part of the signature for the purpose of overloading.
Thus changing one of the above to have an int return value will still result in a compile time error, but this time indicating that a method cannot be redefined with a different return type.
Overloaded methods do not have any restrictions on what exceptions can be thrown. That is something to worry about with overriding.
Overloaded methods are differentiated only on the number, type and order of parameters, not on the return type of the method
Overloading Constructors
You can define multiple constructors to provide optional ways to create and initialize instances of the class.
class Test
{
int i;
boolean b=true;
Test(int j, boolean a)
{
i = j;
b = a;
}
Test(int j)
{
i = j;
}
int get()
{
return i;
}
}
This constructor initializes the two properties to the values passed as arguements.
A second constructor passes an initial value to just one of the variables.
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Note that if a class includes one or more explicit constructors, the java
compiler does not create the default constructor. So for the class shown above, the code
Test ex = new Test(); will generate a compiler error stating that no such constructor exists.
Use of this()
From within a constructor, you can also use the this
keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. public class Rectangle {
private int x, y;
private int width, height;
public Rectangle() {
this(0, 0, 0, 0);
}
public Rectangle(int width, int height) {
this(0, 0, width, height);
}
public Rectangle(int x, int y, int width, int height) {
this.x = x;
this.y = y;
this.width = width;
this.height = height;
}
...
}
This class contains a set of constructors. Each constructor initializes some or all of the rectangle's member variables. The constructors provide a default value for any member variable whose initial value is not provided by an argument. For example, the no-argument constructor calls the four-argument constructor, using 0s as default values. As before, the compiler determines which constructor to call, based on the number and the type of arguments.
If present, an explicit constructor invocation must be the first line in the constructor.
5.17 Recursion
Recursion is a process of defining something in terms of itself. Recursion is an attribute that allows a method to call itself. A method that calls itself is said to be recursive.
//A simple example of recursion class Factorial {
//this is a recursive function int fact(int n) { int result;
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if(n == 1) return 1; result = fact(n-1) * n; return result;
}
} class Recursion { public static void main(String args[]) {
Factorial f = new Factorial();
System.out.println(“Factorial of 3 is “+f.fact(3));
}
}
5.18 Native Methods
The native modifier is used only for methods and indicates that the body of the code is written in a language other than Java such as C or C++. Native methods are often written for platform specific purposes such as accessing some item of hardware that the Java Virtual Machine is not aware of. Another reason is where greater performance is required.
A native method ends with a semicolon rather than a code block. Thus the following would call an external routine, written perhaps in C++.
public native void fastcalc();
The implementation of native is out of the scope of this course.
5.19 Arrays of Objects
Arrays can hold reference types as well as primitive types. You create such an array in much the same way you create an array with primitive types. Here's a small program,
ArrayOfStringsDemo that creates an array containing three string objects then prints the strings in all lower case letters. public class ArrayOfStringsDemo {
public static void main(String[] args) {
String[] anArray = { "String One", "String Two",
"String Three" };
for (int i = 0; i < anArray.length; i++) {
System.out.println(anArray[i].toLowerCase());
}
}
}
This program creates and populates the array in a single statement. However, you can create an array without putting any elements in it. This brings us to a potential stumbling block, often encountered by new programmers, when using arrays that contain objects. Consider this line of code:
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String[] anArray = new String[5];
After this line of code is executed, the array called anArray
exists and has enough room to hold
5 string objects. However, the array doesn't contain any strings yet. It is empty. The program must explicitly create strings and put them in the array. This might seem obvious, however, many beginners assume that the previous line of code creates the array and creates 5 empty strings in it. Thus they end up writing code like the following, which generates a
NullPointerException:
String[] anArray = new String[5]; for (int i = 0; i < anArray.length; i++) {
// ERROR: the following line gives a runtime error
System.out.println(anArray[i].toLowerCase());
}
Arrays of Arrays
ArrayOfArraysDemo creates an array and uses an initializer to populate it with four sub-arrays. public class ArrayOfArraysDemo {
public static void main(String[] args) {
String[][] cartoons =
{
{ "Flintstones", "Fred", "Wilma", "Pebbles", "Dino" },
{ "Rubbles", "Barney", "Betty", "Bam Bam" },
{ "Jetsons", "George", "Jane", "Elroy", "Judy", "Rosie",
“Astro" },
{ "Scooby Doo Gang", "Scooby Doo", "Shaggy", "Velma",
"Fred", "Daphne" }
};
for (int i = 0; i < cartoons.length; i++) {
System.out.print(cartoons[i][0] + ": ");
for (int j = 1; j < cartoons[i].length; j++) {
System.out.print(cartoons[i][j] + " ");
}
}
}
}
System.out.println();
Notice that the sub-arrays are all of different lengths. The names of the sub-arrays are cartoons[0]
, cartoons[1]
, and so on.
5.20 Nested and Inner Classes
You can define a class as a member of another class. Such a class is called a nested class and is illustrated here:
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class EnclosingClass {
...
class ANestedClass {
...
}
}
You use nested classes to reflect and to enforce the relationship between two classes. You should define a class within another class when the nested class makes sense only in the context of its enclosing class or when it relies on the enclosing class for its function. For example, a text cursor might make sense only in the context of a text component.
As a member of its enclosing class, a nested class has a special privilege: It has unlimited access to its enclosing class's members, even if they are declared private. However, this special privilege isn't really special at all. It is fully consistent with the meaning of private and the other access specifiers. The access specifiers restrict access to members for classes outside the enclosing class. The nested class is inside its enclosing class so that it has access to its enclosing class's members.
Like other members, a nested class can be declared static (or not). A static nested class is called just that: a static nested class. A nonstatic nested class is called an inner class. class EnclosingClass {
...
static class StaticNestedClass {
...
}
class InnerClass {
...
}
}
For example, suppose a class named WireFrameModel represents a set of lines in three- dimensional space. (Such models are used to represent three-dimensional objects in graphics programs.) Suppose that the WireFrameModel class contains a static nested class, Line, that represents a single line. Then, outside of the class WireFrameModel, the Line class would be referred to as WireFrameModel.Line. Of course, this just follows the normal naming convention for static members of a class. The definition of the WireFrameModel class with its nested Line class would look, in outline, like this: public class WireFrameModel {
. . . // other members of the WireFrameModel class
static public class Line {
// Represents a line from the point (x1,y1,z1)
// to the point (x2,y2,z2) in 3-dimensional space.
double x1, y1, z1;
double x2, y2, z2;
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} // end class Line
. . . // other members of the WireFrameModel class
} // end WireFrameModel
Inside the
WireframeModel
class, a
Line
object would be created with the constructor " new
Line()
". Outside the class, " new WireFrameModel.Line()
" would be used.
A static nested class has full access to the members of the containing class, even to the private members. This can be another motivation for declaring a nested class, since it lets you give one class access to the private members of another class without making those members generally available to other classes.
When you compile the above class definition, two class files will be created. Even though the definition of
Line
is nested inside
WireFrameModel
, the compiled
Line
class is stored in a separate file. The name of the class file for
Line
will be
WireFrameModel$Line.class
.
As with static methods and variables, which we call class methods and variables, a static nested class is associated with its enclosing class. And like class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class — it can use them only through an object reference.
As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's instance variables and methods. Also, because an inner class is associated with an instance, it cannot define any static members itself.
To help further differentiate the terms nested class and inner class, it's useful to think about them in the following way. The term nested class reflects the syntactic relationship between two classes; that is, syntactically, the code for one class appears within the code of another. In contrast, the term inner class reflects the relationship between objects that are instances of the two classes. Consider the following classes: class EnclosingClass {
...
class InnerClass {
...
}
}
The interesting feature about the relationship between these two classes is not that
InnerClass
is syntactically defined within
EnclosingClass
. Rather, it's that an instance of
InnerClass
can exist only within an instance of
EnclosingClass
and that it has direct access to the instance variables and methods of its enclosing instance. The next figure illustrates this idea.
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Any non-static member of a class is not really part of the class itself (although its source code is contained in the class definition). This is true for non-static nested classes, just as it is for any other non-static part of a class. The non-static members of a class specify what will be contained in objects that are created from that class. The same is true -- at least logically -- for non-static nested classes. It's as if each object that belongs to the containing class has its own copy of the nested class. This copy has access to all the instance methods and instance variables of the object. Two copies of the nested class in different objects differ because the instance variables and methods they refer to are in different objects. In fact, the rule for deciding whether a nested class should be static or non-static is simple: If the class needs to use any instance variable or instance method, make it non-static. Otherwise, it might as well be static.
From outside the containing class, a non-static nested class has to be referred to as variableName.NestedClassName, where variableName is a variable that refers to the object that contains the class. This is actually rather rare, however. A non-static nested class is generally used only inside the class in which it is nested, and there it can be referred to by its simple name.
In order to create an object that belongs to a non-static nested class, you must first have an object that belongs to the containing class. (When working inside the class, the object " this
" is used implicitly.) The nested class object is permanently associated with the containing class object, and it has complete access to the members of the containing class object. Looking at an example will help, and will hopefully convince you that non-static nested classes are really very natural.
Consider a class that represents poker games. This class might include a nested class to represent the players of the game. This structure of the
PokerGame
class could be:
class PokerGame { // Represents a game of poker.
class Player {
// Represents one of the players in this game.
.
.
.
} // end class Player
private Deck deck;
// A deck of cards for playing the game.
private int pot;
// The amount of money that has been bet.
.
.
.
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} // end class PokerGame
If game
is a variable of type
PokerGame
, then, conceptually, game
contains its own copy of the
Player
class. In an an instance method of a
PokerGame
object, a new
Player
object would be created by saying " new Player()
", just as for any other class. (A
Player
object could be created outside the
PokerGame
class with an expression such as " new game.Player()
". Again, however, this is rather rare.) The
Player
object will have access to the deck
and pot
instance variables in the
PokerGame
object. Each
PokerGame
object has its own deck
and pot
and
Players
. Players of that poker game use the deck and pot for that game; playes of another poker game use the other game's deck and pot. That's the effect of making the
Player
class non-static. This is the most natural way for players to behave. A
Player
object represents a player of one particular poker game. If
Player
were a static nested class, on the other hand, it would represent the general idea of a poker player, independent of a particular poker game.
You may encounter nested classes of both kinds in the Java platform API and be required to use them.
To define a non-static nested class either in a class or method scope:
Place the class definition (for the nested class) inside another class definition (the outer class) or a method.
Write code in a non-static method of the outer class to construct an instance of the nested class.
Inner x = new Inner(); constructs an instance of Inner where Inner is a nested class defined in the current class.
Write code to construct an instance on a nested class where either no this object exists, or the current this object is not an instance of the outer class.
You must create an instance of the outer class first. Given a class, Outer, containing a nested class Inner:
Outer.Inner y = new Outer().new Inner();
The above creates an instance of Inner called y, but it had to construct an instance of Outer first.
The following example creates the Outer instance on a separate line, the syntax in the second line is the one you use when you already have an instance of the outer class.
Outer x = new Outer();
Outer.Inner y = x.new Inner();
If Inner is static, you can use:
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Outer.Inner I= new Outer.Inner();
5.21 Command-Line Arguments
A Java application can accept any number of arguments from the command line. Command-line arguments allow the user to affect the operation of an application for one invocation. For example, an application might allow the user to specify verbose mode--that is, specify that the application display a lot of trace information. This is done using the command-line argument
-verbose
.
Purity Tip: Programs that use command-line arguments are not 100% Pure Java because some
systems, like the Mac OS, don't normally have a command line or command-line arguments.
Consider using properties instead so that your programs fit more naturally into the environment.
Arrays have lengths and you can access that length by referencing the variable arrayname.length
You test the length of the args
array as follows.
// This is the Hello program in Java class Hello {
public static void main (String args[]) {
if (args.length > 0) {
System.out.println("Hello " + args[0]);
}
}
}
System.out.println(args[0])
was wrapped in a conditional test, if (args.length > 0) {
}
. The code inside the braces,
System.out.println(args[0])
, now gets executed if and only if the length of the args array is greater than zero.
You can derive the number of command-line arguments with the array's length attribute: numberOfArgs = args.length;
In Java, you always know the name of the application because it's the name of the class in which the main method is defined. So the Java runtime system does not pass the class name you invoke to the main method. Rather, it passes only the items on the command line that appear after the class name. For example, consider the following statement, which is used to invoke a Java application: java diff file1 file2
The following simple application displays each of its command-line arguments on a line by itself: public class Echo {
public static void main (String[] args) {
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for (int i = 0; i < args.length; i++)
System.out.println(args[i]);
}
}
Here's an example of how to invoke the application. You enter the words that are shown here in a different font:
java Echo Drink Hot Java
Drink
Hot
Java
Note that the application displays each word--
Drink
,
Hot
, and
Java
--on a line by itself. This is because the space character separates command-line arguments. If you want
Drink
,
Hot
, and
Java
to be interpreted as a single argument, you would join them by enclosing them within double quotation marks.
java Echo "Drink Hot Java"
Drink Hot Java
Parsing Numeric Command-Line Arguments
If your program needs to support a numeric command-line argument, it must convert a
String argument that represents a number, such as "34", to a number. Here's a code snippet that converts a command-line argument to an int
: int firstArg; if (args.length > 0)
firstArg = Integer.parseInt(args[0]); parseInt
throws a
NumberFormatException
if the format of args[0]
isn't valid. All of the
Number
classes--
Integer
,
Float
,
Double
, and so on--have parseXXX
methods that convert a
String
representing a number to an object of their type.
5.22 Enumerated types
Enumerated types is a new feature introduced in J2SE 5.0. An enumerated type is a type whose legal values consist of a fixed set of constants. Common examples include compass directions, which take the values North, South, East and West and days of the week, which take the values
Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday.
In the Java programming language, you define an enumerated type by using the enum keyword.
For example, you would specify a days of the week enumerated type as: enum Days { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY,
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FRIDAY, SATURDAY };
Notice that by convention the names of an enumerated type's values are spelled in uppercase letters.
You should use enumerated types any time you need to represent a fixed set of constants. That includes natural enumerated types such as the planets in our solar system, the days of the week, and the suits in a deck of cards as well as sets where you know all possible values at compile time, for example the choices on a menu, rounding modes, command line flags, and so on.
Java programming language enumerated types are much more powerful than their counterparts in other languages, which are just glorified integers. The enum declaration defines a class (called an enum type). These are the most important properties of enum types
:
Printed values are informative.
They are typesafe.
They exist in their own namespace.
The set of constants is not required to stay fixed for all time.
You can switch on an enumeration constant.
They have a static values
method that returns an array containing all of the values of the enum type in the order they are declared. This method is commonly used in combination with the for-each construct to iterate over the values of an enumerated type.
You can provide methods and fields, implement interfaces, and more.
They provide implementations of all the
Object
methods. They are
Comparable
and
Serializable
, and the serial form is designed to withstand changes in the enum type.
In the following example,
Planet
is an enumerated type that represents the planets in the solar system. A
Planet
has constant mass and radius properties. Each enum constant is declared with values for the mass and radius parameters that are passed to the constructor when it is created.
Note that the constructor for an enum type is implicitly private. If you attempt to create a public constructor for an enum type, the compiler displays an error message. public enum Planet {
MERCURY (3.303e+23, 2.4397e6),
VENUS (4.869e+24, 6.0518e6),
EARTH (5.976e+24, 6.37814e6),
MARS (6.421e+23, 3.3972e6),
JUPITER (1.9e+27, 7.1492e7),
SATURN (5.688e+26, 6.0268e7),
URANUS (8.686e+25, 2.5559e7),
NEPTUNE (1.024e+26, 2.4746e7),
PLUTO (1.27e+22, 1.137e6);
private final double mass; //in kilograms
private final double radius; //in meters
Planet(double mass, double radius) {
this.mass = mass;
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this.radius = radius;
}
public double mass() { return mass; }
public double radius() { return radius; }
//universal gravitational constant (m3 kg-1 s-2)
public static final double G = 6.67300E-11;
public double surfaceGravity() {
return G * mass / (radius * radius);
}
public double surfaceWeight(double otherMass) {
return otherMass * surfaceGravity();
}
}
In addition to its properties,
Planet
has methods that allow you to retrieve the surface gravity and weight of an object on each planet. Here is a sample program that takes your weight on earth
(in any unit) and calculates and prints your weight on all of the planets (in the same unit): public static void main(String[] args) {
double earthWeight = Double.parseDouble(args[0]);
double mass = earthWeight/EARTH.surfaceGravity();
for (Planet p : Planet.values()) {
System.out.printf("Your weight on %s is %f%n",
p, p.surfaceWeight(mass));
}
}
Here's the output: java Planet 175
Your weight on MERCURY is 66.107583
Your weight on VENUS is 158.374842
Your weight on EARTH is 175.000000
Your weight on MARS is 66.279007
Your weight on JUPITER is 442.847567
Your weight on SATURN is 186.552719
Your weight on URANUS is 158.397260
Your weight on NEPTUNE is 199.207413
Your weight on PLUTO is 11.703031
There's one limitation of enum types: although enum types are classes, you cannot define a hierarchy of enums. In other words, it's not possible for one enum type to extend another enum type.
Enumerated types in switch statement
An example of using enumerated type in switch statement.
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public class SwitchEnumDemo {
public enum Month { JANUARY, FEBRUARY, MARCH, APRIL,
MAY, JUNE, JULY, AUGUST, SEPTEMBER,
OCTOBER, NOVEMBER, DECEMBER }
public static void main(String[] args) {
Month month = Month.FEBRUARY;
int year = 2000;
int numDays = 0;
switch (month) {
case JANUARY:
case MARCH:
case MAY:
case JULY:
case AUGUST:
case OCTOBER:
case DECEMBER:
numDays = 31;
break;
case APRIL:
case JUNE:
case SEPTEMBER:
case NOVEMBER:
numDays = 30;
break;
case FEBRUARY:
if ( ((year % 4 == 0) && !(year % 100 == 0))
|| (year % 400 == 0) )
numDays = 29;
else
numDays = 28;
break;
default:
numDays=0;
break;
}
System.out.println("Number of Days = " + numDays);
}
5.23 Garbage Collection
Some object-oriented languages require that you keep track of all the objects you create and that you explicitly destroy them when they are no longer needed. Managing memory explicitly is tedious and error prone. The Java platform allows you to create as many objects as you want
(limited, of course, by what your system can handle), and you don't have to worry about destroying them. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is called garbage collection.
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An object is eligible for garbage collection when there are no more references to that object.
References that are held in a variable are usually dropped when the variable goes out of scope.
Or, you can explicitly drop an object reference by setting the variable to the special value null.
Remember that a program can have multiple references to the same object; all references to an object must be dropped before the object is eligible for garbage collection.
The Garbage Collector
The Java runtime environment has a garbage collector that periodically frees the memory used by objects that are no longer referenced. The garbage collector does its job automatically, although, in some situations, you may want to run the garbage collection explicitly by calling the gc
method in the
System
class. For instance, you might want to run the garbage collector after a section of code that creates a large amount of garbage or before a section of code that needs a lot of memory.
Finalization
Before an object gets garbage-collected, the garbage collector gives the object an opportunity to clean up after itself through a call to the object's finalize
method. This process is known as
finalization.
Most programmers don't have to worry about implementing the finalize method. In rare cases, however, a programmer might have to implement a finalize method to release resources, such as native peers, that aren't under the control of the garbage collector.
The finalize
method is a member of the
Object
class, which is the top of the Java platform's class hierarchy and a superclass of all classes. A class can override the finalize
method to perform any finalization necessary for objects of that type. If you override finalize
, your implementation of the method should call super.finalize
as the last thing it does.
Summary
A class definition has two parts: a class declaration and a class body. The class body contains member variables, methods, and constructors for the class. A class uses member variables to contain state and uses methods to implement behavior.
A class method or class variable is associated with a particular class. The runtime system allocates a class variable once per class, no matter how many instances exist of that class. You access class variables and methods through the class.
An instance method or instance variable is associated with a particular object (an instance of a class). Every time you create an object, the new object gets a copy of every instance variable defined in its class. You access instance variables and methods through objects.
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member. Class variables are shared by all instance of a class and can be accessed through the class name. Instances of a class get their own copy of each instance variable, which must be accessed through an instance reference.
You create an object from a class by using the new
operator and a constructor. The new operator returns a reference to the object that was created. You can assign the reference to a variable or use it directly.
A class controls access to its instance variables and methods by using the Java platform’s access mechanism. Instance variables and methods that are accessible to code outside of the class that they are declared in can be referred to by using a qualified name. The qualified name of an instance variable looks like this:
objectReference.variableName
The qualified name of a method looks like this:
objectReference.methodName(argumentList)
or
objectReference.methodName()
Constructors initialize a new instance of a class and have the same name as the class.
You can control access to member variables and methods in the same way: by using an access specifier, such as private
or public
, in the member's declaration.
A class defined within another class is called a nested class. Like other members of a class, a nested class can be declared static or not. A nonstatic nested class is called an inner class. An instance of an inner class can exist only within an instance of its enclosing class and has access to its enclosing class's members even if they are declared private.
The garbage collector automatically cleans up unused objects. An object is unused if the program holds no more references to it. You can explicitly drop a reference by setting the variable holding the reference to null
.
Questions
1. class GRC7 {public void main(String[] args) {}} // 1
class GRC8 {public void main(String []args) {}} // 2
class GRC9 {public void main(String args[]) {}} // 3
What is the result of attempting to compile and run the above programs? a. Compile-time error at line 1.
b. Compile-time error at line 2.
c. Compile-time error at line 3.
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d. An attempt to run GRC7 from the command line fails.
e. An attempt to run GRC8 from the command line fails.
f. An attempt to run GRC9 from the command line fails.
2. class GRC1 {public static void main(String[] args) {}} // 1
class GRC2 {protected static void main(String[] args) {}} // 2
class GRC3 {private static void main(String[] args) {}} // 3
What is the result of attempting to compile each of the three class declarations and invoke each main method from the command line? a. Compile-time error at line 1.
b. Compile-time error at line 2.
c. Compile-time error at line 3.
d. An attempt to run GRC1 from the command line fails.
e. An attempt to run GRC2 from the command line fails.
f. An attempt to run GRC3 from the command line fails.
3. class GFM12 { static int x; // 1 public static void main(String[] args) { // 2
int y; // 3
System.out.println("x="+x); // 4
System.out.println("y="+y); // 5
}
}
What is the result of attempting to compile and run the program? a. Compile-time error at line 1.
b. Compile-time error at line 2.
c. Compile-time error at line 3.
d. Compile-time error at line 4.
e. Compile-time error at line 5.
f. Run-time error g. None of the above
4. class GFM13 { static byte a; static short b; static char c; static int d; static long e; static String s; public static void main(String[] args) {
}
System.out.println(a+b+c+d+e+s);
}
What is the result of attempting to compile and run the program? a. Prints: 00000null b. Prints: 00000 c. Prints: 0null
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d. Prints: 0 e. Prints: null f. Compile-time error g. Run-time error h. None of the above
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5. class GFM16 { static int m1 (int i1, int i2) { int i3; if (i1 > 0) {i3 = i1 + i2;} return i3;
} public static void main(String[] args) {
System.out.println(m1(1,2));
}
}
What is the result of attempting to compile and run the program? a. Prints: 0 b. Prints: 1 c. Compile-time error d. Run-time error e. None of the above
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6. class GRC10 { public static void main (String[] s) {
System.out.print(s[1] + s[2] + s[3]);
}
} java GRC10 A B C D E F
What is the result of attempting to compile and run the program using the specified command line? a. Prints: ABC b. Prints: BCD c. Prints: CDE d. Prints: A B C e. Prints: B C D f. Prints: C D E g. Compile-time error h. Run-time error i. None of the above
7. class GFC215 { static String m(float i) {return "float";} static String m(double i) {return "double";} public static void main (String[] args) { int a1 = 1; long b1 = 2;
System.out.print(m(a1)+","+ m(b1));
}
}
What is the result of attempting to compile and run the program? a. Prints: float,float b. Prints: float,double c. Prints: double,float d. Prints: double,double e. Compile-time error f. Run-time error g. None of the above
8. class GFC301 { private String name; public GFC301(String name) {this.name = name;} public void setName(String name) {this.name = name;} public String getName() {return name;} public static void m1(GFC301 r1, GFC301 r2) { r1.setName("Bird"); r2 = r1;
} public static void main (String[] args) {
GFC301 pet1 = new GFC301("Dog");
GFC301 pet2 = new GFC301("Cat");
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m1(pet1,pet2);
System.out.println(pet1.getName() + "," + pet2.getName());
}
}
What is the result of attempting to compile and run the program? a. Prints: Dog,Cat b. Prints: Dog,Bird c. Prints: Bird,Cat d. Prints: Bird,Bird e. Run-time error f. Compile-time error g. None of the above
9. class Maroon { public static void main (String[] args) {
int a = 1; // 1
short b = 1; // 2
long c = 1; // 3
a = c + a; // 4
c = b + a; // 5
}
}
A compile-time error is generated at which line? a. 1 b. 2 c. 3 d. 4 e. 5 f. None of the above
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10. class GFC304 { static void m1(int[] i1, int[] i2) { int[] i3 = i1; i1 = i2; i2 = i3;
} public static void main (String[] args) {
} int[] i1 = {1}, i2 = {3}; m1(i1, i2);
System.out.print(i1[0] + "," + i2[0]);
}
What is the result of attempting to compile and run the program? a. Prints: 1,1 b. Prints: 1,3 c. Prints: 3,1 d. Prints: 3,3 e. Run-time error f. Compile-time error g. None of the above
11. class GFC100 { public static void main(String[] args) { final short s1 = 1; // 1 final char c1 = 1; // 2 byte b1 = s1; // 3 byte b2 = c1; // 4 byte b3 = 1; // 5 byte b4 = 1L; // 6 byte b5 = 1.0; // 7 byte b6 = 1.0d; // 8
}
}
Compile-time errors are generated at which lines? a. 1 b. 2 c. 3 d. 4 e. 5 f. 6 g. 7 h. 8
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12. class JSC201 { static byte m1() {
final char c1 = '\u0001';
return c1; // 1
} static byte m2(final char c2) {return c2;} // 2 public static void main(String[] args) { char c3 = '\u0003';
System.out.print(""+m1()+m2(c3)); // 3
}
}
What is the result of attempting to compile and run the program? a. Prints: 13 b. Prints: 4 c. Compile-time error at 1 d. Compile-time error at 2 e. Run-time error f. None of the above
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Chapter 6 : Inheritance
6.1 Inheritance Basics
Inheritance is the capability of a class to use the properties and methods of another class while adding its own functionality. An example of where this could be useful is with an employee records system. You could create a generic employee class with states and actions that are common to all employees. Then more specific classes could be defined for salaried, commissioned and hourly employees. The generic class is known as the parent (or superclass or base class) and the specific classes as children (or subclasses or derived classes). The concept of inheritance greatly enhances the ability to reuse code as well as making design a much simpler and cleaner process.
Java uses the extends keyword to set the relationship between a child class and a parent class.
The Circle is a simple class that distinguishes circle objects only by their radii. Suppose, instead, that we want to represent circles that have both a size and a position. For example, a circle of radius 1.0 centered at point 0,0 in the Cartesian plane is different from the circle of radius 1.0 centered at point 1,2. To do this, we need a new class, which we'll call PlaneCircle. We'd like to add the ability to represent the position of a circle without losing any of the existing functionality of the Circle class. This is done by defining PlaneCircle as a subclass of Circle, so that
PlaneCircle inherits the fields and methods of its superclass, Circle. class Circle { double radius; public double area() { return Math.PI * radius * radius;
}
} public class PlaneCircle extends Circle {
// We automatically inherit the fields and methods of Circle,
// so we only have to put the new stuff here.
// New instance fields that store the center point of the circle
double cx, cy;
// The area() method is inherited from Circle
// A new instance method that checks whether a point is inside
// the circle
// Note that it uses
the inherited instance field radius public boolean isInside(double x, double y) { double dx = x - cx, dy = y - cy;
// Distance from center double distance = Math.sqrt(dx*dx + dy*dy);
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// Pythagorean theorem return (distance < radius);
// Returns true or false
} public static void main(String args[]) {
PlaneCircle pc=new PlaneCircle(); pc.radius = 10.0; pc.cx = 50.0; pc.cy = 40.0;
System.out.println(“Area of the circle :” + pc.area());
//check whether a point (60,60) is within the circle if(pc.isInside(60,60)) {
System.out.println(“Point 60,60 is inside the circle “);
} else {
System.out.println(“Point 60,60 is outside the circle“);
}
}
}
A subclass inherits all the member variables and methods from its superclass. However, the subclass might not have access to an inherited member variable or method. For example, a subclass cannot access a private member inherited from its superclass. One might say, then, that the item was not inherited at all. But the item is inherited. This becomes important when using an inner class, which does have access to its enclosing class's private members. Note that constructors are not members and so are not inherited by subclasses.
The Object class, defined in the java.lang package, defines and implements behavior that every class needs. Normally, when a class is defined in Java, the inheritance from the Object class is implicit. Therefore, the following two declarations of class "MyClass" are equivalent: public class MyClass{
} public class MyClass extends Object {
}
As depicted in the following figure, many classes derive from
Object
, many classes derive from those classes, and so on, forming a hierarchy of classes.
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At the top of the hierarchy, Object is the most general of all classes. Classes near the bottom of the hierarchy provide more specialized behavior. A subclass derives from another class. The term superclass refers to a class's direct ancestor or to any of its ascendant classes. Every class has one and only one immediate superclass.
Java was designed without multiple inheritance. The overall design of Java supports the solution of problems commonly solved with multiple inheritance in other ways. In particular, the singly rooted hierarchy (with Object as the ultimate ancestor of all classes) and Java interfaces solves most problems that are commonly solved using multiple inheritance in C++.
Example of Multi-level Inheritance
public class AstronomicalObject { double magnitude;
} public class Star extends AstronomicalObject { double surfaceTemperature;
} public class RedDwarf extends Star {
String subtype;
}
Using superclass variable to access subclass object
A superclass variable upwards in the hierarchy of Inheritance till the Object class can be used to used to assign the subclass object. For example, in the above code the object of RedDwarf can be created as follows:
R edDwarf rd = new RedDrawf();
Star rd = new RedDrawf();
AstronomicalObject rd = new RedDrawf();
Object rd = new RedDrawf();
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6.2 Understanding how Constructors are called
A constructor for the base class is always called in the constructor for a derived class, chaining upward so that a constructor for every base class is called. This makes sense because the constructor has a special job: to see that the object is built properly. A derived class has access to its own members only, and not to those of the base class (whose members are typically private). Only the base-class constructor has the proper knowledge and access to initialize its own elements. Therefore, it’s essential that all constructors get called, otherwise the entire object wouldn’t be constructed properly. That’s why the compiler enforces a constructor call for every portion of a derived class. It will silently call the default constructor if you don’t explicitly call a base-class constructor in the derived-class constructor body. If there is no default constructor, the compiler will complain. (In the case where a class has no constructors, the compiler will automatically synthesize a default constructor.)
Let’s take a look at an example that shows the effects of composition, inheritance, and polymorphism on the order of construction:
// Sandwich.java
// Order of constructor calls class Meal {
Meal() { System.out.println("Meal()"); }
} class Bread {
Bread() { System.out.println("Bread()"); }
} class Cheese {
Cheese() { System.out.println("Cheese()"); }
} class Lettuce {
Lettuce() { System.out.println("Lettuce()"); }
} class Lunch extends Meal {
Lunch() { System.out.println("Lunch()");}
} class PortableLunch extends Lunch {
PortableLunch() {
System.out.println("PortableLunch()");
}
}
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class Sandwich extends PortableLunch {
Bread b;
Cheese c;
Lettuce l;
Sandwich() {
b = new Bread();
c = new Cheese();
l = new Lettuce();
System.out.println("Sandwich()");
} public static void main(String[] args) {
new Sandwich();
}
}
This example creates a complex class out of other classes, and each class has a constructor that announces itself. The important class is Sandwich, which reflects three levels of inheritance (four, if you count the implicit inheritance from Object) and three member objects. When a Sandwich object is created in main( ), the output is:
Meal()
Lunch()
PortableLunch()
Bread()
Cheese()
Lettuce()
Sandwich()
This means that the order of constructor calls for a complex object is as follows:
1. The base-class constructor is called. This step is repeated recursively such that the root of the hierarchy is constructed first, followed by the next-derived class, etc., until the most-derived class is reached.
2. Member initializers are called in the order of declaration.
3. The body of the derived-class constructor is called.
The order of the constructor calls is important. When you inherit, you know all about the base class and can access any public and protected members of the base class. This means that you must be able to assume that all the members of the base class are valid when you’re in the derived class. In a normal method, construction has already taken place, so all the members of all parts of the object have been built. Inside the constructor, however, you must be able to assume that all members that you use have been built. The only way to guarantee this is for the base-class constructor to be called first.
Then when you’re in the derived-class constructor, all the members you can
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access in the base class have been initialized. “Knowing that all members are valid” inside the constructor is also the reason that, whenever possible, you should initialize all member objects (that is, objects placed in the class using composition) at their point of definition in the class (e.g.: b, c, and l in the example above). If you follow this practice, you will help ensure that all base class members and member objects of the current object have been initialized.
Use of super keyword
You can also use super within a constructor to invoke a superclass's constructor. Calling the constructor for the superclass must be the first statement in the body of a constructor. If you are satisfied with the default constructor in the superclass, there is no need to make a call to it because it will be supplied automatically.
An object has the fields of its own class plus all fields of its parent class, grandparent class, all the way up to the root class Object. It's necessary to initialize all fields, therefore all constructors must be called! The Java compiler automatically inserts the necessary constructor calls in the process of constructor chaining, or you can do it explicitly.
The Java compiler inserts a call to the parent constructor (super) if you don't have a constructor call as the first statement of you constructor. public class Point {
int m_x;
int m_y;
//============ Constructor
public Point() {
m_x = 10;
m_y = 20;
}
}
The following is the equivalent of the constuctor above.
//============ Constructor (same as in above example)
public Point() {
super(); // Calls the constructor of Object class
m_x = 10;
m_y = 20;
}
Normally, you don't explicitly write the constructor for your parent class, but there are two cases where this is necessary:
•
Passing parameters. You want to call a parent constructor which has parameters (the
default construct has no parameters).
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•
No parameterless constructor. There is no parent constructor with no parameters.
Example of class without parameterless constructor
/////////////////// class without a parameterless constructor.
// If any constructor is defined, the compiler doesn't
// automatically create a default parameterless constructor.
class Parent {
int _x;
Parent(int x) { // constructor
_x = x;
}
}
////////////////// class that must call super in constructor class Child extends Parent {
int _y;
Child(int y) { // WRONG, needs explicit call to super.
_y = y;
}
}
In the example above, there is no explicit call to a constructor in the first line of constructor, so the compiler will insert a call to the parameterless constructor of the parent, but there is no parameterless parent constructor! Therefore this produces a compilation error. This problem can be solved as follows :
Parent class can define a parameterless constructor.
/////////////////// class without a parameterless constructor.
// If any constructor is defined, the compiler doesn't
// automatically create a default parameterless constructor.
class Parent {
int _x;
Parent(int x) { // constructor with parameter
_x = x;
}
Parent() { // constructor without parameters
_x = 0;
}
}
A better way to define the parameterless constructor is to call the parameterized constructor so that any changes that are made only have to be made in one constructor.
Parent() { // constructor without parameters
this(0);
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}
}
Note that each of these constructors implicitly calls the parameterless constructor for its parent class, etc, until the Object class is finally reached.
Example of passing parameters to the base class constructor. class Circle { double radius;
Circle(double radius) { this.radius = radius;
}
} class Cylinder extends Circle { double height; //radius is inherited
Cylinder(double height, double radius) { super (radius); this.height = height;
} public static void main(String args[]) {
Cylinder obj = new Cylinder(30,40);
}
}
6.3 Overriding Methods
In a derived class, if you include a method definition that has the same name and exactly the same number and types of parameters as a method already defined in the base class, this new definition replaces the old definition of the method.
A subclass inherits methods from a superclass. Sometimes, it is necessary for the subclass to modify the methods defined in the superclass. This is referred to as method overriding. The following example demonstrates method overriding.
class Circle {
//declaring the instance variable protected double radius;
Circle(double radius) { this.radius=radius;
}
// other method definitions here public double getArea() {
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return Math.PI*radius*radius;
}//this method returns the area of the circle
}// end of class circle
When the getArea method is invoked from an instance of the Circle class, the method returns the area of the circle.
The next step is to define a subclass to override the getArea() method in the Circle class. The derived class will be the Cylinder class. The getArea() method in the Circle class computes the area of a circle, while the getArea method in the Cylinder class computes the surface area of a cylinder. The Cylinder class is defined below.
class Cylinder extends Circle {
//declaring the instance variable protected double length;
Cylinder(double radius,double length) { super(radius); this.length = length;
}
// other method definitions here public double getArea() { // method overriden here return 2*super.getArea()+2*Math.PI*radius*length;
} //this method returns the cylinder surface area public static void main(String args[]) {
Circle myCircle; myCircle = new Circle(1.20);
System.out.println(“Area of circle :”+ myCircle.getArea();
Cylinder myCylinder; myCylinder = new Cylinder(1.20,2.50);
System.out.println(“Surface area of cylinder :” + myCylinder.getArea());
}
}// end of class Cylinder
When the overriden method (getArea) is invoked for an object of the Cylinder class, the new definition of the method is called and not the old definition from the superclass(Circle).
The ability of a subclass to override a method in its superclass allows a class to inherit from a superclass whose behavior is "close enough" and then override methods as needed.
For example, all classes are descendents of the Object class. Object contains the toString method, which returns a String object containing the name of the object's class and its hash code.
Most, if not all, classes will want to override this method and print out something meaningful for that class.
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public class Employee
{
private String name;
// overrides Object's toString method
public String toString() { return name;
}
public static void main(String args[]) {
Employee e1 = new Employee(); e1.name = “John Smith”;
System.out.println(e1);
}
}
The toString() method is called implicitly when you try to print the object.
The return type, method name, and number and type of the parameters for the overriding method must match those in the overridden method. The overriding method can have a different throws clause as long as it doesn't declare any types not declared by the throws clause in the overridden method. Also, the access specifier for the overriding method can allow more access than the overridden method, but not less. For example, a protected
method in the superclass can be made public
but not private
.
Calling the Overridden Method
Sometimes, you don't want to completely override a method. Rather, you want to add more functionality to it. To do this, simply call the overridden method using the super
keyword. For example, super.overriddenMethodName();
Consider this class,
Superclass
: public class Superclass {
public boolean aVariable;
public void aMethod() {
aVariable = true;
}
}
Now, here's a subclass, called
Subclass
, that overrides aMethod
.
public class Subclass extends Superclass { public void aMethod() {
//overrides aMethod in Superclass
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aVariable = false;
super.aMethod();
System.out.println(aVariable);
}
} super keyword cannot be used in a static reference.
Methods a Subclass Cannot Override
A subclass cannot override methods that are declared final in the superclass (by definition, final methods cannot be overridden). These are discussed later.
Also, a subclass cannot override methods that are declared static
in the superclass. In other words, a subclass cannot override a class method. A subclass can hide a static
method in the superclass by declaring a static
method in the subclass with the same signature as the static method in the superclass.
Let's look at an example to see why. This example contains two classes. The first is Animal, which contains one instance method and one class method:
} public class Animal { public static void hide() {
System.out.println("The hide method in Animal.");
} public void override() {
System.out.println("The override method in Animal.");
}
The second class, a subclass of
Animal
, is called
Cat
: public class Cat extends Animal { public static void hide() {
System.out.println("The hide method in Cat.");
} public void override() {
System.out.println("The override method in Cat.");
} public static void main(String[] args) {
Cat myCat = new Cat();
Animal myAnimal = (Animal)myCat;
myAnimal.hide();
myAnimal.override();
}
}
The Cat class overrides the instance method in Animal called override and hides the class method in Animal called hide. The main method in this class creates an instance of Cat, casts it
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to a Animal reference, and then calls both the hide and the override methods on the instance. The output from this program is as follows:
The hide method in Animal.
The override method in Cat.
The version of the hidden method that gets invoked is the one in the superclass, and the version of the overridden method that gets invoked is the one in the subclass. For class methods, the runtime system invokes the method defined in the compile-time type of the reference on which the method is called. In the example, the compile-time type of myAnimal
is
Animal
. Thus, the runtime system invokes the hide
method defined in
Animal
. For instance methods, the runtime system invokes the method defined in the runtime type of the reference on which the method is called. In the example, the runtime type of myAnimal
is
Cat
. Thus, the runtime system invokes the override
method defined in
Cat
.
An instance method cannot override a static method, and a static method cannot hide an instance method.
6.4 Dynamic Method Dispatch
Dynamic Inheritance is the ability of an object to inherit classes at runtime and provides several key abilities. It is a mechanism by which a call to an overridden function is resolved at run time, rather than compile time (run-time polymorphism). The principle is a superclass reference variable can refer to a subclass object. It is the type of the object being referred to (not the type of of the reference variable) that determines which version of an overridden method will be executed class A { void callme() {
System.out.println(“Inside A’s callme method”);
}
} class B extends A {
//override callme void callme() {
System.out.println(“Inside B’s callme method”);
}
} class C extends A {
// override callme void callme() {
System.out.println(“Inside C’s callme method”);
}
} class Dispatch { public static void main (String args [] ) {
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A a = new A(); //object of type A
B b = new B(); //object of type B
C c = new C(); // object of type C
A r; // reference variable of type a r = a; r.callme();
// r refers to an A object
//calls A’s version of callme r = b; r.callme(); r = c; r.callme();
}
}
6.5 Annotations
// r refers to B object
//calls B’s version of callme
//refers to C object
//calls B’s version of callme
Introduced in J2SE 5.0, annotations provide a way for programmers to instruct the compiler on how to handle certain situations. For example, an annotation can indicate that a particular method or class should be overridden in a subclass or that certain compiler error messages shouldn't be printed. There are three useful annotations built-in to release 5.0 that will be discussed.
Annotations use the form @annotation and can be applied to methods or classes. For example:
@Override class Cat extends Pet {
} or
@Override void feedTheDog() { }
The following example illustrates all three built-in annotation types, using methods: import java.util.List; class Food {} class Hay extends Food {} class Animal {
Food getPreferredFood() { return null;
}
@Deprecated static void deprecatedMethod() {
}
}
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class Horse extends Animal {
Horse() {
return;
}
@Override //compiler error if getPreferredFood
//overloaded, not overridden
//Notice the return type is different
//(covariant return type).
Hay getPreferredFood() {
return new Hay();
}
@SuppressWarnings({"deprecation", "unchecked"})
void useDeprecatedMethod(List raw) {
//deprecation warning - suppressed
Animal.deprecateMethod();
//unchecked warning - suppressed raw.add(new Horse());
}
}
java.lang.Override
The
@Override
annotation indicates that a method defined in the current class must override a method is one of its superclasses. In the preceding example, the override annotation is used to indicate that the getPreferredFood
method in the
Horse
class is overriding the getPreferredFood
method in the
Animal
class. Note that this method is returning a different type (
Hay
) than the superclass (
Food
) — this practice is called covariant return types.
If a method marked with
@Override
does not override the same method in one of its superclasses, the compiler will generate an error message.
java.lang.Deprecated
The
@Deprecated
annotation indicates that the marked method should no longer be used.
Note: The
@deprecated
tag used by the Javadoc
TM
tool achieves the same result as
@Deprecation
. As of J2SE 5.0, the compiler-based
@Deprecation
annotation replaces the
Javadoc
@deprecation
tag.
In the preceding example, the deprecatedMethod
in the
Animal
class is marked with the deprecated annotation. Invoking or overriding the deprecatedMethod
method will generate a compile-time warning, unless that warning is suppressed in the subclass.
java.lang.SuppressWarnings
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The
@SuppressWarnings
annotation indicates that the named compiler warnings should be suppressed.
Note: This annotation can be used as of release 5.0 and some Java compiler implementations
may suppress compiler warnings, but this feature won't be fully implemented in javac
until release 6.0.
In the preceding example, the useDeprecatedMethod
method in the
Horse
class uses the suppress warnings annotation to suppress both the unchecked and deprecation warnings that would otherwise occur.
It is strongly recommended that you suppressed warnings only where necessary. For example, using the SuppressWarnings annotation on the entire Horse class is not recommended because genuine errors in the code might be hidden.
6.6 Hiding Member Variables
Member variables defined in the subclass hide member variables that have the same name in the superclass.
One interesting feature of Java member variables is that a class can access a hidden member variable through its superclass. Consider the following superclass and subclass pair: class Super {
Number aNumber;
} class Subbie extends Super {
Float aNumber;
}
The aNumber
variable in
Subbie
hides aNumber
in
Super
. But you can access
Super
's aNumber from
Subbie
with super.aNumber
super
keyword allows a method to refer to hidden variables.
Another example of variable hiding.
class SuperA {
int x = 10;
} class SubA extends SuperA {
int x = 20; // hides x in superclass void methodA() {
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System.out.println(“ x in SuperA :” + super.x);
System.out.println(“ x in SuperA :” +
((SuperA)this).x);
System.out.println(“ x in SubA :” + x);
} public static void main(String args[]) { new SubA().methodA();
}
}
You can access a hidden variable by casting the object to its superclass, with the syntax
((Superclass)object).name
.
6.7 Abstract Classes
Sometimes, a class that you define represents an abstract concept and, as such, should not be instantiated. Take, for example, food in the real world. Have you ever seen an instance of food?
No. What you see instead are instances of carrot, apple, and chocolate. Food represents the abstract concept of things that we all can eat. It doesn't make sense for an instance of food to exist.
Similarly in object-oriented programming, you may want to model an abstract concept without being able to create an instance of it. For example, the Number class in the java.lang package represents the abstract concept of numbers. It makes sense to model numbers in a program, but it doesn't make sense to create a generic number object. Instead, the Number class makes sense only as a superclass to classes like Integer and Float, both of which implement specific kinds of numbers. A class such as Number, which represents an abstract concept and should not be instantiated, is called an abstract class. An abstract class is a class that can only be subclassed-- it cannot be instantiated.
To declare that your class is an abstract class, use the keyword abstract before the class keyword in your class declaration: abstract class Number {
. . .
}
If you attempt to instantiate an abstract class, the compiler displays an error similar to the following and refuses to compile your program:
AbstractTest.java:6: class AbstractTest is an abstract class.
It can't be instantiated.
new AbstractTest();
^
1 error
Abstract class can contain static methods.
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public abstract class test { public static void main(String [] args) { int x = 3; int y = 1; if (x == y)
System.out.println("Not equal");
}
} else
System.out.println("Equal");
6.8 Abstract Methods
An abstract class may contain abstract methods, that is, methods with no implementation. In this way, an abstract class can define a complete programming interface, thereby providing its subclasses with the method declarations for all of the methods necessary to implement that programming interface. However, the abstract class can leave some or all of the implementation details of those methods up to its subclasses.
Let's look at an example of when you might want to create an abstract class with an abstract method in it. In an object-oriented drawing application, you can draw circles, rectangles, lines,
Bezier curves, and so on. Each of these graphic objects share certain states (position, bounding box) and behavior (move, resize, draw). You can take advantage of these similarities and declare them all to inherit from the same parent object--
GraphicObject
.
However, the graphic objects are also substantially different in many ways: drawing a circle is quite different from drawing a rectangle. The graphics objects cannot share these types of states or behavior. On the other hand, all
GraphicObject s must know how to draw themselves; they just differ in how they are drawn. This is a perfect situation for an abstract superclass.
First you would declare an abstract class, GraphicObject, to provide member variables and methods that were wholly shared by all subclasses, such as the current position and the moveTo method. GraphicObject also declares abstract methods for methods, such as draw, that need to be implemented by all subclasses, but are implemented in entirely different ways (no default implementation in the superclass makes sense). The GraphicObject class would look something like this: abstract class GraphicObject {
int x, y;
. . .
void moveTo(int newX, int newY) {
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. . .
}
abstract void draw();
}
Each non-abstract subclass of
GraphicObject
, such as
Circle
and
Rectangle
, would have to provide an implementation for the draw
method. class Circle extends GraphicObject {
void draw() {
. . .
}
} class Rectangle extends GraphicObject {
void draw() {
. . .
}
}
An abstract class is not required to have an abstract method in it. But any class that has an abstract method in it or that does not provide an implementation for any abstract methods declared in its superclasses must be declared as an abstract class.
A subclass must override methods that are declared abstract in the superclass, or the subclass itself must be abstract.
An abstract method cannot be static, synchronized or a final method.
public abstract class Shape { public abstract double area(); // Abstract methods: note public abstract double circumference();
// semicolon instead of body.
} class Circle extends Shape { public static final double PI = 3.14;
protected double r;
// Instance data public Circle(double r) { this.r = r; } //Constructor
public double getRadius() { return r; } //Accessor
// implementation abstract methods.
public double area() { return PI*r*r; } public double circumference() { return 2*PI*r; }
} class Rectangle extends Shape { protected double w, h; // Instance data
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public Rectangle(double w, double h) {
this.w = w; this.h = h;
} public double getWidth() { return w; } public double getHeight() { return h; } public double area() { return w*h; } public double circumference() { return 2*(w + h); }
} class ShapeExample { public static void main(String args[]) {
Shape[] shapes = new Shape[3];
// Create an array to hold shapes shapes[0] = new Circle(2.0);
// Fill in the array shapes[1] = new Rectangle(1.0, 3.0); shapes[2] = new Rectangle(4.0, 2.0); double total_area = 0; for(int i = 0; i < shapes.length; i++) total_area += shapes[i].area();
// Compute the area of the shapes
}
}
6.9 Final Classes
You can declare that your class is final, that is, that your class cannot be subclassed. There are
(at least) two reasons why you might want to do this: to increase system security by preventing system subversion, and for reasons of good object-oriented design.
Security:
One mechanism that hackers use to subvert systems is to create a subclass of a class and then substitute their class for the original. The subclass looks and feels like the original class but does vastly different things, possibly causing damage or getting into private information. To prevent this kind of subversion, you can declare your class to be final and thereby prevent any subclasses from being created. The
String
class in the java.lang
package is a final class for just this reason. This class is so vital to the operation of the compiler and the interpreter that the Java system must guarantee that whenever a method or object uses a
String
it gets exactly a java.lang.String
and not some other string. This ensures that all strings have no strange, inconsistent, undesirable, or unpredictable properties.
If you try to compile a subclass of a final class, the compiler prints an error message and refuses to compile your program. In addition, the bytecode verifier ensures that the subversion is not
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taking place at the bytecode level. It does this by checking to make sure that a class is not a subclass of a final class.
Design:
You may also wish to declare a class as final for object-oriented design reasons. You may think that your class is "perfect" or that, conceptually, your class should have no subclasses.
To specify that your class is final, use the keyword final before the class keyword in your class declaration.
For example, if you wanted to declare your (perfect) ChessAlgorithm class as final, its declaration should look like this: final class ChessAlgorithm {
. . .
}
Any subsequent attempts to subclass
ChessAlgorithm
will result in a compiler error such as the following:
Chess.java:6: Can't subclass final classes: class ChessAlgorithm class BetterChessAlgorithm extends ChessAlgorithm {
^
1 error
Another example of final class class SmallBrain {}
} final class Dinosaur { int i = 7; int j = 1;
SmallBrain x = new SmallBrain(); void f() {}
// class Further extends Dinosaur {}
// error: Cannot extend final class 'Dinosaur' public class Jurassic { public static void main(String[] args) {
Dinosaur n = new Dinosaur(); n.f(); n.i = 40; n.j++;
}
}
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6.10 Final Methods
Does creating a final class seem heavy-handed for your needs? Do you really just want to protect some of your class's methods from being overridden? You can use the final
keyword in a method declaration to indicate to the compiler that the method cannot be overridden by subclasses. As just shown, the
Object
class does this; some of its methods are final and some are not.
You might wish to make a method final if it has an implementation that should not be changed and it is critical to the consistent state of the object. For example, instead of making your
ChessAlgorithm class final, you might want instead to make the nextMove method final: class ChessAlgorithm {
. . .
final void nextMove(ChessPiece pieceMoved,
BoardLocation newLocation)
{
...
}
. . .
}
Final methods cannot be abstract methods. All methods of final class are implicitly final.
Final Arguments
class Gizmo { public void spin() {}
} public class FinalArguments { void with(final Gizmo g) {
// g = new Gizmo(); // Illegal -- g is final g.spin();
} void without(Gizmo g) { g = new Gizmo(); // OK -- g not final g.spin();
}
// void f(final int i) { i++; } // Can't change
// You can only read from a final primitive: int g(final int i) { return i + 1; } public static void main(String[] args) {
FinalArguments bf = new FinalArguments(); bf.without(null); bf.with(null);
}
}
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If a method is not final it can be overridden in the subclass. As seen above the object of the super class can use dynamic method
6.11 Access control and Inheritance
When you are creating a subclass make sure that the super class is present in the same package.
If not, you will have to import the package using import statement discussed in the next chapter.
If you are not using packages, then take care to save the subclass in the same directory as the baseclass as the directory acts as an unnamed package.
The private members in the base class are not inherited in the subclass. Hence private methods in the base class are not overridden even though there is a method with the same name and same signature in the subclass.
The protected members are inherited by the subclasses. If the base class is in the different package import the package and you can use the protected members.
The members with no access specifier are of the type “package private”. Hence they can be used only by the classes in that package or by the classes in the unnamed package. Hence if the classes are of package type then they cannot be used outside the package. If you want to use them outside the package declare them as public.
6.12 Anonymous Inner Class
Anonymous classes are classes that don’t have a name (hence the term anonymous). Because they don’t have a name, there is no way to refer to them. For this reason, their declaration must be given at creation time, as part of the new statement.
That requires yet another form of the new statement, as follows: new <class or interface> <class’ body>
This form of the new statement declares a new anonymous class that extends a given class. It also creates a new instance of that class and returns it as the result of the statement.
If the anonymous class extends another class, its body can access the class’s members, override its methods, and so on, like any other regular class. If the anonymous class implements an interface, its body must implement the interface’s methods.
Notice that the declaration of the anonymous class is made at compile time, and the instantiation at runtime. That means that a new statement inside a for loop, for example, creates several instances of the same anonymous class, and not one instance of several different anonymous classes.
Technically, anonymous classes are considered nonstatic inner classes, so they have the same privileges and limitations of nonstatic inner classes declared inside a method.
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Anonymous classes are great when you need to perform some task that needs an object, but that doesn’t justify creating a whole new class, maybe because the needed class is too simple or because it's used only inside a single method. Anonymous classes are particularly useful for quickly creating event handlers in Swing applications.
To define, in method scope, an anonymous nested class that implements a specified interface:
An anonymous nested class is defined where is it instantiated (in a method). An anonymous nested class must either implement an interface or extend a class, but the implements or extends keywords are not used. For example the following line causes the method to return an object which is an instance of an anonymous nested class: return new SomeClass() { /*body of the anonymous class goes here*/ };
You might like to think of an anonymous nested class as part of a really long new
statement, which happens to contain a class definition, and which is why it has a ";" at the end. The following example calls someMethod()
, passing an instance of the anonymous nested class: someMethod(new SomeClass() { /*body of the anonymous class goes here*/ });
In both cases
SomeClass()
is not the name of the anonymous class (anonymous means it has no name) rather is it the name of the class that you are extending or the interface you are implementing. These classes cannot define a constructor, as they do not have a name that you can use to declare the constructor method. If SomeClass() is a class, the default constructor of that class is called, if you want to use a non-default constructor instead, you supply arguments e.g.: return new SomeClass(12) { /*body of the anonymous class goes here*/ }; will call the SomeClass constructor which takes an int.
Examples: public MyClass makeObj(final String name)
{
return new SubClassOfMyClass() {
public String toString()
{
return "My name is "+name;
}
};
}
Only final members of the outer class can be accessed in the methods of the anonymous inner class.
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public Object makeObj(String name)
{
final String objName = "My name is " + name;
return new Object() {
public String toString()
{
return objName;
}
};
}
6.13 The Object class
Methods of Object class
protected native Object clone() throws CloneNotSupportedException
Creates a new object of the same class as this object. It then initializes each of the new object's fields by assigning it the same value as the corresponding field in this object. No constructor is called.
The clone method of class Object will only clone an object whose class indicates that it is willing for its instances to be cloned. A class indicates that its instances can be cloned by declaring that it implements the Cloneable interface.
An example of cloning an object: class Car extends Cloneable {
String color;
Car(String color) { this.color = color;
} public String toString() { return color + “Colored Car”;
}
} class CloneExample { public static void main(String args[]) throws CloneNotSupportedException {
Car c1 = new Car(“Red”);
System.out.println(c1);
Car c2 = (Car)c1.clone();
System.out.println(c2);
}
}
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public boolean equals(Object obj)
Compares two Objects for equality. The equals method implements an equivalence relation:
It is reflexive: for any reference value x
, x.equals(x)
should return true
.
It is symmetric: for any reference values x
and y
, x.equals(y)
should return true
if and only if y.equals(x)
returns true
.
It is transitive: for any reference values x
, y
, and z
, if x.equals(y)
returns true
and y.equals(z)
returns true
, then x.equals(z)
should return true
.
It is consistent: for any reference values x
and y
, multiple invocations of x.equals(y) consistently return true
or consistently return false
.
For any reference value x
, x.equals(null)
should return false
.
The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any reference values x and y, this method returns true if and only if x and y refer to the same object (x==y has the value true). protected void finalize() throws Throwable
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides the finalize
method to dispose of system resources or to perform other cleanup.
Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored. The finalize method in Object does nothing. public final native Class getClass()
Returns the runtime class of an object. public native int hashCode()
Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable
. The general contract of hashCode is:
Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode
method must consistently return the same integer. This integer need not remain consistent from one execution of an application to another execution of the same application.
If two objects are equal according to the equals
method, then calling the hashCode method on each of the two objects must produce the same integer result. public final native void notify()
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Wakes up a single thread that is waiting on this object's monitor. A thread waits on an object's monitor by calling one of the wait
methods.
This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:
By executing a synchronized instance method of that object.
By executing the body of a synchronized
statement that synchronizes on the object.
For objects of type
Class,
by executing a synchronized static method of that class.
Only one thread at a time can own an object's monitor. public final native void notifyAll()
Wakes up all threads that are waiting on this object's monitor. A thread waits on an object's monitor by calling one of the wait
methods.
This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor. public String toString()
Returns a string representation of the object. In general, the toString
method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.
The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. public final void wait() throws InterruptedException
Waits to be notified by another thread of a change in this object. The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
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public final native void wait(long timeout) throws
InterruptedException
Waits to be notified by another thread of a change in this object.
The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until either of the following two conditions has occurred:
Another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify
method or the notifyAll
method.
The timeout period, specified by the timeout
argument in milliseconds, has elapsed.
The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor. public final void wait(long timeout, int nanos) throws
InterruptedException
Waits to be notified by another thread of a change in this object.
This method is similar to the wait method of one argument, but it allows finer control over the amount of time to wait for a notification before giving up.
The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until either of the following two conditions has occurred:
Another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify
method or the notifyAll
method.
The timeout period, specified by timeout
milliseconds plus nanos
nanoseconds arguments, has elapsed.
The thread then waits until it can re-obtain ownership of the monitor and resumes execution
This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
Summary
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Extending an existing class often provides an easy way to create a new type. This is primarily true when an existing class creates a type whose features are close to, but not identical to the features needed in the new type. When an existing class is extended to define a new class, the existing class is often called the superclass and the new class is often called the subclass. The subclass inherits all of the variables and all of the methods defined in the superclass and its superclasses. Inheritance provides a formal mechanism for code reuse.
Except for the
Object
class, a class has exactly one direct superclass. A class inherits member variables and methods from all its superclasses, whether direct or indirect. A subclass can override methods that it inherits, or it can hide variables or methods that it inherits. The Object class is the top of the class hierarchy. All classes are descendants from this class and inherit methods from it. Useful methods inherited from Object include toString, equals, clone, getClass, wait, notify, and notifyAll. You can prevent a class from being subclassed by using the final keyword in the class's declaration. Similarly, you can prevent a method from being overridden by subclasses by declaring it as a final method.
An abstract class can only be subclassed; it cannot be instantiated. An abstract class can contain abstract methods — methods that are declared but not implemented. Subclasses provide the implementations for abstract methods.
Questions
1. class Color {}
class Red extends Color {}
class Blue extends Color {}
class A { public static void main (String[] args) {
}
}
Color color1 = new Red(); Red color2 = new Red(); boolean b1 = color1 instanceof Color; boolean b2 = color1 instanceof Blue; boolean b3 = color2 instanceof Blue;
System.out.print(b1+","+b2+","+b3);
What is the result of attempting to compile and run the program? a. false,false,false b. false,false,true c. false,true,false d. false,true,true e. true,false,false f. true,false,true g. true,true,false h. true,true,true i. Run-time error j. Compile-time error k. None of the above
2. Which of the following modifiers can be applied to the declaration of a field? a. abstract b. final c. private d. protected
155
e. public
3. class A {A(int i) {}} // 1
class B extends A {} // 2
Which of the following statements are true? a. The compiler attempts to create a default constructor for class A.
b. The compiler attempts to create a default constructor for class B.
c. Compile-time error at 1.
d. Compile-time error at 2
4. Which of the follow statements is true. a. An anonymous class can be declared abstract.
b. A local class can be declared abstract.
c. An abstract class can be instantiated.
d. An abstract class is implicitly final.
e. An abstract class must declare at least one abstract method.
f. An abstract class can not extend a concrete class.
5. public class Basics {} // 1
class Basics1 {} // 2
protected class Basics2 {} // 3
private class Basics3 {} // 4
Class Basics4 {} // 5
Suppose these are top-level class declarations and not nested class declarations; and suppose that all of the declarations are contained in one file named Basics.java. Compile-time errors are generated at which lines? a. 1 b. 2 c. 3 d. 4
6. class A11 {public String toString() {return "A11";}}
class A12 { public static void main(String[] arg) {
A11[] a1 = new A11[1]; // 1
A11[][] a2 = new A11[2][]; // 2
A11[][][] a3 = new A11[3][][]; // 3 a1[0] = new A11(); // 4 a2[0] = a2[1] = a1; // 5 a3[0] = a3[1] = a3[2] = a2; // 6
System.out.print(a3[2][1][0]); // 7
}
} e. 5
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What is the result of attempting to compile and run the program? a. Prints: null b. Prints: A11 c. Compile-time error at 1.
d. Compile-time error at 2.
e. Compile-time error at 3.
f. Compile-time error at 4.
g. Compile-time error at 5.
h. Compile-time error at 6.
i. Compile-time error at 7.
j. Run-time error k. None of the above
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Chapter 7 : Packages and Interfaces
7.1 Packages
To make classes easier to find and to use, to avoid naming conflicts, and to control access, programmers bundle groups of related classes and interfaces into packages.
Definition: A package is a collection of related classes and interfaces providing access
protection and namespace management.
The classes and interfaces that are part of the Java platform are members of various packages that bundle classes by function: fundamental classes are in java.lang, classes for reading and writing (input and output) are in java.io, and so on. You can put your classes and interfaces in packages, too.
Let's look at a set of classes and examine why you might want to put them in a package.
Suppose that you write a group of classes that represent a collection of graphic objects, such as circles, rectangles, lines, and points. You also write an interface,
Draggable
, that classes implement if they can be dragged with the mouse by the user:
//in the Graphic.java file public abstract class Graphic {
. . .
}
//in the Circle.java file public class Circle extends Graphic implements Draggable {
. . .
}
//in the Rectangle.java file public class Rectangle extends Graphic implements Draggable {
. . .
}
//in the Draggable.java file public interface Draggable {
. . .
}
You should bundle these classes and the interface in a package for several reasons:
•
•
•
You and other programmers can easily determine that these classes and interfaces are related.
You and other programmers know where to find classes and interfaces that provide graphics-related functions.
The names of your classes wont conflict with class names in other packages, because the package creates a new namespace.
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You can allow classes within the package to have unrestricted access to one another yet still restrict access for classes outside the package.
Package declarations
Each file must have a package declaration which precedes all non-comment code. The package name must be the same as the enclosing directory. For example, here are two files in the packagetest directory.
//ClassA.java
package packagetest; class ClassA {
public static void main(String[] args) {
ClassB.greet();
}
}
//ClassB.java
package packagetest; class ClassB {
static void greet() {
System.out.println("Hi");
}
}
Note that these source files must be named ClassA.java and ClassB.java (case matters) and they must be in a directory named packagetest.
Package names are identifiers - they can use mixed case and be of any length. Conventionally, package names start with a lowercase letter.
While the Java Language specification does not put a limit on the length of a package name, it does recommend that package names be used as directory names. UNIX operating systems typically require that the full path to a file, from root through file name, be limited to 1024 characters total. It is recommended to use package names of a dozen characters or less.
If the package declaration is not given, then the class in the source file is placed in the unnamed package. The unnamed package is often used when creating classes for small applications.
However, for larger projects all code should be placed in a named package - such as one named after the application. This helps avoid potential name space collisions.
Another example of creating a package
package world;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
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}
}
Create a folder world in c: drive and save the class file as HelloWorld.java
Compiling and running packages from a command line
To compile the above example, you must be outside the packagetest directory. To compile the classes: javac packagetest/ClassB.java
javac packagetest/ClassA.java
To run the main program in ClassA. java packagetest.ClassA
or java packagetest/ClassA
In windows the "/" can be replaced by the "\" in the javac command, but not in the java command. Generally use a forward slash ("/") because it is used more commonly than the backslash in other places as well.
Setting up the CLASSPATH
If the HelloWorld class is saved as described above the classpath can be set as follows. On the command prompt type this set CLASSPATH=.;C:\;
We set the
CLASSPATH
to point to 2 places, . (dot) and C:\ directory.
If you used to play around with DOS or UNIX, you may be familiar with . (dot) and .. (dot dot).
We use . as an alias for the current directory and .. for the parent directory. In our
CLASSPATH
we include this for convenient reason. Java will find our class file not only from C: directory but from the current directory as well. Also, we use ; (semicolon) to separate the directory location in case we keep class files in many places.
When compiling
HelloWorld
class, we just go to the world directory and type the command:
C:\world>javac HelloWorld.java
If you try to run this
HelloWorld
using
java HelloWorld
, you will get the following error:
C:\world>java HelloWorld
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong name: world/HelloWorld)
at java.lang.ClassLoader.defineClass0(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:442)
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at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:101)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:248)
at java.net.URLClassLoader.access$1(URLClassLoader.java:216)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:191)
at java.lang.ClassLoader.loadClass(ClassLoader.java:290)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:286)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
The reason is right now the
HelloWorld
class belongs to the package world
. If we want to run it, we have to tell JVM about its fully-qualified class name
(world.HelloWorld)
instead of its plain class name
(HelloWorld)
.
C:\world>java world.HelloWorld
C:\world>Hello World
Note: fully-qualified class name is the name of the java class that includes its package name
Nesting packages
Package declarations may be a string of names, separated by dots, to create nested packages.
This is useful for creating a hierarchy of packages. For instance, there may be a project "abc" package in which are found several nested packages for "vis" work, "database" work, and so forth. Within the "vis" package may be several more nested packages to do data management, rendering, etc.
package abc.vis.renderer; class MyClass
{
}
7.2 Using package members
Only public package members are accessible outside the package in which they are defined. To use a public package member from outside its package, you must do one or more of the following:
•
•
•
Refer to the member by its long (qualified) name
Import the package member
Import the member's entire package
Referring to a Package Member by Name
So far, the examples in this book have referred to types by their simple names, such as
R ectangle, Circle etc
. You can use a package member's simple name if the code you are writing is in the same package as that member or if that member has been imported.
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However, if you are trying to use a member from a different package and that package has not been imported, you must use the member's qualified name, which includes the package name.
This is the qualified name for the ClassA class declared in the packagetest package in the previous example: packagetest.ClassA
You could use this long name to create an instance of packagetest.
packagetest.ClassA obj = new packagetest.ClassA();
You'll find that using long names is okay for one-shot uses. But you'd likely get annoyed if you had to write packagetest.ClassA
again and again. Also, your code would get messy and difficult to read. In such cases, you can just import the member instead.
Importing a Package Member
To import a specific member into the current file, put an import
statement at the beginning of your file before any class or interface definitions but after the package
statement, if there is one.
Here's how you would import the
ClassA
class from the packagetest package created in the previous section: import packagetest.ClassA;
Now you can refer to the
ClassA
class by its simple name:
ClassA obj = new ClassA();
This approach works well if you use just a few members from the packagetest
package. But if you use many types from a package, you can import the entire package.
Importing an Entire Package
To import all the types contained in a particular package, use the import
statement with the asterisk
(*)
wildcard character: import packagetest.*;
Now you can refer to any class or interface in the graphics
package by its short name:
ClassA obj = new ClassA();
ClassB objB = new ClassB();
The asterisk in the import
statement can be used only to specify all the classes within a package, as shown here. It cannot be used to match a subset of the classes in a package. For example, the following does not match all the classes in the packagetest package that begin with
C
:
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import packagetest.C*; // does not work
Instead, it generates a compiler error. With the import
statement, you generally import only a single package member or an entire package.
Note: Another, less common form of import allows you to import only a class and its public
inner classes. For example, if the packagetest.ClassB
class contained useful inner classes, like
ClassB.Rectangle and ClassB
.Square
, you could import ClassB and its inner classes this way: import packagetest.ClassB.*;
For your convenience, the Java compiler automatically imports three entire packages:
The default package (the package with no name)
The java.lang
package
Note: Packages aren't hierarchical. For example, importing
java.util.*
doesn't let you refer to the
Pattern
class as regex.Pattern
. You must always refer to it as either java.util.regex.Pattern
or (if you import java.util.regex.*
) simply
Pattern
.
Disambiguating a Name
If by some chance a member in one package shares the same name with a member in another package and both packages are imported, you must refer to each member by its qualified name.
For example, lets consider that you have created this package package graphics; class Rectangle {
}
The java.awt
package also contains a
Rectangle
class. If both graphics
and java.awt
have been imported, the following is ambiguous:
Rectangle rect;
In such a situation, you have to be more specific and use the member's qualified name to indicate exactly which
Rectangle
class you want: graphics.Rectangle rect;
Avoid duplicating import statement. Also there is no need to import the classes in the same package. Don’t import unnecessary classes.
If a class contains package declaration, import statement and class definition they should be written only in the same given order.
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7.3 Interfaces
Interfaces are part of what is known as “programming by contract”. This means that a programmer creates something that obliges other programmers to follow a set of conditions.
Interfaces are also considered to be the way Java gains some of the benefits of multiple inheritance without the drawbacks.
In Java an interface is similar to an abstract class in that its members are not implemented. In interfaces, _none_ of the methods are implemented. There is no code at all associated with an interface. For example public interface Comparable
{ boolean less(Object m); boolean greater(Object m); boolean lessEqual(Object m); boolean greaterEqual(Object m);
}
All instance methods are implicitly public and abstract. You can mark them as such, but are discouraged from doing so as the marking is considered obsolete practice. The interfaces themselves need not be public and several interfaces in the standard libraries are not public and thus used only internally.
All constant values defined in an interface are implicitly public
, static
, and final
. Member declarations in an interface disallow the use of some declaration modifiers; you cannot use transient
, volatile
, or synchronized
in a member declaration in an interface. Also, you may not use the private
and protected
specifiers when declaring members of an interface.
An interface creates a protocol that classes may implement. Note that one can extend an interface
(to get a new interface) just as you can extend a class. One can actually extend several interfaces.
Interfaces thus enjoy the benefits of multiple inheritance. (Classes do not.) There are almost no disadvantages to multiple inheritance of interface (small name conflict problems are one exception).
The Polynomial class that implements Comparable will need to implement all of the functions declared in the interface. public class Polynomial implements Comparable
{ . . .
boolean less(Object m){ . . . } boolean greater(Object m){ . . . } boolean lessEqual(Object m){ . . . } boolean greaterEqual(Object m){ . . . }
Polynomial multiply(Polynomial P){ . . . }
. . .
}
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A class may choose to implement any number of interfaces. A class that implements an interface must provide bodies for all methods of that interface. Also, an abstract class can choose to implement part of an interface leaving the rest for non-abstract subclasses.
The usefulness of interfaces goes far beyond simply publishing protocols for other programmers.
Consider the simple example of a class Car that implements interface Vehicle. Interface Vehicle has a single method called start(). Class Car will implement the interface by providing a start() method. Other functionality in the Car class has been left out for the sake of clarity. interface Vehicle {
// All vehicle implementations must implement the start
//method public void start();
} class Car implements Vehicle{
// Required to implement Vehicle public void start(){ ... }
}
Having laid the foundations of the Car object, we can create another object called Driver. It is the
Driver's job to start the Car and bring it to the restaurant patron. The Driver object can be written without interfaces, as follows: class Driver {
public Car getCar( Car c){ ... }
}
The Driver object has a method called getCar that returns a Car object. This code example satisfies the functional requirements of the system, but it forever links the Driver object with that of the Car. In this situation, the two objects are said to be tightly coupled. The Driver object requires knowledge of the Car object and has access to all public methods and variables contained within that object. It is best to avoid such tight coupling of code because it increases dependencies and reduces flexibility.
To code the Driver object using interfaces, the following implementation could be used: class Driver{
public Vehicle getVehicle( Vehicle c) { ... }
}
While the code changes are fairly minor -- changing the references from Car to Vehicle -- the effects on the development cycle are considerable. Using the second implementation, the Driver has knowledge only of the methods and variables defined in the Vehicle interface. Any other public methods and data contained in the specific implementation of the Vehicle interface are hidden from the user of the Vehicle object.
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This simple code change has ensured the proper concealment of information and implementation from other objects, and has therefore eliminated the possibility that developers will use undesirable methods.
Defining object types with interfaces
The Java interface keyword declares a type and the implements keyword guarantees that a class either implements or defers the implementation of all operations for each type specified in the implements clause. That guarantees that objects instantiated from concrete classes possess implementation code for all the operations of each type specified in the implements clause.
Consider the following interface and class definitions: interface Foo
{
String m3();
} public class Base
{ public String m1()
{ return "Base.m1()";
} public String m2( String s )
{ return "Base.m2(" + s + ")";
}
} private String p()
{ return "Base.p()";
} public class Derived extends Base implements Foo
{
public String m1()
{
return "Derived.m1";
}
public String m3()
{
return "Derived.m3()";
}
}
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Interface Foo declares a new type, Foo, with a single operation, m3(). The definition of class
Derived differs from before only by the addition of the implements Foo clause.
The additional implements Foo clause means type Derived also subtypes Foo. Thus you achieve multiple type inheritance: type Derived subtypes type Base and type Foo. Let's examine the ramifications of these type definitions.
Derived derived = new Derived();
Foo foo = derived;
Base base = derived; foo = base; // REJECT
The first statement attaches a type Derived variable to a type Derived object-creation expression.
The second statement attaches a variable of type Foo to the created object. The type-checker enforces conformance between the type Derived variable and the type Foo variable by verifying that Derived actually subtypes Foo. The third statement similarly passes type-checking. Note that these conformance checks consider the variable type, not the attached object type. The typechecker rejects the fourth statement since Foo does not subtype Base, even though foo and base point to the same object. Incompatible variable types prevent this attempt to attach a variable to the very object to which it is already attached.
Now let's take a look at method invocation via the different variable types.
Derived derived = new Derived();
Foo foo = derived;
Base base = derived;
// call m3()
System.out.println( derived.m3() );
System.out.println( foo.m3() );
System.out.println( base.m3() ); // REJECT
// call m1()
System.out.println( derived.m1() );
System.out.println( foo.m1() ); // REJECT
System.out.println( base.m1() );
Section 1 calls method m3() through three different reference variable types. Since type Base does not possess operation m3(), the type-checker rejects the section's third statement. Similarly, the type-checker rejects the second statement in Section 2 since type Foo does not possess operation m1(). Remarkably, the three variables remain attached to the same object for every method call, and that object has implementation for both the m1() and m3() methods. In the rejected statements, the type-checker overrules the underlying Derived object's ability to perform the specified method call by enforcing variable type conformance.
Implementing multiple inheritance in Java
If you want to write something similar to:
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//this code will give compile time error public class Employee extends Person, Employment {
// detail omitted
}
Here, Person is a concrete class that represents a person, while Employment is another concrete class that represents the details of a person who is employed. If you could only put them together, you would have everything necessary to define and implement an Employee class.
Inheriting implementation from more than one superclass - multiple implementation inheritance - is not a feature of Java. Java allows a class to have a single superclass and no more.
On the other hand, a class can implement multiple interfaces. In other words, Java supports multiple interface inheritance. Suppose the PersonLike interface is: public interface PersonLike {
String getName(); int getAge();
} and the EmployeeLike interface is: public interface EmployeeLike { float getSalary(); java.util.Date getHireDate();
}
You can create an Employee class like this : public class Employee implements PersonLike,EmploymentLike {
//details omitted
}
Difference between an Interface and an Abstract class
An Abstract class declares have at least one instance method that is declared abstract which will be implemented by the subclasses. An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior.
Subinterfaces
One interface may be described as a subinterface of another if it extends its properties (i.e., contains additional methods.)
In Java, each subinterface must be explicitly listed as extending one or more other interfaces, implicitly inheriting all of the super-interface operations. For example: interface File {
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public void open(String name);
public void close();
} interface ReadableFile extends File {
public byte readByte();
} interface WritableFile extends File {
public void writeByte(byte b);
} interface ReadWriteFile extends ReadableFile, WritableFile {
public void seek(int position);
}
7.4 Static Import
Many classes, including many in the Java core libraries, contain static constants that are used within the class and are also useful outside the class. For example, the java.lang.Math
class contains the constants
PI
and
E
for pi and e, respectively.
Prior to Java version 5.0, the only way to access those constants was by fully spelling out the names
Math.PI
,
Math.E
,
BorderLayout.NORTH
, etc. in your code. With static imports, you can use just
PI
,
E
, and
NORTH
without all the extra typing. To use static imports, add the static keyword to the import statement as follows:
import static java.lang.Math.PI;
Then in the code body you can use
double area = PI * radius * radius; instead of
double area = Math.PI * radius * radius;
Wildcards also work in the import static statement:
import static java.lang.Math.*;
For occasional use, simply typing the
Math.PI
is probably easier, but if constants from another class are used extensively, then the static import feature will reduce the code and also make it easier to read.
In addition to the constants, all the many static methods in the
Math
class are also available when you use the wildcard static import line above. For example,
double logarithm = log (number);
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instead of
double logarithm = Math.log (number);
Interfaces with Constants Only
Before the static import became available, a trick for avoiding the need to write the class name for static constants was to create an interface whose only purpose is to hold constants.
For example, say that you need the constants
ALPHA
and
OMEGA
for several different programs.
The first option is to create a utility class that holds the constants. e.g.: public class MyConstants
{
final static double ALPHA = 1.1;
final static double OMEGA = 901.0
}
Then in your other classes, you would refer to these constants with
MyConstants.ALPHA
MyConstants.OMEGA
However, placing the
MyConstants
class name wherever these constants appear can be tedious and also makes the code less readable, especially in a formula like
x = (3.1 * MyConstants.ALPHA)/(1.0 + MyConstants.OMEGA);
An alternative is to put the constants into an interface. This will not interfere with the class design since there is no limit to the number of interfaces that a class can implement.
public interface MyConstantsImpl
{
static final double ALPHA = 1.1;
static final double OMEGA = 901.0
}
Any class that needs these constants can implement
MyConstantsImpl
. The methods in the program will then refer to the constants simply with
ALPHA
and
OMEGA
.
Then a method in the class that implements
MyConstantsImpl
could use the constants in an equation without the class reference:
x = (3.1*ALPHA)/(1.0 + OMEGA); whcih is far more readable.
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This technique, however, violates the object-oriented design of the language. That is, you are not really implementing anything. You would not think of a class that implements
MyConstantsImpl as a
MyConstants
object in the sense of taking on an identifiable behavior of an interface.
So constants-only interfaces are ill advised and considered bad programming style . Use a class instead and then statically import the class.
7.5 strictfp
The keyword "strictfp" is used to control certain aspects of floating-point arithmetic.
You can use strictfp as a modifier of class, interface, and method declarations, like this:
// legal uses of strictfp
strictfp interface A {}
public strictfp class FpDemo1 {
strictfp void f() {}
}
You cannot use strictfp on constructors or methods within interfaces:
// illegal uses of strictfp
interface A {
strictfp void f();
}
public class FpDemo2 {
strictfp FpDemo2() {}
}
The strictfp keyword is used to designate an expression as "FP-strict." If a class, interface, or method is declared using strictfp, then the class, interface, or method is FP-strict. So too are all classes, interfaces, methods, constructors, instance initializers, variable initializers, and static initializers within the declaration.
An expression is FP-strict if it occurs anywhere within one of these FP-strict declarations, or if it is a compile-time constant expression. In practical terms, this means that if a class or method is declared with strictfp, any expression that occurs within the class or method is an FP-strict expression.
Because constructors cannot be declared using strictfp, they are FP-strict only if their defining class is FP-strict. Methods within interfaces cannot be declared using strictfp because this is an implementation rather than an interface property.
So what does FP-strict actually mean? Consider the following example:
public strictfp class FpDemo3 {
public static void main(String[] args) {
double d = 8e+307;
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System.out.println(4.0 * d * 0.5);
System.out.println(2.0 * d);
}
}
The maximum value of a double (Double.MAX_VALUE) is approximately 1.8e+308. In the
FpDemo3 example, the first expression is evaluated as:
(4.0 * d) * 0.5
because the Java programming language guarantees a left-to-right order of evaluation. When the first part of the expression is evaluated, the result is 32e+307 or 3.2e+308, which is larger than
Double.MAX_VALUE.
Because the expression is FP-strict, the implementation is required to evaluate the whole expression as resulting in positive infinity (which the program prints as "Infinity"). This is true even though the later multiplication by 0.5 produces a final value for the expression of 1.6e+308, which is less than Double.MAX_VALUE.
By contrast, if the expression is not FP-strict, an implementation is allowed to use an extended exponent range to represent intermediate results. In the FpDemo3 example, this could keep the expression from overflowing, and produce a final result that is within range. An implementation is not required to do this; it can, in effect, use FP-strict rules everywhere.
Note that multiplication in this example is not associative, in that:
(4.0 * d) * 0.5 != 4.0 * (d * 0.5) strictfp is important because its use guarantees common behavior across different Java implementations. In other words, you can know that the floating-point arithmetic in your application behaves the same when you move your application to a different Java implementation or hardware platform.
Summary
Packages are important as they act as a namespace. They help to avoid conflict in classes and you can group the related classes together as a single package. Interfaces are used as a programming discipline. They are also helpful while using multiple inheritance. The static import is a new concept of J2SE 5.0. It helps you to use the class members without using the class name.
Questions
1. interface I1 {} interface I2 {} class Base implements I1 {} class Sub extends Base implements I2 {} class Red { public static void main(String args[]) {
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Sub s1 = new Sub();
I2 i2 = s1; // 1
I1 i1 = s1; // 2
Base base = s1; // 3
Sub s2 = (Sub)base; // 4
}
}
A compile-time error is generated at which line? a. 1 b. 2 c. 3 d. 4 e. None of the above
173
2. package com.dan.chisholm; public class A {
public void m1() {System.out.print("A.m1, ");}
protected void m2() {System.out.print("A.m2, ");}
private void m3() {System.out.print("A.m3, ");}
void m4() {System.out.print("A.m4, ");}
} class B {
public static void main(String[] args) {
A a = new A();
a.m1(); // 1
a.m2(); // 2
a.m3(); // 3
a.m4(); // 4
}}
Assume that the code appears in a single file named A.java. What is the result of attempting to compile and run the program? a. Prints: A.m1, A.m2, A.m3, A.m4, b. Compile-time error at 1.
c. Compile-time error at 2.
d. Compile-time error at 3.
e. Compile-time error at 4.
f. None of the above
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Chapter 8 : Assertions and Exception handling
8.1 What is an Exception?
The Java language uses exceptions to provide error-handling capabilities for its programs. An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions.
The term exception is shorthand for the phrase "exceptional event."
Many kinds of errors can cause exceptions--problems ranging from serious hardware errors, such as a hard disk crash, to simple programming errors, such as trying to access an out-of-bounds array element. When such an error occurs within a Java method, the method creates an exception object and hands it off to the runtime system. The exception object contains information about the exception, including its type and the state of the program when the error occurred. The runtime system is then responsible for finding some code to handle the error. In
Java terminology, creating an exception object and handing it to the runtime system is called
throwing an exception.
After a method throws an exception, the runtime system leaps into action to find someone to handle the exception. The set of possible "someones" to handle the exception is the set of methods in the call stack of the method where the error occurred. The runtime system searches backwards through the call stack, beginning with the method in which the error occurred, until it finds a method that contains an appropriate exception handler. An exception handler is considered appropriate if the type of the exception thrown is the same as the type of exception handled by the handler. Thus the exception bubbles up through the call stack until an appropriate handler is found and one of the calling methods handles the exception. The exception handler chosen is said to catch the exception.
If the runtime system exhaustively searches all of the methods on the call stack without finding an appropriate exception handler, the runtime system (and consequently the Java program) terminates.
By using exceptions to manage errors, Java programs have the following advantages over traditional error management techniques:
•
•
•
Advantage 1: Separating Error Handling Code from "Regular" Code
Advantage 2: Propagating Errors Up the Call Stack
Advantage 3: Grouping Error Types and Error Differentiation
Advantage 1: Separating Error Handling Code from "Regular" Code
In pseudo-code, your function might look something like this: readFile {
175
open the file;
determine its size;
allocate that much memory;
read the file into memory;
close the file;
}
At first glance this function seems simple enough, but it ignores all of these potential errors:
•
•
•
•
•
What happens if the file can't be opened?
What happens if the length of the file can't be determined?
What happens if enough memory can't be allocated?
What happens if the read fails?
What happens if the file can't be closed?
To answer these questions within your read_file
function, you'd have to add a lot of code to do error detection, reporting and handling. Your function would end up looking something like this: errorCodeType readFile {
initialize errorCode = 0;
open the file;
if (theFileIsOpen) {
determine the length of the file;
if (gotTheFileLength) {
allocate that much memory;
if (gotEnoughMemory) {
read the file into memory;
if (readFailed) {
errorCode = -1;
}
} else {
errorCode = -2;
}
} else {
errorCode = -3;
}
close the file;
if (theFileDidntClose && errorCode == 0) {
errorCode = -4;
} else {
errorCode = errorCode and -4;
}
} else {
errorCode = -5;
}
return errorCode;
}
There's so much error detection, reporting, and returning that the original 7 lines of code are lost in the clutter. And worse yet, the logical flow of the code has also been lost in the clutter,
176
making it difficult to tell if the code is doing the right thing: Is the file really being closed if the function fails to allocate enough memory? It's even more difficult to ensure that the code continues to do the right thing after you modify the function three months after writing it. Many programmers "solve" this problem by simply ignoring it--errors are "reported" when their programs crash.
Java provides an elegant solution to the problem of error management: exceptions. Exceptions enable you to write the main flow of your code and deal with the, well, exceptional cases elsewhere. If your read_file function used exceptions instead of traditional error management techniques, it would look something like this:
readFile {
try {
open the file;
determine its size;
allocate that much memory;
read the file into memory;
close the file;
} catch (fileOpenFailed) {
doSomething;
} catch (sizeDeterminationFailed) {
doSomething;
} catch (memoryAllocationFailed) {
doSomething;
} catch (readFailed) {
doSomething;
} catch (fileCloseFailed) {
doSomething;
}
}
Advantage 2: Propagating Errors Up the Call Stack
A second advantage of exceptions is the ability to propagate error reporting up the call stack of methods. Suppose that the readFile
method is the fourth method in a series of nested method calls made by your main program: method1
calls method2
, which calls method3
, which finally calls readFile
. method1 {
call method2;
} method2 {
call method3;
} method3 {
call readFile;
}
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Suppose also that method1
is the only method interested in the errors that occur within readFile
. Traditional error notification techniques force method2
and method3
to propagate the error codes returned by readFile
up the call stack until the error codes finally reach method1
-the only method that is interested in them.
method1 {
errorCodeType error;
error = call method2;
if (error)
doErrorProcessing;
else
proceed;
}
errorCodeType method2 {
errorCodeType error;
error = call method3;
if (error)
return error;
else
proceed;
}
errorCodeType method3 {
errorCodeType error;
error = call readFile;
if (error)
return error;
else
proceed;
}
The Java runtime system searches backwards through the call stack to find any methods that are interested in handling a particular exception. A Java method can "duck" any exceptions thrown within it, thereby allowing a method further up the call stack to catch it. Thus only the methods that care about errors have to worry about detecting errors.
method1 {
try {
call method2;
} catch (exception) {
doErrorProcessing;
}
}
method2 throws exception {
call method3;
}
method3 throws exception {
call readFile;
}
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However, as you can see from the pseudo-code, ducking an exception does require some effort on the part of the "middleman" methods. Any checked exceptions that can be thrown within a method are part of that method's public programming interface and must be specified in the throws
clause of the method. Thus a method informs its callers about the exceptions that it can throw, so that the callers can intelligently and consciously decide what to do about those exceptions.
Note again the difference in the bloat factor and code obfuscation factor of these two error management techniques. The code that uses exceptions is more compact and easier to understand.
Advantage 3: Grouping Error Types and Error Differentiation
Often exceptions fall into categories or groups.
Java exceptions must be instances of Throwable or any Throwable descendant. As for other Java classes, you can create subclasses of the Throwable class and subclasses of your subclasses.
Each "leaf" class (a class with no subclasses) represents a specific type of exception and each
"node" class (a class with one or more subclasses) represents a group of related exceptions.
ArrayException
is a subclass of
Exception
(a subclass of
Throwable
) and has three subclasses.
InvalidIndexException
,
ElementTypeException
, and
NoSuchElementException
are all leaf classes. Each one represents a specific type of error that can occur when manipulating an array.
One way a method can catch exceptions is to catch only those that are instances of a leaf class.
For example, an exception handler that handles only invalid index exceptions has a catch statement like this: catch (InvalidIndexException e) {
. . .
}
ArrayException
is a node class and represents any error that can occur when manipulating an array object, including those errors specifically represented by one of its subclasses. A method can catch an exception based on its group or general type by specifying any of the exception's superclasses in the catch
statement. For example, to catch all array exceptions regardless of their specific type, an exception handler would specify an
ArrayException
argument: catch (ArrayException e) {
. . .
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}
This handler would catch all array exceptions including
InvalidIndexException
,
ElementTypeException
, and
NoSuchElementException
. You can find out precisely which type of exception occurred by querying the exception handler parameter e
. You could even set up an exception handler that handles any
Exception
with this handler: catch (Exception e) {
. . .
}
Exception handlers that are too general, such as the one shown here, can make your code more error prone by catching and handling exceptions that you didn't anticipate and therefore are not correctly handled within the handler.
8.2 Types of Exceptions
All exception classes are the subclass of java.lang.Throwable class.
Java has different types of exceptions, including I/O Exceptions, runtime exceptions, and exceptions of your own creation, to name a few. Runtime exceptions are those exceptions that occur within the Java runtime system. This includes arithmetic exceptions (such as when dividing by zero), pointer exceptions (such as trying to access an object through a null reference), and indexing exceptions (such as attempting to access an array element through an index that is too large or too small).
Runtime exceptions can occur anywhere in a program and in a typical program can be very numerous. The cost of checking for runtime exceptions often exceeds the benefit of catching or specifying them. Thus the compiler does not require that you catch or specify runtime exceptions, although you can. Checked exceptions are exceptions that are not runtime exceptions and are checked by the compiler; the compiler checks that these exceptions are caught or specified.
The Throwable Class and Its Subclasses
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Throwable
has two direct descendants:
Error
and
Exception.
Errors
When a dynamic linking failure or some other "hard" failure in the virtual machine occurs, the virtual machine throws an
Error
. Typical Java programs should not catch
Error s. In addition, it's unlikely that typical Java programs will ever throw
Error s either.
Exceptions
Most programs throw and catch objects that derive from the Exception class.
The Exception class has many descendants defined in the Java packages. These descendants indicate various types of exceptions that can occur. For example, IllegalAccessException signals that a particular method could not be found, and NegativeArraySizeException indicates that a program attempted to create an array with a negative size.
One Exception subclass is RuntimeException.
Runtime Exceptions
The RuntimeException class represents exceptions that occur within the Java virtual machine
(during runtime). An example of a runtime exception is NullPointerException, which occurs when a method tries to access a member of an object through a null reference. A
NullPointerException can occur anywhere a program tries to dereference a reference to an object. The cost of checking for the exception often outweighs the benefit of catching it.
The Java packages define several RuntimeException classes. You can catch these exceptions just like other exceptions. However, a method is not required to specify that it throws
RuntimeExceptions.
You can create your own
RuntimeException
subclasses.
Runtime exceptions can occur anywhere in a program and in a typical program can be very numerous. Typically, the cost of checking for runtime exceptions exceeds the benefit of catching or specifying them. Thus the compiler does not require that you catch or specify runtime exceptions, although you can.
What does it cost you if you throw a RuntimeException or create a subclass of
RuntimeException just because you don't want to deal with specifying it? Simply, you get the ability to throw an exception without specifying that you do so. In other words, it is a way to avoid documenting the exceptions that a method can throw. When is this good? Well, when is it ever good to avoid documenting a method's behavior? The answer is "hardly ever."
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8.3 Catching and Handling Exceptions
The first step in writing an exception handler is to enclose the statements that might throw an exception within a try
block. The try
block is said to govern the statements enclosed within it and defines the scope of any exception handlers (established by subsequent catch
blocks) associated with it.
Next, you associate exception handlers with a try
block by providing one or more catch
blocks directly after the try
block.
Java's finally
block provides a mechanism that allows your method to clean up after itself regardless of what happens within the try
block. Use the finally
block to close files or release other system resources.
The try Block
The first step in constructing an exception handler is to enclose the statements that might throw an exception within a try
block. In general, a try
block looks like this: try {
Java statements
}
The segment of code labelled Java statements is composed of one or more legal Java statements that could throw an exception.
You could put each statement that might potentially throw an exception within its own try statement, and provide separate exception handlers for each try
. Or you could put all of the statements within a single try
statement and associate multiple handlers with it. The following listing uses one try
statement for the entire method because the code tends to be easier to read.
PrintWriter out = null;
try {
System.out.println("Entering try statement");
out = new PrintWriter(new FileWriter("OutFile.txt"),true);
for (int i = 0; i < size; i++)
out.println("Value at: " + i + " = " + victor.elementAt(i));
}
The try
statement governs the statements enclosed within it and defines the scope of any exception handlers associated with it. In other words, if an exception occurs within the try statement, that exception is handled by the appropriate exception handler associated with this try
statement.
A try statement must be accompanied by at least one catch block or one finally block.
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The catch Block(s)
The try
statement defines the scope of its associated exception handlers. You associate exception handlers with a try
statement by providing one or more catch
blocks directly after the try
block: try {
. . .
} catch ( . . . ) {
. . .
} catch ( . . . ) {
. . .
} .
. .
There can be no intervening code between the end of the try
statement and the beginning of the first catch
statement. The general form of Java's catch
statement is: catch (SomeThrowableObject variableName) {
Java statements
}
As you can see, the catch
statement requires a single formal argument. The argument to the catch
statement looks like an argument declaration for a method. The argument type,
SomeThrowableObject, declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable class defined in the java.lang package.
When Java programs throw an exception they are really just throwing an object, and only objects that derive from
Throwable
can be thrown.
The catch
block contains a series of legal Java statements. These statements are executed if and when the exception handler is invoked. The runtime system invokes the exception handler when the handler is the first one in the call stack whose type matches that of the exception thrown.
try {
. . .
} catch (ArrayIndexOutOfBoundsException e) {
System.err.println("Caught ArrayIndexOutOfBoundsException:" + e.getMessage());
} catch (IOException e) {
System.err.println("Caught IOException: "+e.getMessage());
}
When you have to write multiple catch blocks and the exception classes in the blocks are related to each other then take care to write the catch block for the subclass first and then the catch block for the superclass. If the superclass catch block is written first then the catch block of subclass becomes unreachable and gives compile-time error.
An example of using multiple catch blocks
183
class MultiCatch { public static void main (String args[]) { try { int a = args.length;
System.out.println("a = " + a); int b = 42 / a ; int c[] ={ 1 }; c[42] = 99;
} catch (ArithmeticException e) {
System.out.println ("Divide by 0:" + e);
} catch (ArrayIndexOutOfBoundsException e) {
System.out.println ("Array Index oob:" + e);
}
System.out.println ("After try/catch blocks.");
}
}
Catching Multiple Exception Types with One Handler
Each handles only one type of exception. The Java language allows you to write general exception handlers that handle multiple types of exceptions.
Your exception handler can be written to handle any class that inherits from
Throwable
.
The closest common ancester of IOException and ArrayIndexOutOfBoundsException is the
Exception class. An exception handler that handles both types of exceptions looks like this: try {
. . .
} catch (Exception e) {
System.err.println("Exception caught:"+e.getMessage());
}
Handlers that can catch most or all exceptions are typically useless for error recovery because the handler has to determine what type of exception occurred anyway to determine the best recovery strategy. Also, exception handlers that are too general can make code more error prone by catching and handling exceptions that weren't anticipated by the programmer and for which the handler was not intended.
The finally Block
The final step in setting up an exception handler is providing a mechanism for cleaning up the state of the method before (possibly) allowing control to be passed to a different part of the program. You do this by enclosing the cleanup code within a finally
block.
184
The runtime system always executes the statements within the finally
block regardless of what happens within the try
block. Regardless of whether control exits the writeList
method's try block due to one of the three scenarios listed previously, the code within the finally
block will be executed.
This is the finally block for the writeList method. It cleans up and closes the PrintWriter. finally {
if (out != null) {
System.out.println("Closing PrintWriter");
out.close();
} else {
System.out.println("PrintWriter not open");
}
}
Displaying a description of an exception
Throwable overrides the toString() so that it returns a string containing a description of the exception. You can display this description in a println() statement by simply passing the exception as an argument.
You access the instance variables and methods of exceptions in the same manner that you access the instance variables and methods of other objects. getMessage
is a method provided by the
Throwable
class that prints additional information about the error that occurred. The
Throwable class also implements two methods for filling in and printing the contents of the execution stack when the exception occurred.
Accessing Stack Trace Information
Definition: A stack trace provides information on the execution history of the current thread
and lists the names of the classes and methods that were called at the point when the exception occurred. A stack trace is a useful debugging tool that you'll normally take advantage of when an exception has been thrown.
The following code shows how to call the getStackTrace
method on the exception object: catch (Exception cause) {
StackTraceElement elements[] = cause.getStackTrace();
for (int i = 0; n = elements.length; i < n; i++) {
System.err.println(elements[i].getFileName() + ":"
+ elements[i].getLineNumber() + ">> " +
elements[i].getMethodName() + "()");
}
}
Nested try statements
185
Each time a try statement is entered, the context of that exception is pushed on the stack. If an inner try statement does not have a catch handler for a particular exception, the stack is unwound and the next try statement’s catch handlers are inspected for a match. This continues until one of the catch statements succeeds, or until the entire nested try statements are exhausted. If no catch statement matches, then the Java runtime system will handle the exception.
Nesting of try statements can occur in less obvious ways when method calls are involved. You can enclose a call to a method within a try block. Inside that method is another try statement. In this case, the try statement within the method is still nested inside the outer try block, which calls the method.
//An Example of nested try statements.
class NestTry
{ public static void main (String args[])
{ try
{ int a = args.length;
/* if no command-line args are present
the following statement will generate
a divide-by-zero exception*/ int b = 42 / a;
System.out.println(“a = “ + a); try //nested try block
{
/* if one command line arg is used ,
then a divide-by-zero exception
will be generated by the following
code*/
if (a == 1) a = a / (a – a);
// division by zero
/* if two command line arg is used ,
then generate an out of bounds exception .*/
if( a = 2 )
{
int c [] = { 1 };
c[ 42 ] = 99;
//generate an out of bound exception
}
} catch (ArrayIndexOutOfBoundsException e)
{
System.out.println(e);
}
} catch (ArithmeticException e)
{
186
}
}
}
Sample outputs:
1)
C:\> java NestTry
Divide by zero.
System.out.println(“Divide by zero”);
2)
C:\> java NestTry One
a = 1
divide by zero
3)
C:\> java NestTry One Two a = 2
Array index out of bounds.
8.4 The throw Statement
All Java methods use the throw
statement to throw an exception. The throw
statement requires a single argument: a throwable object. In the Java system, throwable objects are instances of any subclass of the Throwable class. Here's an example of a throw
statement: throw someThrowableObject;
If you attempt to throw an object that is not throwable, the compiler refuses to compile your program and displays an error message similar to the following: testing.java:10: Cannot throw class java.lang.Integer; it must be a subclass of class java.lang.Throwable.
throw new Integer(4); public Object pop() throws EmptyStackException {
Object obj;
if (size == 0)
throw new EmptyStackException();
obj = objectAt(size - 1);
setObjectAt(size - 1, null);
size--;
return obj;
}
The
EmptyStackException
class is defined in the java.util
package.
187
// Demonstrate throw.
class ThrowDemo { static void demoproc() { try { throw new NullPointerException("demo");
} catch(NullPointerException e) {
System.out.println("Caught inside demoproc."); throw e; // re-throw the exception
}
}
public static void main(String args[]) { try { demoproc();
} catch(NullPointerException e) {
System.out.println("Recaught " );
}
}
}
The resulting output is
Caught inside demoproc.
Recaught
One more example using finally.
class FinallyDemo {
// Through an exception out of the method.
static void procA() { try {
System.out.println("inside procA"); throw new RuntimeException("demo");
} finally {
System.out.println("procA's finally");
}
}
// Return from within a try block.
static void procB() { try {
System.out.println("inside procB"); return;
} finally {
System.out.println("procB's finally");
}
}
188
// Execute a try block normally.
static void procC() { try {
System.out.println("inside procC");
} finally {
System.out.println("procC's finally");
}
}
} public static void main(String args[]) { try {
procA();
} catch (Exception e) {
System.out.println("Exception caught");
} procB(); procC();
}
The resulting Output is: inside procA procA's finally
Exception caught inside procB procB's finally inside procC procC's finally
8.5 The throws Clause
The declaration of the pop
method contains this clause: throws EmptyStackException
The throws
clause specifies that the method can throw an
EmptyStackException
.
Java requires that a method either catch or specify all checked exceptions that can be thrown within the scope of the method.
A method can catch an exception by providing an exception handler i.e. using a try..catch block for that type of exception.
189
If a method chooses not to catch an exception, the method must specify that it can throw that exception. The throws clause specifies that if an exception occurs it is not explicitly handled.
Specifying the Exceptions Thrown by a Method
public void writeList() {
PrintWriter out = new PrintWriter(new FileWriter("OutFile.txt"), true);
for (int i = 0; i < size; i++)
out.println("Value at: " + i + " = " + victor.elementAt(i));
}
The throws clause is composed of the throws keyword followed by a comma-separated list of all the exceptions thrown by that method. The throws clause goes after the method name and argument list and before the curly bracket that defines the scope of the method. Here's an example: public void writeList() throws IOException,
ArrayIndexOutOfBoundsException {
Remember that
ArrayIndexOutofBoundsException
is a runtime exception, so you don't have to specify it in the throws
clause, although you can.
class ThrowsDemo { static void throwOne() throws IllegalAccessException {
System.out.println("Inside throwOne."); throw new IllegalAccessException("demo");
} public static void main(String args[]) { try { throwOne();
} catch (IllegalAccessException e) {
System.out.println("Caught “);
}
}
}
The resulting Output is
Inside throwOne.
Caught
8.6 Overriding methods that throw exceptions
An overriding method in a subclass may only throw exceptions declared in the parent class or children of the exceptions declared in the parent class. This is only true for overriding methods not overloading methods. Thus if a method has exactly the same name and arguments it can only throw exceptions declared in the parent class, or exceptions that are children of exceptions in the
190
parent declaration. It can however throw fewer or no exceptions. Thus the following example will not compile import java.io.*; class Base{ public static void amethod()throws FileNotFoundException{}
} public class ExcepDemo extends Base{
//Will not compile, exception not in base version of method
public static void amethod()throws IOException{}
}
If it were the method in the parent class that was throwing IOException and the method in the child class that was throwing FileNotFoundException this code would compile. Again, remember that this only applies to overridden methods, there are no similar rules to overloaded methods. Also an overridden method in a sub class may throw Exceptions.
8.7 Java’s built in exceptions
Inside the standard package java.lang, Java defines several exception classes. The most general of these are subclasses of RuntimeException. Since, java.lang is implicitly imported to all Java programs, most exceptions derived from RuntimeException are automatically available. They need not be included in any method’s throws list. They are called unchecked exceptions because the compiler does not check to see if a method handles, or throws these exceptions. Checked exceptions are those defined by java.lang that must be included in a method’s throws list if that method can generate one of these exceptions and does not handle it itself.
RuntimeException subclasses (unchecked):
ArithmeticException
ArrayIndexOutOfBoundsException
ArrayStoreException
ClassCastException
IllegalArgumentException
IllegalMonitorStateException
IllegalStateException
IllegalThreadStateException
IndexOutOfBoundsException
NegativeArraySizeException
NullPointerException
NumberFormatException
SecurityException
191
StringIndexOutOfBoundsException
UnsupportedOperationException
Checked Exceptions defined in java.lang
ClassNotFoundException
CloneNotSupportedException
IllegalAccessException
InstantiationException
InterruptedException
NoSuchfieldException
NoSuchMethodException
ArithmeticException
Attempting to divide an integer by zero or take a modulus by zero throw the
ArithmeticException--no other arithmetic operation in Java throws an exception.
For example, the following code causes an ArithmeticException to be thrown: class Arith {
public static void main(String args[]) {
int j = 0;
j = j / j;
}
}
NullPointerException
An attempt to access a variable or method in a null object or a element in a null array throws a
NullPointerException. For example, the accesses o.length and a[0] in the following class declaration throws a NullPointerException at runtime.
class Null {
public static void main(String args[]) {
String o = null;
int a[] = null;
o.length();
a[0] = 0;
}
}
It is interesting to note that if you throw a null object you actually throw a NullPointerException.
IncompatibleClassChangeException
192
In general the IncompatibleClassChangeException is thrown whenever one class's definition changes but other classes that reference the first class aren't recompiled. Four specific changes that throw a IncompatibleClassChangeException at runtime are:
•
•
•
•
A variable's declaration is changed from static to non-static in one class but other classes that access the changed variable aren't recompiled.
A variable's declaration is changed from non-static to static in one class but other classes that access the changed variable aren't recompiled.
A field that is declared in one class is deleted but other classes that access the field aren't recompiled.
A method that is declared in one class is deleted but other classes that access the method aren't recompiled.
ClassCastException
A ClassCastException is thrown if an attempt is made to cast an object O into a class C and O is neither C nor a subclass of C.
The following class declaration results in a ClassCastException at runtime: class ClassCast {
public static void main(String args[]) {
Object o = new Object();
String s = (String)o; // the cast attempt
s.length();
}
}
NegativeArraySizeException
A NegativeArraySizeException is thrown if an array is created with a negative size. For example, the following class definition throws a NegativeArraySizeException at runtime:
class NegArray {
public static void main(String args[]) {
int a[] = new int[-1];
a[0] = 0;
}
}
OutOfMemoryException
An OutOfMemoryException is thrown when the system can no longer suppy the application with memory. The OutOfMemoryException can only occur during the creation of an object, i.e., when new is called. For example, the following code results in an OutOfMemoryException at runtime: class Link {
int a[] = new int[1000000];
193
Link l;
}
class OutOfMem {
public static void main(String args[]) {
Link root = new Link();
Link cur = root;
while(true) {
cur.l = new Link();
cur = cur.l;
}
}
}
NoClassDefFoundException
A NoClassDefFoundException is thrown if a class is referenced but the runtime system cannot find the referenced class.
For example, class NoClass is declared: class NoClass {
public static void main(String args[]) {
C c = new C();
}
}
When NoClass is run, if the runtime system can't find C.class it throws the
NoClassDefFoundException.
C class must have existed at the time NoClass is compiled.
IncompatibleTypeException
An IncompatibleTypeException is thrown if an attempt is made to instantiate an interface. For example, the following code causes an IncompatibleTypeException to be thrown. interface I {
}
class IncompType {
public static void main(String args[]) {
I r = (I)new("I");
}
}
ArrayIndexOutOfBoundsException
194
An attempt to access an invalid element in an array throws an
ArrayIndexOutOfBoundsException. For example: class ArrayOut {
public static void main(String args[]) {
int a[] = new int[0];
a[0] = 0;
}
}
UnsatisfiedLinkException
An UnsatisfiedLinkException is thrown if a method is declared native and the method cannot be linked to a routine in the runtime. class NoLink {
static native void foo();
public static void main(String args[]) {
foo();
}
}
8.8 Chained Exceptions
Chained exceptions allow you to rethrow an exception, providing additional information without losing the original cause of the exception. The chained exception API was introduced in 1.4 by adding a cause property of type
Throwable
to exceptions. Two methods and two constructors were added to
Throwable
, the class from which all exceptions inherit. Since every
Throwable can have a cause, each exception can have a cause, which itself can have a cause, and so on.
The methods and constructors in Throwable that support chained exceptions are:
Throwable getCause()
Throwable initCause(Throwable)
Throwable(String, Throwable)
Throwable(Throwable)
The Throwable argument to initCause and the Throwable constructors is the exception that caused the current exception. getCause returns the exception that caused the current exception, and initCause returns the current exception.
The following example shows how to use a chained exception: try {
...
} catch (IOException e) { throw new SampleException("Other IOException", e);
195
}
In this example, when an
IOException
is caught, a new
SampleException
exception is created with the original cause attached and the chain of exceptions is thrown up to the next higher level exception handler.
What is a cause?
A cause is a reference to another Throwable object. The intent is that this object will be interpreted as the thing that caused this throwable to get thrown in the first place.
However, you could encapsulate any Throwable object in a new Throwable object, whether or not it had anything to do with the true cause. All that you are required to do is pass a Throwable object's reference to the constructor for a new Throwable object, or invoke the initCause method on an existing Throwable object, passing a reference to another Throwable object as a parameter. It then becomes a cause.
Two ways to encapsulate a cause
As suggested above, you can associate a cause with a Throwable in two different ways. One way is to invoke one of the constructors that accepts a Throwable as a parameter. This assumes, of course, that the class from which you are instantiating the new object has such a constructor.
The other way to associate a cause with a Throwable is to invoke the initCause method on an existing Throwable object's reference, passing a reference to another Throwable object as a parameter. This works even when you are instantiating a new object from a class that doesn't have a constructor that accepts a parameter of type Throwable. import java.io.*; class Excep20{ public static void main(String[] args){ try{ new Class01().meth01();
} catch(NewEx01 e){
}//end Excep20
System.out.println("In main catch block");
System.out.println("Msg is:\n" + e.getMessage());
System.out.println("Cause is:\n" + e.getCause());
System.out.println();//blank line
System.out.println("Print StackTrace"); e.printStackTrace();
}//end catch
}//end main
//This is a new exception class class NewEx01 extends Exception{ public NewEx01() {
196
} public NewEx01(String message){ super(message);
} public NewEx01(Throwable throwable){ super(throwable);
} public NewEx01(String message,Throwable throwable){ super(message, throwable);
}
}//end NewEx01
//This is a new exception class class NewEx02 extends Exception{ public NewEx02() {
} public NewEx02(String message){ super(message);
} public NewEx02(Throwable throwable){ super(throwable);
} public NewEx02(String message,Throwable throwable){ super(message, throwable);
}
}//end NewEx02 class Class01{ void meth01() throws NewEx01{ try{ meth02();
} catch(NewEx02 e){
System.out.println("In meth01 catch block");
System.out.println("Msg is:\n" + e.getMessage());
System.out.println("Cause is:\n" + e.getCause());
System.out.println();//blank line throw new NewEx01("Msg from meth01",e);
}//end catch
}//end meth01 void meth02() throws NewEx02{ try{ meth03();
197
} catch(RuntimeException e){
System.out.println("In meth02 catch block");
System.out.println("Msg is:\n" + e.getMessage());
System.out.println("Cause is:\n" + e.getCause());
System.out.println(); throw new NewEx02("Msg from meth02",e);
}//end catch
}//end meth02 void meth03(){ try{ int x = 3/0;
} catch(ArithmeticException e){
IndexOutOfBoundsException ex = new IndexOutOfBoundsException("Msg from metho03"); ex.initCause(e);
throw ex;
}//end catch
}//end meth03
}//end Class01
8.9 Creating Your Own Exception Classes
Choosing the Exception Type to Throw
When faced with choosing the type of exception to throw, you have two choices:
1. Use one written by someone else. The Java development environment provides a lot of exception classes that you could use.
2. Write one of your own.
You should go to the trouble of writing your own exception classes if you answer "yes" to any of the following questions. Otherwise, you can probably get away with using someone else's:
•
•
Do you need an exception type that isn't represented by those in the Java development environment?
Would it help your users if they could differentiate your exceptions from those thrown by classes written by other vendors?
Does your code throw more than one related exception?
•
If you use someone else's exceptions, will your users have access to those exceptions? A similar question is: Should your package be independent and self-contained?
Example public class DivideByZeroException extends Exception
{
198
public DivideByZeroException()
{
super("Dividing by Zero!");
}
public DivideByZeroException(String message)
{
super(message);
}
} public class DivisionExample { public static void main(String args[]) { try { int x = Integer.parseInt(args[0]); int y = Integer.parseInt(args[1]); if(y == 0) { throw new DivideByZeroException();
} else {
System.out.println(x/y);
}
} catch(ArrayIndexOutOfBoundsException e) {
System.out.println(e);
} catch(DivideByZeroException e) {
System.out.println(e);
}
}
}
Choosing a Superclass
However, the java.lang package
provides two
Throwable
subclasses that further divide the type of problems that can occur within a Java program:
Error s and
Exception s. Most of the applets and applications that you write will throw objects that are
Exception s. (
Error s are reserved for serious hard errors that occur deep in the system.)
Runtime exceptions don't have to be specified in the throws
clause of a method. The bottom line is that you shouldn't subclass
RuntimeException
unless your class really is a runtime exception!
Naming Conventions
It's good practice to append the word "Exception" to the end of all classes that inherit (directly or indirectly) from the
Exception
class. Similarly, classes that inherit from the
Error
class should end with the string "Error".
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8.10 Assertions
Assertions were added to Java with the release of JDK1.4. Assertions are a feature of other
Object Orientated languages and there has been pressure for a while for them to be added to
Java.
Why Assertions exist?
Assertions are a fairly simple concept where you write a statement that should always be true, but in a form that can be removed from the finally compiled version of the code so they cause no runtime overhead. It would be perfectly possible to write code using the constructs available in
Java prior to JDK1.4 that simulates the functionality of assertions but it would be hard to do so in such a way that they would be turned off at runtime.
How assertions are used?
Where and how you use assertions is a matter of judgment in a similar way to where and how you use comments.
Assertions can be considered an extension of comments in that comments are often used to tell a person reading the code that a particular statement or piece of code should always be true. With assertions, instead of indicating by a comment that a statement should always be true you can assert that it should always be true.
If you then run the code with assertions enables you do not have to rely on a close reading of the code as you would with comments but the running of the code itself will check your assertions are true, and if they are not an assert error will be thrown.
By default, assert statements are disabled during normal program run.
As the name implies assertions are used to assert something that should always be true. When a program is running normally assertions are disabled and cause no performance overhead. When a programmer is investigating an issue assertions can be enabled and if any of the assert statements are not true an assert exception will be thrown. Assertions are a key part of JDK 1.4 and require no additional import statements in the source code. However because programmers in the past have used the word assert in creating their own versions of assertions the compilation process requires a command line parameter to tell it that it will be using the genuine JDK 1.4 version of assertions. This takes the form javac -source1.4 Myprog.java
If you then run the program normally in the form java Myprog
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assertions are disabled and no assert exceptions will be thrown. If you subsequently have an issue you want to investigate and confirm that all of the assertions of items that should always be true, really are true you can run the program with assertions enabled as follows.
java -enableassertions Myprog or java –ea Myprog
What should you assert to be true?
Assertions should be used for anything you believe should always be true. For example it should always be true that a person has an age greater than zero. If a person has an age less than zero your program or its input has a significant problem. For another example if you were recording the date of a persons death you program (or your morality) might have a problem if you had the date of death in the future, therefore you could assert that the date of death is in the future.
For example if you are falling through a case statement or a set of if/else statements you might believe that the code should always exit before it reaches the final test. Imagine if you had an application dealing with media types. Your application might be expecting to deal with jpg, mpg, avi or gif files. You set up a case statement that branch according to the type of file. Because you believe that the type will always be one of those file types there is definitely a problem if you get to the end of the case statement without branching and you can place an assert statement at the location of the default option.
Where should you use assertions?
Assertions should not be used to enforce the public interface of a program. One of the most public interfaces of a program is its command line parameters. Thus traditionally a programmer will inspect the command line passed to a Java program by looking at the value in the String args array passed from the command line. Typically if this array does not contain the expected type of values the program will exit and print a message indicating what the correct format of the command line should be. The introduction of the assert mechanism does not change this. It is not appropriate to use assert to check the command line parameters of a program because assertions will not always be enabled.
It is not appropriate to use assertions to check the parameters passed to public methods. Because your public methods may be used in programs written by other people you cannot be certain that they will have assertions enabled and thus the normal running of the program may be faulty.
However it is appropriate to use assertions for checking the parameters to private methods as these will generally only be called by code written by people who have access to the source of those methods. The same assumption may be made for code in protected or in package protected methods.
Assert syntax
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The assert statement has two formats
The simple assert somebooleatest and assert somebooleantest : someinformatinvemethod
In the first simpler version the assert tests that something is true and if it is not an
AssertionError
is thrown. For example if you were testing that a persons age was greater than zero you might create an assert in the form assert (iAge > 0);
The more complex version might be of the form assert (iAge > 0) :"age must be greater than zero";
This example is simple in that the right hand side of the expression is a simple string, but this could be any method call that returns a value, i.e. a method with any return type except void.
public class Foo
{
public void m1( int value )
{
assert 0 <= value;
System.out.println( "OK" );
}
public static void main( String[] args )
{
Foo foo = new Foo();
System.out.print( "foo.m1( 1 ): " );
foo.m1( 1 );
System.out.print( "foo.m1( -1 ): " );
foo.m1( -1 );
}
}
If the assertions are disabled the output is foo.m1(1) : OK foo.m1( -1) : OK
If the assertions are enabled the output is
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foo.m1(1) : OK foo.m1( -1)
(Runtime Exception – AssertionError)
Summary
An exception is an abnormal condition arised during the program execution that disrupts the normal flow of program and terminates the application. There are built-in api exception classes in java.lang. You can also define your own exception class. All exception classes are subclasses of Throwable class. There are two types of exception types: checked and runtime exceptions.
Questions
1. class A {A() throws Exception {}} // 1
class B extends A {B() throws Exception {}} // 2
class C extends A {} // 3
Which of the following statements is true? a. Compile-time error at 1.
b. Compile-time error at 2.
c. Compile-time error at 3.
d. None of the above
2. class A {
public static void main (String[] args) {
Error error = new Error();
Exception exception = new Exception();
System.out.print((exception instanceof Throwable) + ",");
System.out.print(error instanceof Throwable);
}
}
What is the result of attempting to compile and run the program? a. Prints: false,false b. Prints: false,true c. Prints: true,false d. Prints: true,true e. Compile-time error f. Run-time error g. None of the above
3. class Level1Exception extends Exception {}
class Level2Exception extends Level1Exception {}
class Level3Exception extends Level2Exception {}
class Purple { public static void main(String args[]) { int a,b,c,d,f,g,x; a = b = c = d = f = g = 0; x = 1; try {
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}
} try { switch (x) { case 1: throw new Level1Exception(); case 2: throw new Level2Exception(); case 3: throw new Level3Exception();
} a++;
} catch (Level2Exception e) {b++;} finally {c++;}
} catch (Level1Exception e) { d++;} catch (Exception e) {f++;} finally {g++;}
System.out.print(a+","+b+","+c+","+d+","+f+","+g);
What is the result of attempting to compile and run the program? a. Prints: 0,0,0,1,0,0 b. Prints: 0,0,1,1,0,1 d. Prints: 1,0,1,1,0,1 e. Prints: 1,1,1,1,0,1 g. Run-time error h. None of the above c. Prints: 0,1,1,1,0,1 f. Compile-time error
4. class A { void m1(int i) { int j = i % 3; switch (j) { case 0: System.out.print("0"); break; case 1: System.out.print("1"); break; default: assert j == 2;
System.out.print(j);
}
} public static void main (String[] args) {
A a = new A(); for (int i=5; i >= -1; i--) {a.m1(i);}
}
}
Which statements are true? a. With assertions enabled it prints 210210-1 followed by an AssertionError message.
b. With assertions enabled it prints 210210 followed by an AssertionError message.
c. With assertions enabled it prints only 210210.
d. With assertions enabled it prints nothing.
e. With assertions disabled it prints 210210-1 f. With assertions disabled it prints only 210210 g. Assertions should not be used within the default case of a switch statement.
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Chapter 9 : Multithreaded programming
9.1 Multitasking
Multitasking is performing two or more tasks at the same time. Nearly all operating systems are capable of multitasking by using one of two multitasking techniques: process-based multitasking and thread-based multitasking.
Process-based multitasking is running two programs concurrently. Programmers refer to a program as a process. Therefore, you could say that process-based multitasking is program-based multitasking.
Thread-based multitasking is having a program perform two tasks at the same time. For example, a word processing program can check the spelling of words in a document while you write the document. This is thread-based multitasking.
A good way to remember the difference between process-based multitasking and thread-based multitasking is to think of process-based as working with multiple programs and thread-based as working with parts of one program.
The objective of multitasking is to utilize the idle time of the CPU. Think of the CPU as the engine of your car. Your engine keeps running regardless of whether the car is moving. Your objective is to keep your car moving as much as possible so you can get the most miles from a gallon of gas. An idling engine wastes gas.
The same concept applies to the CPU in your computer. You want your CPU cycles to be processing instructions and data rather than waiting for something to process. A CPU cycle is somewhat similar to your engine running.
Process-based multitasking has a larger overhead than thread-based multitasking. In processbased multitasking, each process requires its own address space in memory. The operating system requires a significant amount of CPU time to switch from one process to another process.
Programmers call this context switching, where each process (program) is a context. Additional resources are needed for each process to communicate with each other.
In comparison, the threads in thread-based multitasking share the same address space in memory because they share the same program. This also has an impact on context switching, because switching from one part of the program to another happens within the same address space in memory. Likewise, communication among parts of the program happens within the same memory location.
9.2 What Is a Thread?
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All programmers are familiar with writing sequential programs. You've probably written a program that displays "Hello World!". This is a sequential program. That is, each has a beginning, an execution sequence, and an end. At any given time during the runtime of the program, there is a single point of execution.
A single thread also has a beginning, a sequence, and an end and at any given time during the runtime of the thread, there is a single point of execution. However, a thread itself is not a program; it cannot run on its own. Rather, it runs within a program. The following figure shows this relationship.
Definition: A thread is a single sequential flow of control within a program.
A thread is part of a program that is running. Thread-based multitasking has multiple threads running at the same time (that is, multiple parts of a program running concurrently). Each thread is a different path of execution. This is illustrated by the following figure:
The Java run-time environment manages threads, unlike in process-based multitasking where the operating system manages switching between programs. Threads are processed asynchronously.
This means that one thread can pause while other threads continue to process.
The HotJava Web browser is an example of a multithreaded application. Within the HotJava browser you can scroll a page while it's downloading an applet or image, play animation and sound concurrently, print a page in the background while you download a new page, or watch three sorting algorithms race to the finish. You are used to life operating in a concurrent fashion...so why not your browser?
Some texts use the name lightweight process instead of thread. A thread is similar to a real process in that a thread and a running program are both a single sequential flow of control.
However, a thread is considered lightweight because it runs within the context of a full-blown program and takes advantage of the resources allocated for that program and the program's environment.
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As a sequential flow of control, a thread must carve out some of its own resources within a running program. (It must have its own execution stack and program counter for example.) The code running within the thread works only within that context. Thus, some other texts use
execution context as a synonym for thread.
Basic support for threads in the Java platform is in the class java.lang.Thread. It provides a thread API and provides all the generic behavior for threads. (The actual implementation of concurrent operations is system-specific. For most programming needs, the underlying implementation doesn’t matter.) These behaviors include starting, sleeping, running, yielding, and having a priority.
To implement a thread using the
Thread
class, you need to provide it with a run
method that performs the thread's task.
There are two ways to provide the run
method.
•
Subclass the
Thread
class and override the run
method.
•
Provide a class that implements the
Runnable
interface and therefore implements the run method. In this case, a
Runnable
object provides the run
method to the thread.
Daemon threads
A daemon thread is a thread that runs continuously to perform a service, without having any connection with the overall state of the program. For example, the thread that runs the garbage collector in Java is a daemon thread. The thread that processes mouse events for a Java program is also a daemon thread. In general, threads that run application code are not daemon threads, and threads that run system code are daemon threads. If a thread dies and there are no other threads except daemon threads alive, the Java virtual machine stops.
A
Thread
object has a boolean
attribute that specifies whether or not a thread is a daemon thread. The daemon attribute of a thread is set when the
Thread
object is created, by passing an argument to the constructor that creates the
Thread
object. If the daemon attribute is not explicitly specified, the
Thread
inherits the daemon attribute of its parent
Thread
object.
The daemon attribute is queried using the isDaemon()
method; it is set using the setDaemon() method.
Controlling groups of threads
Sometimes it is necessary to control multiple threads at the same time. Java provides the
ThreadGroup
class for this purpose. Every
Thread
object belongs to a
ThreadGroup
object. By passing an argument to the constructor that creates the
Thread
object, the
ThreadGroup
of a thread can be set when the
Thread
object is created. If an explicit
ThreadGroup
is not specified, the
Thread
belongs to the same
ThreadGroup
as its parent
Thread
object.
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9.3 The Thread class
The class java.lang.Thread is used to create and control threads. To create a thread, a new instance of this class must be created. However, the thread does not start running right away.
Thread.start() must be called to actually make the thread run. When Thread.start() is called, the thread begins executing in the run() method of the target class. A new Thread class always starts running the public void run() method of a class.
The
Thread
class has seven constructors. All of them create a new thread. The thread does not start running until
Thread.start()
is called. When
Thread.start()
is called, the new thread starts running in the run()
method of an object. The constructors are the following:
Thread()
Thread(Runnable)
Thread(ThreadGroup)
Thread(String)
Thread(ThreadGroup,String)
Thread(Runnable,String)
Thread(ThreadGroup,Runnable,String)
The constructors can use three possible parameters:
•
String
The name of the new thread is the parameter
String
. A thread can get its name by calling
Thread.getName()
.
•
ThreadGroup
The new thread will belong to the group specified by the parameter
ThreadGroup
. A
ThreadGroup
can be used to organize a thread.
•
Runnable
The
Runnable
parameter is an object that has implemented the
Runnable interface. The thread will start executing in the run()
method of the
Runnable
parameter when
Thread.start()
has been called.
There are many methods in the
Thread
class.
Some of the methods that control the thread execution are the following:
• start()
This method starts the thread. It starts executing in the run()
method of its
Runnable
target that was set when the constructor was called. This method can be called only once.
• suspend()
This method suspends the execution of the thread. It remains suspended until resume()
is called.
• resume()
This method resumes the execution of a suspended thread. It has no effect on a thread that is not suspended.
• stop()
This method stops and kills a running thread. Currently, the thread does not stop unless it is running. If it is suspended, it does not die until it starts running again. This is a deprecated method.
• sleep(int m)/sleep(int m,int n)
The thread sleeps for
m
milliseconds, plus
n
nanoseconds.
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9.4 Using the main thread
When a Java program starts up, one thread begins running immediately. This is usually called the
main thread of your program, because it is the one that is executed when your program begins.
The main thread is important because :
It is the thread from which other “child” threads will be spawned.
Often it must be the last thread to finish execution because it performs various shutdown actions.
Although the main thread is created automatically when your program is started, it can be controlled through a Thread object. To do so you must obtain the reference to it by calling the method currentThread(), which is a public static member of Thread. Its general form is shown below: static Thread currentThread()
This method returns a reference to the thread in which it is called. Once, you have a reference to the main thread, you can control it just like any other thread.
Example,
//controlling the main thread class CurrentThreadDemo { public static void main(String args[]) {
Thread t = Thread.currentThread();
System.out.println(“Current thread :“+ t);
// change the name of the thread t.setName(“My thread”);
System.out.println(“After name change :” +t); try { for(int n = 5; n > 0; n -- ) {
System.out.println(n);
Thread.sleep(1000);
}
} catch(InterruptedException e) {
System.out.println(“Main thread interrupted”);
}
}
}
The output of the program is as follows :
5
4
Current thread : Thread[main,5,main]
After name change : Thread[My Thread, 5,main]
3
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2
1
When you try to print t, it displays the name of the thread, its priority and the name of its group.
By default, the name of the main thread is main. Its priority is 5, which is the default value, and
main is also the name of the group of threads to which this thread belongs. A thread group is a data structure that controls the state of a collection of threads as a whole. This process is managed by particular runtime environment.
The sleep method causes the thread from which it is called to suspend execution for the specific period of milliseconds. Its general form is static void sleep(long milliseconds) throws InterruptedException static void sleep(long milliseconds, int nanoseconds) throws InterruptedException
You can set the name of the thread by using setName(). You can obtain the name of the thread by calling getName(). The syntax is given below: final void setName(String threadName) final String getName()
9.5 Creating a thread
Subclassing Thread and Overriding run
The first way to customize a thread is to subclass Thread (itself a Runnable object) and override its empty run method so that it does something. Let's look at the SimpleThread class, the first of two classes in this example, which does just that: public class SimpleThread extends Thread {
public SimpleThread(String str) {
super(str);
}
public void run() {
for (int i = 0; i < 10; i++) {
System.out.println(i + " " + getName());
try {
sleep((long)(Math.random() * 1000));
} catch (InterruptedException e) {}
}
System.out.println("DONE! " + getName());
}
}
The first method in the SimpleThread class is a constructor that takes a String as its only argument. This constructor is implemented by calling a superclass constructor and is interesting to us only because it sets the Thread's name, which is used later in the program.
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The next method in the SimpleThread class is the run method. The run method is the heart of any
Thread and where the action of the Thread takes place. The run method of the SimpleThread class contains a for loop that iterates ten times. In each iteration the method displays the iteration number and the name of the Thread, then sleeps for a random interval of up to 1 second. After the loop has finished, the run method prints DONE! along with the name of the thread. That's it for the SimpleThread class. Let’s put it to use in TwoThreadsTest.
The TwoThreadsTest class provides a main method that creates two SimpleThread threads:
Jamaica and Fiji. public class TwoThreadsTest {
public static void main (String[] args) {
new SimpleThread("Jamaica").start();
new SimpleThread("Fiji").start();
}
}
The main method starts each thread immediately following its construction by calling the start method, which in turn calls the run method. Compile and run the program. You should see output similar to this:
Note how the output from each thread is intermingled with the output from the other. The reason is that both
SimpleThread
threads are running concurrently. So both run
methods are running, and both threads are displaying their output at the same time. When the loop completes, the thread stops running and dies.
Implementing Runnable interface
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The Runnable interface is a built-in interface in java.lang package and has only one method in it
– that is public void run()
When you are implementing the Runnable interface you have to override the run method
//Implementing Runnable public class ThreadedClass implements Runnable
{
int data;
public ThreadedClass(){
data = 0;
}
public void run(){
//this method runs when start() is invoked on the thread
System.out.println( ++data );
}
public Thread getNewThread(){
Thread t = new Thread( this );
return t;
}
public static void main( String[] args ){
ThreadedClass threadedClass = new ThreadedClass();
Thread t1 = threadedClass.getNewThread();
t1.start();
Thread t2 = threadedClass.getNewThread();
t2.start();
}
}
In the main()
method of the code above, only one instance of
ThreadedClass
is created. This one instance is used to spawn a multitude of threads (each executing the run() method of the same object).
9.6 The Java Thread Model
Java uses threads to enable the entire environment to be asynchronous. This helps reduce inefficiency by preventing the waste of CPU cycles.
Single-threaded systems use an approach called an event loop, with polling. In this model, a single thread of control runs in an infinite loop, polling a single event queue to decide what to do next. Once this polling mechanism returns with, say, a signal that a network file is ready to be read, then the event loop dispatches the control to the appropriate event handler. Until this event handler returns, nothing else can happen in the system. This wastes CPU time. It can also result
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in one program dominating the system and preventing any other events from being processed. In single-threaded environment, when a thread blocks because it is waiting for some resource, the entire program stops running.
The benefit of Java’s multithreading is that the main loop/polling mechanism is eliminated. One thread can pause without stopping other parts of your program.
The states of a Thread
A thread can be in one of these states:
•
Running A thread is said to be in running state when it is being executed. This thread
has access to CPU.
•
Suspended Execution is paused and can be resumed where it left off.
•
Resumed. A suspended thread is started.
•
Blocked A resource cannot be accessed because it is being used by another thread.
•
Terminated or Dead State A thread reaches "dead" state when the run method has
finished execution. This thread cannot be executed now.
•
Ready State A thread in this state is ready for execution, but is not being currently
executed. Once a thread in the ready state gets access to the CPU, it gets converted to running state.
A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object’s lock, or by invoking an object’s wait() method.
It can also enter the waiting state by invoking its (deprecated) suspend() method.
A CPU intensive operation being executed may not allow other threads to be executed for a
"large" period of time. To prevent this it can allow other threads to execute by invoking the yield() method. The thread on which yield() is invoked would move from running state to ready state.
The Life cycle of a thread
The following figure shows the states that a thread can be in during its life and illustrates which method calls cause a transition to another state. A simple diagram is shown below:
A more detailed diagram follows:
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Testing Thread State
Release 5.0 introduced the Thread.getState method. When called on a thread, one of the following Thread.State values is returned:
•
•
•
•
•
•
NEW
RUNNABLE
BLOCKED
WAITING
TIMED_WAITING
TERMINATED
The API for the Thread class also includes a method called isAlive. The isAlive method returns true if the thread has been started and not stopped. If the isAlive method returns false, you know that the thread either is a New Thread or is Dead. If the isAlive method returns true, you know that the thread is either Runnable or Not Runnable.
Prior to release 5.0, you couldn't differentiate between a New Thread or a Dead thread. Nor could you differentiate between a Runnable thread and a Not Runnable thread.
9.7 Thread priority
A thread's priority is specified with an integer from 1 (the lowest) to 10 (the highest), Constants
Thread.MIN_PRIORITY and Thread.MAX_PRIORITY can also be used. By default, the setPriority() method sets the thread priority to 5, which is the Thread.NORM_PRIORITY.
Thread aThread = Thread.currentThread(); int currentPriority; currentPriority = aThread.getPriority();
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aThread.setPriority( currentPriority + 1 );
Setting priorities may not always have the desired effect because prioritization schemes may be implemented differently on different platforms. However, if you cannot resist messing with priorities, use higher priorities for threads that frequently block (sleeping or waiting for I/O). Use medium to low-priority for CPU-intensive threads to avoid hogging the processor down.
class Clicker implements Runnable { int click = 0;
Thread t; private volatile boolean running = true; public Clicker(int p) { t = new Thread(this); t.setPriority(p);
} public void run () { while (running) { click++;
}
} public void stop() { running = false;
} public void start () { t.start();
}
} class HiLoPri { public static void main (String args [ ]) {
Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
Clicker hi = new Clicker (Thread.NORM_PRIORITY + 2);
Clicker lo = new Clicker (Thread.NORM_PRIORITY - 2); lo.start(); hi.start(); try {
Thread.sleep(10000);
} catch (InterruptedException e) {
System.out.println("Main Thread Interrupted");
} lo.stop(); hi.stop(); try { hi.t.join(); lo.t.join();
}
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catch (InterruptedException e) {
System.out.println("InterruptedException Caught");
}
System.out.println("Low-Priority Thread:" + lo.click);
System.out.println("High-Priority Thread:" + hi.click);
}
}
The output of this program, shown as follows when run under Windows 2000, indicates that the threads did context switch even though neither voluntarily yielded the CPU nor blocked for I/O.
The higher priority thread got approx. 90% of the CPU time.
Low-Priority Thread:4408112
High-Priority Thread:589626904
The output depends on the speed of your CPU and the number of other tasks running in the system.
Volatile ensures that the value of running is examined each time the following loop iterates: while(running) { click++;
}
9.8 Using the Thread yield method.
Because of the platform dependent nature of Java threading you cannot be certain if a thread will ever give up its use of CPU resources to other threads. On some operating systems the threading algorithm may automatically give different threads a share of the CPU time, on others one thread might simply hog processor resources. For this reason the Java Thread class has a static method called yield, which causes the currently running thread to yield its hold on CPU cycles. This thread returns to the "ready to run" state and the thread scheduling system has a chance to give other threads the attention of the CPU. If no other threads are in a "ready to run state" the thread that was executing may restart running again.
Example of using yield() method public class TestRunner implements Runnable
{
public void run()
{
System.out.println(Thread.currentThread().getName() +
" In run");
Thread.yield();
System.out.println(Thread.currentThread().getName() +
" Leaving run");
}
}
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public class TestYield
{
public static void main (String[] args)
{
TestRunner r1 = new TestRunner();
Thread t2 = new Thread(r1);
t2.setName("BlahBlah");
Thread t1 = new Thread(r1);
t1.setName("TestThreadHaah");
t1.start();
t2.start();
}
}
9.9 Stopping a Thread
suspend()
and stop()
methods of Thread provide asynchronous methods of stopping a thread.
However, these methods have been deprecated because they are very unsafe. Using them often results in deadlocks and incorrect resource cleanup.
Instead of using stop, a thread should arrange for its own death by having a run method that terminates naturally. For example, the while loop in this run method is a finite loop: It will iterate
100 times and then exit: public void run() {
int i = 0;
while (i < 100) {
i++;
System.out.println("i = " + i);
}
}
A thread with this run method dies naturally when the loop completes and the run method exits.
If you don’t want to use the deprecated method you can set a variable that the thread checks occasionally. When the thread detects that the variable is set, it should return from the run() method.
class MyThread extends Thread {
boolean allDone = false;
// This method is called when the thread runs
public void run() { try { for(int i=1; i<= 10;i++) { sleep(500); if(!allDone)
System.out.println(i); else return;
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}
} catch(InterruptedException e) {
System.out.println(e);
}
}
} class ThreadStopDemo { public static void main(String args[]) {
// Create and start the thread
MyThread thread = new MyThread(); thread.start(); try {
Thread.sleep(2000);
} catch(InterruptedException e) {
System.out.println(e);
}
// Stop the thread thread.allDone = true;
}
}
Why is stop deprecated?
Because it is inherently unsafe. Stopping a thread causes it to unlock all the monitors that it has locked. (The monitors are unlocked as the ThreadDeath exception propagates up the stack.) If any of the objects previously protected by these monitors were in an inconsistent state, other threads may now view these objects in an inconsistent state. Such objects are said to be damaged.
When threads operate on damaged objects, arbitrary behavior can result. This behavior may be subtle and difficult to detect, or it may be pronounced. Unlike other unchecked exceptions,
ThreadDeath kills threads silently; thus, the user has no warning that his program may be corrupted. The corruption can manifest itself at any time after the actual damage occurs, even hours or days in the future.
9.10 Determining When a Thread Has Finished
Typically, the main thread is the last thread to finish in a program. However, there isn’t any guarantee that the main thread won’t finish before a child thread finishes.
Programmers use two other techniques to ensure that the main thread is the last thread to terminate. These techniques involve calling the isAlive() method and the join() method. Both of these methods are defined in the Thread class.
The isAlive() method determines whether a thread is still running. If it is, the isAlive() method returns a boolean true value; otherwise, a boolean false is returned. You can use the isAlive() method to examine whether a child thread continues to run. The join() method works differently than the isAlive() method. The join() method waits until the child thread terminates and “joins”
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the main thread. In addition, you can use the join() method to specify the amount of time you want to wait for a child thread to terminate.
class ThreadDemo { public static void main(String args[]) {
// Create and start a thread
MyThread thread = new MyThread(); thread.start();
// Check if the thread has finished in a non-blocking way if (thread.isAlive()) {
System.out.println(“Thread has not finished”);
} else {
System.out.println(“Finished”);
}
// Wait for the thread to finish try { thread.join();
} catch (InterruptedException e) {
System.out.println(“Thread was interrupted”);
} if (thread.isAlive()) {
System.out.println(“Thread has not finished”);
} else {
System.out.println(“Finished”);
}
}
}
9.11 Thread Scheduling
Many computer configurations have a single CPU. Hence, threads run one at a time in such a way as to provide an illusion of concurrency. Execution of multiple threads on a single CPU in some order is called scheduling. The Java runtime environment supports a very simple, deterministic scheduling algorithm called fixed-priority scheduling. This algorithm schedules threads on the basis of their priority relative to other Runnable threads.
Thread scheduling is implementation dependent and cannot be relied on to act the same way on every JVM
When a thread is created, it inherits its priority from the thread that created it. At any given time, when multiple threads are ready to be executed, the runtime system chooses for execution the
Runnable thread that has the highest priority. Only when that thread stops, yields, or becomes
Not Runnable will a lower-priority thread start executing. If two threads of the same priority are
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waiting for the CPU, the scheduler arbitrarily chooses one of them to run. The chosen thread runs until one of the following conditions is true:
•
•
•
A higher priority thread becomes runnable.
It yields, or its run
method exits.
On systems that support time-slicing, its time allotment has expired.
Then the second thread is given a chance to run, and so on, until the interpreter exits.
The Java runtime system's thread scheduling algorithm is also preemptive. If at any time a thread with a higher priority than all other Runnable threads becomes Runnable, the runtime system chooses the new higher-priority thread for execution. The new thread is said to preempt the other threads.
Rule of thumb: At any given time, the highest priority thread is running. However, this is not
guaranteed. The thread scheduler may choose to run a lower priority thread to avoid starvation.
For this reason, use thread priority only to affect scheduling policy for efficiency purposes. Do not rely on it for algorithm correctness.
Time slicing/preemptive
Each thread gets a set amount of CPU time for executing. Once it has used up its time with the
CPU, it is removed from accessing the CPU and any other waiting Threads get a chance at CPU time. When each thread has had its chance with the CPU the cycle starts again. The beauty of this approach is that you can be confident that each thread will get at least some time executing.
In a pre-emptive system one program can "pre-empt" another to get its share of CPU time. In a time sliced system each thread gets a "slice" of the CPU time and then gets moved to the ready state. This ensures against a single thread getting all of the CPU time. The downside is that you cannot be certain how long a Thread might execute or even when it will be running. Although
Java defines priorities for threads from the lowest at 1 to the highest at 10, some platforms will accurately recognise these priorities whereas others will not.
The notify method will wake up one thread waiting to reacquire the monitor for the object. You cannot be certain which thread gets woken. If you have only one waiting thread then you do not have a problem. If you have multiple waiting threads then it will be probably the thread that has been waiting the longest that will wake up. However you cannot be certain, and the priorities of the threads will influence the result. As a result you are generally advised to use notifyAll instead of notify, and not to make assumptions about scheduling or priorities. Of course this is not always possible and you may have to try to test your code on as many platforms as possible.
Non time slicing/Cooperative
A priority system is used to decide which thread will run. A thread with the highest priority gets time with the CPU. A program under this system needs to be created in such a way that it
"voluntarily" yield access to the CPU.
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Relinquishing the CPU
As you can imagine, writing CPU-intensive code can have negative repercussions on other threads running in the same process. In general, try to write well-behaved threads that voluntarily relinquish the CPU periodically and give other threads an opportunity to run.
A thread can voluntarily yield the CPU by calling the yield method. The yield method gives other threads of the same priority a chance to run. If no equal-priority threads are Runnable, the yield is ignored.
9.12 Thread Synchronization
So far the examples in this chapter have contained independent, asynchronous threads. Each thread contained all the data and methods required for its execution and didn’t require any outside resources or methods. Also, the threads in those examples ran at their own pace without concern for the state or activities of any other concurrently running threads.
A method in a Java object is said to be thread safe if it can be safely run in a multithreaded environment. To achieve this safety, there must be a mechanism by which multiple threads running the same method can synchronize their operations, such that only one of them is allowed to proceed when accessing the same object or lines of code. This synchronization requires the threads to communicate with each other using objects called semaphores.
One specific type of semaphore is called a mutual exclusion semaphore or a mutex. As the name indicates, ownership of this semaphore object is mutually exclusive, in that only one thread can own the mutex at any given time. Any other thread that tries to acquire ownership will be blocked and must wait until the owning thread releases the mutex. If multiple threads are waiting in line for the same mutex, only one of them will get it when it is released by the current owner; the others will continue to block.
In the early 1970s, C.A.R. Hoare and others developed a concept known as a monitor. A monitor is a body of code whose access is guarded by a mutex. Any thread wishing to execute this code must acquire the associated mutex at the top of the code block and release it at the bottom.
Because only one thread can own a mutex at a given time, this effectively ensures that only the owing thread can execute a monitor block of code. (The guarded code need not be contiguous -- for example, every object in the Java language has a single monitor associated with it.)
In many interesting situations, separate, concurrently running threads share data and must consider the state and activities of other threads. In one such set of programming situations, called producer-consumer scenarios, the producer generates a stream of data that a consumer uses.
For example, imagine an application in which one thread (the producer) writes data to a file while a second thread (the consumer) reads data from the same file. Or, as you type characters on the keyboard, the producer thread places mouse events in an event queue and the consumer thread reads the events from the same queue. Both of these examples use concurrent threads that
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share a common resource: The first shares a file, and the second shares an event queue. Because the threads share a common resource, they must be synchronized.
The synchronized keyword
The synchronized keyword can be used to mark a statement or block of code so that only one thread may execute an instance of the code at a time. Entry to the code is protected by a monitor lock around it. This process is implemented by a system of locks. You may also see the words monitor, or mutex (mutually exclusive lock) used. A lock is assigned to the object and ensures only one thread at a time can access the code. Thus when a thread starts to execute a synchronized block it grabs the lock on it. Any other thread will not be able to execute the code until the first thread has finished and released the lock. Note that the lock is based on the object and not on the method.
For a method the synchronized keyword is placed before the method thus synchronized void amethod() { /* method body */}
For a block of code the synchronized keyword comes before opening and closing brackets thus.
synchronized (Object Reference) { /* Block body */ }
The value in parentheses indicates the object or class whose monitor the code needs to obtain. It is generally more common to synchronize the whole method rather than a block of code.
When a synchronized block is executed, its object is locked and it cannot be called by any other code until the lock is freed.
Example
//This program is not synchronized class Callme { void call(String msg) {
System.out.print(“[“ + msg); try {
Thread.sleep(1000);
} catch(InterruptedException e) {
System.out.println(“Interrupted”);
}
System.out.println(“]”);
}
} class Caller implements Runnable {
String msg;
Callme target;
Thread t;
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public Caller(Callme targ, String s) { target = targ; msg = s; t =new Thread(this); t.start();
} public void run() { target.call(msg);
}
} class Synch { public static void main(String args[]) {
Callme target = new Callme();
Caller obj1 = new Caller(target, “Hello”);
Caller obj2 = new Caller(target, “Synchronized”); try { obj1.t.join(); obj2.t.join();
} catch(InterruptedException e) {
System.out.println(“Interrupted”);
}
}
}
Output is :
[Hello[Synchronized
]
]
Same example using synchronization:
In the code above just add the keyword synchronized in front of the call method in the Callme class.
synchronized void call(String msg)
This prevents other thread from entering call() while another thread is using it. The output of the program is
[Hello]
[Synchronized]
If the Callme class is a third party class then you cannot change its code. In that case you can use the synchronized block in the run method as follows: public void run() {
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synchronized(target) { target.call(msg);
}
}
Only the run method is changed. The rest of the code is same.
The Producer/Consumer Example
In this example, the Producer generates an integer between 0 and 9 (inclusive), stores it in a
CubbyHole object. To make the synchronization problem more interesting, the
Producer
sleeps for a random amount of time between 0 and 100 milliseconds before repeating the numbergenerating cycle: public class CubbyHole {
private int contents;
public int get() {
return contents;
}
public void put(int value) {
contents = value;
}
} public class Producer extends Thread {
private CubbyHole cubbyhole;
private int number;
public Producer(CubbyHole c, int number) {
cubbyhole = c;
this.number = number;
}
public void run() { for (int i = 0; i < 10; i++) {
cubbyhole.put(number, i);
try { sleep((int)(Math.random() * 100));
} catch (InterruptedException e) { }
}
}
}
The Consumer consumes all integers from the CubbyHole (the exact same object into which the
Producer put the integers in the first place) as quickly as they become available. public class Consumer extends Thread {
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private CubbyHole cubbyhole;
private int number;
public Consumer(CubbyHole c, int number) {
cubbyhole = c;
this.number = number;
}
public void run() {
int value = 0;
for (int i = 0; i < 10; i++) {
value = cubbyhole.get(number);
}
}
}
Producer and Consumer example share data through a common CubbyHole object. Although
Consumer ideally will get each value produced once and only once, neither
Producer
nor
Consumer
makes any effort whatsoever to ensure that happens. The synchronization between these two threads occurs at a lower level, within the get
and put
methods of the
CubbyHole object. However, assume for a moment that these two threads make no arrangements for synchronization, and let’s discuss the potential problems that might arise from this.
One problem arises when the Producer is quicker than the Consumer and generates two numbers before the Consumer has a chance to consume the first one. In this situation, the Consumer misses a number. Part of the output might look like this:
Another problem might arise when the
Consumer
is quicker than the
Producer
and consumes the same value twice. In this situation, the
Consumer
might produce output that looks like this:
Either way, the result is wrong because the Consumer should get each integer produced by the
Producer exactly once. A problem such as this is called a race condition. A race condition is a situation in which two or more threads or processes are reading or writing some shared data, and the final result depends on the timing of how the threads are scheduled. Race conditions can lead to unpredictable results and subtle program bugs. Race conditions in the producer-consumer example are prevented by having the storage of a new integer into the CubbyHole by the
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Producer be synchronized with the retrieval of an integer from the CubbyHole by the Consumer.
The activities of the Producer and the Consumer must be synchronized in two ways.
First, the two threads must not simultaneously access the CubbyHole. A thread can prevent this from happening by locking an object. When an object is locked by one thread and another thread tries to call a synchronized method on the same object, the second thread will block until the object is unlocked.
Second, the two threads must do some simple coordination. That is, the Producer must have a way to indicate to the Consumer that the value is ready, and the Consumer must have a way to indicate that the value has been retrieved. The Object class provides a collection of methods — wait, notify, and notifyAll — to help threads wait for a condition and notify other threads when that condition changes.
Locking an Object
Within a program, the code segments that access the same object from separate, concurrent threads are called critical sections. A critical section can be a block or a method and is identified with the synchronized
keyword. The Java platform associates a lock with every object and the lock is acquired upon entering a critical section.
In the producer-consumer example, the put and get methods of CubbyHole.java are the critical sections. The Consumer should not access the CubbyHole when the Producer is changing it, and the Producer should not modify it when the Consumer is getting the value. So put and get in the
CubbyHole class should be marked with the synchronized keyword.
Here’s a code skeleton for the CubbyHole class: public class CubbyHole {
private int contents;
private boolean available = false;
public synchronized int get(int who) {
...
}
public synchronized void put(int who, int value) {
...
}
}
The method declarations for both put
and get
contain the synchronized
keyword. Whenever control enters a synchronized method, the thread that called the method locks the object whose method has been called. Other threads cannot call a synchronized method on the same object until the object is unlocked.
Thus, when it calls CubbyHole's put method, The Producer locks the CubbyHole, thereby preventing the Consumer from calling the CubbyHole's get method:
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public synchronized void put(int value) {
//CubbyHole locked by the Producer
...
//CubbyHole unlocked by the Producer
}
When the put method returns, the Producer unlocks the CubbyHole. Similarly, when the
Consumer calls CubbyHole's get method, it locks the CubbyHole, thereby preventing the
Producer from calling put: public synchronized int get() {
// CubbyHole locked by the Consumer
...
// CubbyHole unlocked by the Consumer
}
The acquisition and release of a lock is done automatically and atomically by the Java run-time system. This ensures that race conditions cannot occur in the underlying implementation of the threads, thus ensuring data integrity.
Synchronization isn't the whole story. The two threads must also be able to notify one another when they've done their job.
Reaquiring a Lock
The same thread can call a synchronized method on an object for which it already holds the lock, thereby reacquiring the lock. The Java runtime environment allows a thread to reacquire a lock because the locks are reentrant. Reentrant locks are important because they eliminate the possibility of a single thread’s waiting for a lock that it already holds.
Consider this class: public class Reentrant {
public synchronized void a() { b();
System.out.println("here I am, in a()");
}
public synchronized void b() {
System.out.println("here I am, in b()");
}
}
Reentrant
contains two synchronized methods: a
and b
. The first, a
, calls the other, b
. When control enters method a
, the current thread acquires the lock for the
Reentrant
object. Now, a calls b
; because b
is also synchronized, the thread attempts to acquire the same lock again.
Because the Java platform supports reentrant locks, this works. In platforms that don’t support reentrant locks, this sequence of method calls causes deadlock. The current thread can acquire
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the
Reentrant
object's lock again, and both a
and b
execute to conclusion, as is evidenced by the output: here I am, in b() here I am, in a()
9.13 Interthread Communication
The construct of wait/notify plays an important role in the Java language's interthread communication mechanism. The essential idea is that one thread needs a certain condition that can be brought about by another thread to become true. It therefore waits for the condition to be met. Once the condition is true, the causing thread notifies the waiting thread to wake up and proceed from where it left off.
wait and notify should be placed within synchronized code to ensure that the current code owns the monitor
Using the notifyAll and wait Methods in the Producer/ Consumer Example
Let's investigate how the code in CubbyHole's put and get methods helps the Producer and the
Consumer coordinate their activities. The CubbyHole stores its value in a private member variable called contents. CubbyHole has another private member variable, available, that is a boolean. The available variable is true when the value has been put but not yet gotten and is false when the value has been gotten but not yet put. Here's one possible implementation for the put and get methods: public synchronized int get() { //won't work!
if (available == true) { available = false; return contents;
}
} public synchronized void put(int value) {
//won't work!
if (available == false) { available = true; contents = value;
}
}
As implemented, these two methods won't work. Look at the get method. What happens if the
Producer hasn't put anything in the CubbyHole and available isn't true? The get method does nothing. Similarly, if the Producer calls put before the Consumer got the value, put doesn't do anything.
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You really want the Consumer to wait until the Producer puts something in the CubbyHole and the Producer to notify the Consumer when it's done so. Similarly, the Producer should wait until the Consumer takes a value (and notifies the Producer of its activities) before replacing it with a new value. The two threads must coordinate more fully and can use Object's wait and notifyAll methods to do so.
Here are the new get and put implementations that wait on and notify each other of their activities: class CubbyHole { boolean available; int contents; public synchronized int get() { while (available == false) { try {
//wait for Producer to put value wait();
} catch (InterruptedException e) { }
} available = false;
//notify Producer that value has been retrieved notifyAll(); return contents;
} public synchronized void put(int value) { while (available == true) { try {
//wait for Consumer to get value wait();
} catch (InterruptedException e) {
}
} contents = value; available = true;
//notify Consumer that value has been set notifyAll();
}
}
The code in the get method loops until the Producer has produced a new value. Each time through the loop, get calls the wait method. The wait method relinquishes the lock held by the
Consumer on the CubbyHole (thereby allowing the Producer to get the lock and update the
CubbyHole) and then waits for notification from the Producer. When Producer puts something in the CubbyHole, it notifies Consumer by calling notifyAll. The Consumer then comes out of the wait state and the get method returns the value in the CubbyHole.
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The put method works in a similar fashion. It waits for the Consumer thread to consume the current value before allowing the Producer to produce a new one.
The notifyAll method wakes up all threads waiting on the object in question (in this case, the
CubbyHole). The awakened threads compete for the lock. One thread gets it, and the others go back to waiting. The Object class also defines the notify method, which arbitrarily wakes up one of the threads waiting on this object.
There are the three versions of the wait method contained in the Object class:
wait()
Waits indefinitely for notification
wait(long timeout)
Waits for notification or until the
timeout
period has elapsed. timeout
is measured in milliseconds.
wait(long timeout, int nanos)
Waits for notification or until
timeout
milliseconds plus nanos
nanoseconds have elapsed.
Note: Besides using these timed
wait
methods to synchronize threads, you also can use them in place of sleep
. Both wait
and sleep
delay for the requested amount of time. You can easily wake up wait
with a notify
but a sleeping thread cannot be awakened prematurely. This doesn't matter too much for threads that don't sleep for long, but it could be important for threads that sleep for minutes at a time.
Running the Producer-Consumer Example
Here’s a small standalone application, called ProducerConsumerTest, that creates a CubbyHole object, a Producer, and a Consumer and then starts both the Producer and the Consumer: public class ProducerConsumerTest {
public static void main(String[] args) {
CubbyHole c = new CubbyHole();
Producer p1 = new Producer(c, 1);
Consumer c1 = new Consumer(c, 1);
p1.start();
c1.start();
}
}
Here’s the output of
ProducerConsumerTest
:
Producer #1 put: 0
Consumer #1 got: 0
Producer #1 put: 1
Consumer #1 got: 1
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Producer #1 put: 2
Consumer #1 got: 2
Producer #1 put: 3
Consumer #1 got: 3
Producer #1 put: 4
Consumer #1 got: 4
Producer #1 put: 5
Consumer #1 got: 5
Producer #1 put: 6
Consumer #1 got: 6
Producer #1 put: 7
Consumer #1 got: 7
Producer #1 put: 8
Consumer #1 got: 8
Producer #1 put: 9
Consumer #1 got: 9
9.14 Starvation and Deadlock
If you write a program in which several concurrent threads are competing for resources, you must take precautions to ensure fairness. A system is fair when each thread gets enough access to limited resources to make reasonable progress. A fair system prevents starvation and deadlock.
Starvation occurs when one or more threads in your program are blocked from gaining access to a resource and, as a result, cannot make progress. Deadlock, the ultimate form of starvation, occurs when two or more threads are waiting on a condition that cannot be satisfied. Deadlock most often occurs when two (or more) threads are each waiting for the other(s) to do something.
Livelocks
A livelock, unlike a deadlock, happens when threads are actually running, but no work gets done.
This usually happens when the two threads are working at cross-purposes, so what is done by the first thread is undone by another. A simple example is where each thread already holds one object and needs another that is held by the other thread. Now imagine a situation wherein each thread puts down the object it possesses and picks up the object put down by the other thread.
Clearly, these two threads can run forever in lock-step, effectively managing to achieve nothing at all. (A common real world example is when two people approach each other in a narrow corridor. Each person tries to be polite by moving to one side to let the other one pass, but both keep moving to the same side at the same time, thereby ensuring that neither can pass. This continues for some time, with both of them swaying from side to side and no progress being made.)
9.15 Suspending, Resuming & Stopping Threads
Sometimes suspending execution of a thread is useful. For example, a separate thread can be used to display the time of the day. If the user doesn’t want a clock, then its thread can be suspended. Whatever the case, suspending a thread is a simple matter. Once suspended, restarting the thread is also a simpler matter.
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An example using the deprecated methods:
//using suspend() and resume() class NewThread implements Runnable {
String name; //name of thread
Thread t;
NewThread(String threadName) { name = threadName; t = new Thread(this, name);
System.out.println(“New Thread :” + t); t.start(); // start the thread
}
// this is the entry point for Thread public void run() { try { for(int i=15; i > 0;i--) {
System.out.println(name + “:” + i);
Thread.sleep(200);
}
} catch(InterruptedException e) {
System.out.println(e);
}
System.out.println(name + “exiting”);
}
} class SuspendResume { public static void main(String args[]){
NewThread t1 = new NewThread(“One”);
NewThread t2 = new NewThread(“Two”); try {
Thread.sleep(1000); t1.t.suspend();
System.out.println(“Suspending thread one”);
Thread.sleep(1000); t1.t.resume();
System.out.println(“Resuming thread one”); t1.t.join(); t2.t.join();
} catch(InterruptedException e) {
System.out.println(“Main thread interrupted”);
}
System.out.println(“Main thread exiting”);
}
}
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This program creates two threads and suspends one of them. While one thread is suspended the other runs.
The same example code is written below without using deprecated methods.
class NewThread implements Runnable {
String name; //name of thread
Thread t; boolean suspendFlag;
NewThread(String threadName) { name = threadName; t = new Thread(this, name);
System.out.println(“New Thread :” + t); suspendFlag = false; t.start(); // start the thread
}
// this is the entry point for Thread public void run() { try { for(int i=15; i > 0;i--) {
System.out.println(name + “:” + i);
Thread.sleep(200); synchronized(this) { while(suspendFlag) { wait();
}
}
}
} catch(InterruptedException e) {
System.out.println(e);
}
System.out.println(name + “exiting”);
} void mysuspend() { suspendFlag = true;
} synchronized void myresume() { suspendFlag = false; notify();
}
}
In the main method of the previous code instead of calling suspend() call mysuspend() method and instead of resume() call myresume()
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9.16 ThreadGroup
ThreadGroup creates a group of threads. It defines two constructors:
ThreadGroup(String groupName);
ThreadGroup(ThreadGroup parent, String groupName)
ThreadGroup offers a convenient way to manage groups of threads as a unit. This is valuable in situations in which you want to suspend and resume a number of related threads.
Example of ThreadGroup: class NewThread extends Thread {
NewThread(String threadName, ThreadGroup tg) { super(tg,threadName);
System.out.println(“New Thread :”+ this); start(); //start the thread
}
//This is the entry point for the thread public void run() { try { for(int i=15; i > 0; i--) {
System.out.println(getName() + “:”+ i);
Thread.sleep(1000);
}
} catch(Exception e) {
System.out.println(e);
}
System.out.println(getName() + “: exiting”);
}
} class ThreadGroupDemo { public static void main(String args[]) {
ThreadGroup gA = new ThreadGroup(“Group A”);
ThreadGroup gB = new ThreadGroup(“Group B”);
NewThread t1 = new NewThread(“One”,gA);
NewThread t2 = new NewThread(“Two”,gA);
NewThread t3 = new NewThread(“Three”,gB);
NewThread t4 = new NewThread(“Four”,gB); try {
Thread.sleep(5000);
} catch(Exception e) {
System.out.println(e);
} gA.suspend();
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}
}
Summary
try {
Thread.sleep(5000);
} catch(Exception e) {
System.out.println(e);
} gA.resume(); try { t1.join(); t2.join(); t3.join(); t4.join();
} catch(Exception e){
System.out.println(e);
}
A thread is a logical execution unit that has a single sequential flow. Multitasking can be achieved by process based multitasking and thread based multitasking. Thread-based multitasking also called as multithreading is cheaper compared to process based multitasking. In multithreaded application you can execute multiple threads within a single program.
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Chapter 10 : String Handling
The Java development environment provides two classes that store and manipulate character data: String, for immutable strings, and StringBuffer, for mutable strings.
The String class provides for constant strings; you use Strings when you don't want the value of the string to change. For example, if you pass a string data into a method, and you don't want the method to modify the string in any way (which is typical), you would use a String.
The StringBuffer class provides for non-constant strings; you use StringBuffers when you know that the value of the character data will change.
Because they are constants, Strings are typically cheaper than StringBuffers and they can be shared. So it's important to use Strings when they're appropriate.
Both String and StringBuffer are the classes in java.lang package and they implement the
CharSequence interface.
10.1 The String class
Strings in Java are constants. Once they are initialized and populated the value and memory allocation is set (they are immutable, meaning they can not be changed.) If the String is changed in any way a new String object is created in memory for the new value.
In the Java language, you specify literal strings between double quotes
"Hello World!"
You can use literal strings anywhere you would use a String object. For example,
System.out.println()
accepts a String argument, so you could use a literal string in place of a
String there.
System.out.println("And might I add that you look lovely today.");
Because the compiler automatically creates a new String object for every literal string it encounters, you can use a literal string to initialize a String.
String s = "Hola Mundo";
The above construct is equivalent to, but more efficient than, this one
String s = new String("Hola Mundo"); because the compiler ends up creating two Strings instead of one: first when the compiler encounters "Hola Mundo!", and second when it encounters new String()
.
Creating a string
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Strings are created like any other object in Java using the new operator. For example one of the
String constructors allows an array of characters to be taken as input: char letters[] = {'J','A','V','A'};
String str = new String (letters);
// One constructor accepts an array of chars
System.out.println (str); // will print JAVA
One more constructor of String is
String (String obj)
String s = new String(“Java”);
While creating a String object you can also specify the byte values.
String (byte asciiChars[])
String (byte asciiChars[], int startIndex, int numChars)
Class StringExample { public static void main(String args[]) { byte ascii [] = {65, 66, 67, 68, 69, 70};
String s1 = new String(ascii);
System.out.println(s1);
String s2 = new String(ascii, 2, 3);
System.out.println(s2);
}
}
This program prints
ABCDEF
CDE
There are two more constructors of String class
String(StringBuffer)
String(StringBuilder)
The first one creates a string whose value is set from a string buffer. The second c reates a string whose value is set from a string builder.
Comparing Strings
The equals method
Comparing two Strings references to see if they hold identical strings is easily achieved with the equals method. This method is called on one string object and takes another as an input
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parameter, if they are equal it returns the boolean value true, otherwise it returns false. For example:
String str = "hello";
String str2 = "hello";
String str3 = "goodbye";
System.out.println ("str and str2" + str.equals(str2)); // true
System.out.println ("str and str3" + str.equals(str2)); // false
If you try to compare two strings like this: if (str == str2)
System.out.println ("true"); else
System.out.println ("false"); you are actually checking to see whether the two references str and str2 refer to the same object.
It is possible that two strings are equal without them being the same object. So if you want to compare the contents of the string use equal, if you want to check if the two references refer to the same object use ==. Two references which point to the same object will always return true with the equals method, but two references which refer to different objects, both containing the same string, with be false with the == operator.
The equalsIgnoreCase method
This method is similar to the equals method instead it performs the case insensitive comparison of Strings.
boolean equalsIgnoreCase (String str)
The compareTo method
int compareTo(String str)
This method is called in the same way as the equals method but returns an integer result. If the result is less than 0 then the first string is less than the second, if it is equal to 0 they are equal and if the result is greater than 0 then the first is greater than the second.
This short example shows how comparisons are made amongst string: class strCmp
{
public static void main (String args[])
{
String str = "Hello";
String str2 = "Java";
System.out.println (str.equals(str2)); // false
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}
}
System.out.println (str.compareTo(str2));
// a negative number, i.e. str is less than str2
The compareToIgnoreCase method
This method is similar to the compare method the only difference is it performs the case insensitive comparison. int compareToIgnoreCase(String str)
The regionMatches method
boolean regionMatches(int, String, int, int) boolean regionMatches(boolean, int, String, int, int)
This method tests whether the specified region of this string matches the specified region of the
String argument. The boolean argument indicates whether case should be ignored; if true, the case is ignored when comparing characters.
The following program,
RegionMatchesDemo
, uses the regionMatches
method to search for a string within another string: public class RegionMatchesDemo { public static void main(String[] args) {
String searchMe = "Green Eggs and Ham";
String findMe = "Eggs"; int len = findMe.length(); boolean foundIt = false; int i = 0; while (!searchMe.regionMatches(i, findMe, 0, len))
{ i++; foundIt = true;
} if (foundIt) {
System.out.println(searchMe.substring(i, i+len));
}
}
}
The output from this program is
Eggs
.
The program steps through the string referred to by searchMe
one character at a time. For each character, the program calls the regionMatches method to determine whether the substring beginning with the current character matches the string for which the program is looking.
The charAt method
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char charAt (int where)
This method is called on a string object with an integer value as a parameter, it returns a char which is at the location of the parameter passed in the string.
char ch = “abc”.charAt(1); assigns ‘b’ to ch
The length method
public int length();
This method is called on a string object with no parameters and returns an integer number which is the number of characters in the string class strExample
{
public static void main (String args[])
{
String str = "Hello";
String str2 = "Java";
System.out.println (str.charAt(0));
// H, i.e. char is position 0
System.out.println (str.length() + str2.length());
// 5 + 4 = 9
}
}
Altering Strings
Strings in Java are actually rather like constants, once they have been declared the objects can not be altered. Hence, they are called immutable objects. However, there are several methods which appear to need to alter the contents of the string. What they actually do is return a new
String.
The toUpperCase and ToLowerCase method
String toLowerCase()
String toUpperCase()
These two methods are called on a string object and simply convert all letters in the string to the appropriate case.
//demonstrate toUpperCase and toLowerCase class ChangeCase { public static void main(String args[]) {
String s = “ This is a test”;
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System.out.println(“Original :” + s);
String upper = s.toUpperCase();
String lower = s.toLowerCase();
System.out.println(“Uppercase:”+ upper);
System.out.println(“Lowercase:”+ lower);
}
}
The output produced by the program is shown here:
Original : This is a test.
UpperCase : THIS IS A TEST.
LowerCase : this is a test.
The substring method
String substring (int startIndex)
String substring (int startIndex, int endIndex)
This versatile method is overloaded to have two versions, the first accepts an int and the second two. The first type leaves the new string with all letters from the old ranging from the first to the number specified. The second version allows the start of the substring being created to be specified as well as the end.
The trim method
String trim()
A very simple method called on a string object this simply removes all whitespace from the string
String s = “ Hello World ”. trim();
Puts “Hello World” into s.
The concat method
String concat(String str)
This method is called on one string object with another as its parameter. The two are concatenated together with the object on which the method was called holding the new string.
String s1 = “One”;
String s2 = s1.concat(“Two”); puts “OneTwo” into s2.
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The replace method
String replace (char original, char replacement)
This method is called on a string object with two char parameters. The string is searched for the first char parameter, when it is found it is replaced with the second.
String s = “Hello”.replace(‘l’,’w’); puts the string”Hewwo” into s
The valueOf method
static String valueOf (double num) static String valueOf (long num) static String valueOf (Object obj) static String valueOf (char char[]) static String valueOf (char char[], int startIndex, int numChars)
This method is slightly different to the one mentioned above because it is not called on a string object. Instead it is a static method which is overloaded to accept all simple Java types and returns a string.
As a convenience, the String class provides the static method valueOf()
which you can use to convert variables of different types to Strings. For example, to print the value of pi
System.out.println(String.valueOf(Math.PI));
An example of methods which alter Strings class altStr
{
public static void main (String args[])
{
String str = "Hello";
String str2 = "Java"; str = str.toUpperCase(); str2 = str2.toLowerCase();
System.out.println (str); // HELLO
System.out.println (str2);// java
str = str.concat(str2);
// str now equals "HELLO java"
System.out.println (str);
str = str.trim();
// str now equals "HELLOjava"
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System.out.println (str); str = str.substring (5,str.length());
// str = "java"
System.out.println (str); str = str.replace ('a', 'i'); // str = "jivi"
System.out.println (str);
}
} str = String.valueOf (3.141); // str = "3.141"
System.out.println (str);
Searching String
The indexOf and lastIndexOf methods
int indexOf (int ch) int indexOf (String s) int indexOf(int ch, int startIndex) int indexOf(String s, int startIndex) int lastIndexOf(int ch) int lastIndexOf(String s) int lastIndexOf(String s, int startIndex) int lastIndexOf(String s, int startIndex)
These two overloaded methods allow a character or substring to be searched for. These methods allow for the string to be searched either from the beginning (IndexOf) or from the end
(lastIndexOf). The input parameters for these methods is a char or string which is to be found and an optional offset from the beginning (or end) of the string, given as an int. The example below shows possible versions of the two methods.
class indexTest
{ public static void main (String args[])
{
String str = "This string will be searched";
String find = "will";
System.out.println (str.indexOf ('s'));
// find char, no offset
System.out.println (str.lastIndexOf ('s', 4));
// find s, offset by 9
System.out.println (str.indexOf (find));
// find the string find, no offset
System.out.println (str.lastIndexOf(find));
// find the string find in str, no offset
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}
}
The indexOf() and lastIndexOf() methods are frequently used in conjunction with substring() which returns a substring of the string. The following class illustrates the use of lastIndexOf() and substring() to isolate different parts of a filename.
class Filename {
String fullpath;
char pathseparator;
Filename(String str, char sep) { fullpath = str; pathseparator = sep;
}
String extension() { int dot = fullpath.lastIndexOf('.'); return fullpath.substring(dot + 1);
}
String filename() { int dot = fullpath.lastIndexOf('.'); int sep = fullpath.lastIndexOf(pathseparator); return fullpath.substring(sep + 1, dot);
}
String path() { int sep = fullpath.lastIndexOf(pathseparator); return fullpath.substring(0, sep);
}
}
The extension()
method uses lastIndexOf()
to locate the last occurrence of the period ('.') in the filename. Then substring()
uses the return value of lastIndexOf()
to extract the filename extension--that is, the substring from the period ('.') to the end of the string. This code assumes that the filename actually has a period ('.') in it; if the filename does not have a period ('.'), then lastIndexOf()
returns -1, and the substring()
method throws a "string index out of range exception".
Also, notice that extension() uses dot + 1 as the argument to substring(). If the period ('.') character is the last character of the string, then dot + 1 is equal to the length of the string which is one larger than the largest index into the string (because indices start at 0). However, substring() accepts an index equal to (but not greater than) the length of the string and interpret it to mean "the end of the string".
While the methods in the example above uses only one version of the lastIndexOf() method, the
String class supports four different versions of both the indexOf() and lastIndexOf() methods which
returns the index of the first (last) occurrence of the specified character
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returns the index of the first (last) occurrence of the specified character, searching forward
(backward) from the specified index
returns the index of the first (last) occurrence of the specified String.
returns the index of the first (last) occurrence of the specified String, searching forward
(backward) from the specified index
The getBytes method
byte [] getBytes()
This method converts the String to the byte array where each character in the string is converted to its ascii equivalent and stored in the array.
The toCharArray() method
If you want to convert the characters in a string to char type and store it in the character array use this method char [] toCharArray()
The startsWith and endsWith methods
The startsWith method determines whether a given String begins with a specified string. The endsWith determines whether the String ends up with a specified String. boolean startsWith(String str) boolean startsWith(String str, int startIndex) boolean endsWith(String str)
For example,
“Foobar”.endsWith(“bar”);
“Foobar”.startsWith(“Foo”); both return true
“Foobar”.startsWith(“bar”,3) returns true
The split method
J2SE 1.4 added the split()
method to the
String
class to simplify the task of breaking a string into substrings, or tokens. This method uses the concept of a regular expression to specify the delimiters. A regular expression is a remnant from the Unix grep tool ("grep" meaning "general regular expression parser").
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In its simplest form, searching for a regular expression consisting of a single character finds a match of that character. For example, the character 'x' is a match for the regular expression "x".
The split()
method takes a parameter giving the regular expression to use as a delimiter and returns a
String
array containing the tokens so delimited. Using split()
, the first example above becomes
String str = "This is a string object";
String[] words = str.split (" "); for (int i=0; i < words.length; i++)
System.out.println (words[i]);
//To use "*" as a delimiter, simply specify "*" as the regular
//expression:
String str = "A*bunch*of*stars";
String[] starwords = str.split ("*");
For most string splitting tasks, the
String.split()
method is much easier and more natural to use than the
StringTokenizer
class. However,
StringTokenizer
is still useful for some tasks.
For example, an overloaded
StringTokenizer
constructor allows you to specify that the tokens to be returned include the delimiter characters themselves.
The + operator
There is another special piece of string syntax using the + operator. This allows strings to be added together, at the moment it is convenient to know how to use this.
String s = "Hello";
String t = s + "there" + ", how are you" + "today";
// t = "Hello there, how are you today"
The following code will append several values together and output a line of text to the system console.
String CustName = "John Smith";
String CustOrder = "Java Programming Course";
System.out.println(CustName + " ordered a " + CustOrder + ".");
Example of String concatenation with other data types int age = 9;
String s = “He is ” + age + “ years old”;
System.out.println(s);
Here int is converted to String
String s = “four :” + 2 + 2;
System.out.println(s);
This code displays
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four : 22 rather than four : 4
This is because the String concatenation operator has the higher precedence than the arithmetic + operator. Hence to get the desired result use
String s = “four :” + (2 + 2);
10.2 The StringBuffer Class
The StringBuffer class is the second of Java's classes which deal with string handling. While the original String class dealt with strings which were of a fixed length (the length of the number of characters the string contained) and whose contents could not be altered the StringBuffer class deals with strings which are not full of characters and whose contents can be changed. This is useful for a number of operation, such as adding characters into to the end of a string, which is not possible with a normal String. The plus operator which looks like it allows normal Strings to be extended actually uses the StringBuffer class and converts back to a string. The reason Java has too separate classes to deal with strings is simply for performance reasons.
For example : The following code creates String objects and uses the concatenation (+) character to add more characters to them:
String sample1 = new String(“Builder.com”); sample1 += “ is “; sample1 += “the place “; sample1 += “to be.”;
In the end, the system creates four String objects to handle the alterations. The first is created with the text Builder.com. A new object is created every time more text is appended.
The problem with this approach is that too many resources are being used for such a simple procedure. The effects may be minimal in this instance (given the small amount of code), but a larger application with more operations will likely degrade performance. This is the niche filled by the StringBuffer class.
Constructors of StringBuffer class
Constructor
StringBuffer()
Description
Constructs a string buffer with no characters in it and an
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initial capacity of 16 characters.
StringBuffer(int capacity) Constructs a string buffer with no characters in it and an
StringBuffer(String str) initial capacity specified by the length argument.
Constructs a string buffer so that it represents the same sequence of characters as the string argument; in other words, the initial contents of the string buffer is a copy of the argument string.
The StringBuffer class is designed to create and manipulate dynamic string information. The memory allocated to the object is automatically expanded to accommodate additional text.
StringBuffer sb = new StringBuffer();
StringBuffer sb = new StringBuffer(30);
StringBuffer sb = new StringBuffer(“Builder.com”);
The first line creates an object with no text and the default capacity of 16 characters. The second instance of the class has no value and a capacity of 30 characters, and the final line creates an object with the initial value. The StringBuffer class is included in the base java.lang package, so no special imports statement is needed to take advantage of it.
Creating StringBuffer objects
To create a StringBuffer object use the following:
StringBuffer rb = new StringBuffer();
When a StringBuffer object is created it has a capacity, which is the number of characters that the StringBuffer will contain if full. If the default constructor is called with no parameters this capacity is set to 16, otherwise an int may be passed as a parameter to specify the capacity.
Another constructor allows an initial set of characters to be passed as a parameter, in this case the capacity of the StringBuffer will be the number of characters in the initial string plus a further
16. The following example shows the possible ways of building StringBuffer objects.
StringBuffer strbuf = new StringBuffer (); // capacity = 16
StringBuffer strbuf2 = new StringBuffer (25);
// capacity = 25
StringBuffer strbuf3 = new StringBuffer ("Java");
// capacity = 4 + 16 = 20
The length method
int length()
Returns the length (character count) of this string buffer.
Methods concerned with capacity
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The capacity method
int capacity()
This method is called on a StringBuffer object and returns the capacity as an int. Note that the capacity will be different to the length, unless the buffer is full. The following example shows this:
S tringBuffer strbuf = new StringBuffer ("Hello");
System.out.println (strbuf.length()); // prints 5
System.out.println (strbuf.capacity()); // prints 21
The capacity() method differs from length() in that it returns the amount of space currently allocated for the StringBuffer, rather than the amount of space used.
The ensureCapacity method
void ensureCapacity(int capacity)
Once a StringBuffer object has been created the size of the buffer can be resized using the ensureCapacity method, called on the StringBuffer object with the new size (int) as the only parameter. Note that this method ensures a minimum capacity, the actual capacity may be more than you have specified.
The setLength method
void setLength(int length)
This method allows the length of a StringBuffer object to be set, if the value passed (int) is greater than the existing length then the remainder will be filled with zeros.
To reset a StringBuffer (to use it somewhere else) set the length to zero using the setLength() method. This will "clear" out the value in the StringBuffer.
rb.setLength(0);
An example of StringBuffer capacity class altBuf
{
public static void main (String args[])
{
StringBuffer strbuf = new StringBuffer ("Hello");
System.out.println (strbuf); // Hello
strbuf.append ("_world");
System.out.println (strbuf); // Hello_World
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strbuf.setCharAt (5, ' ');
System.out.println (strbuf); // Hello World
strbuf.insert (6, "Java ");
System.out.println (strbuf); // Hello Java World
}
}
The charAt and setCharAt methods
char charAt(int index) void setCharAt(int index, char charValue)
The charAt method works the same as for a String object. The setCharAt method takes two parameters as input, an int to specify the position in the StringBuffer and a char which the character at the given position will be changed to.
//demonstrate charAt and setCharAt class CharAtDemo { public static void main(String args[]) {
StringBuffer sb = new StringBuffer(“Hello”);
System.out.println(“Buffer before = ” + sb);
System.out.println(“charAt(1) before = ” + sb.charAt(1)); sb.setCharAt(1,’i'); sb.setLength(2);
System.out.println(“Buffer after : ” +sb);
System.out.println(“charAt(1) after = ” + sb.charAt(1));
}
}
Here is the output generated by this program:
Buffer before = Hello charAt(1) before = e buffer after = Hi charAt(1) after = i
The insert method
StringBuffer insert(int index, boolean booleanValue)
Inserts the string representation of the boolean argument into this string buffer.
StringBuffer insert(int index, char charValue)
Inserts the string representation of the char argument into this string buffer.
StringBuffer insert(int index, char[] charArray)
Inserts the string representation of the char array argument into this string buffer.
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StringBuffer insert(int index, char[] charArray, int startIndex, int numChars)
Inserts the string representation of a subarray of the str array argument into this string buffer.
StringBuffer insert(int index, double doubleValue)
Inserts the string representation of the double argument into this string buffer.
StringBuffer insert(int index, float floatValue)
Inserts the string representation of the float argument into this string buffer.
StringBuffer insert(int index, int intValue)
Inserts the string representation of the second int argument into this string buffer.
StringBuffer insert(int index, long longValue)
Inserts the string representation of the long argument into this string buffer.
StringBuffer insert(int index, Object obj)
Inserts the string representation of the Object argument into this string buffer.
StringBuffer insert(int index, String str)
Inserts the string into this string buffer.
The insert method is identical to the append method except that the data is added to the string at a specified position. Again this method is called on a StringBuffer object but it takes two parameters, the first (int) is the position where the data is to be added, and the second is the data to be added (of any simple type, or a String).
This example illustrates how you would insert a string into a StringBuffer.
StringBuffer sb = new StringBuffer("Drink Java!"); sb.insert(6, "Hot ");
System.out.println(sb.toString());
This code snippet prints
Drink Hot Java!
With StringBuffer's many insert()
, you specify the index before which you want the data inserted. In the example, "Hot " needed to be inserted before the 'J' in "Java". Indices begin at 0, so the index for 'J' is 6. To insert data at the beginning of a StringBuffer use an index of 0. To add data at the end of a StringBuffer use an index equal to the current length of the StringBuffer or use append()
.
An example of altering the contents of StringBuffer objects class AltBuf
{
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public static void main
{
StringBuffer strbuf = new ("Hello");
System.out.println (strbuf); // Hello strbuf.append ("_world");
System.out.println (strbuf); // Hello_World strbuf.setCharAt (6, " ");
System.out.println (strbuf); // Hello World strbuf.insert (7, "Java ");
System.out.println (strbuf); // Hello Java World
}
}
The toString method
String toString()
Converts to a string representing the data in this string buffer.
To output the StringBuffer value use the toString() method
The append method
StringBuffer append(boolean booleanValue)
Appends the string representation of the boolean argument to the string buffer.
StringBuffer append(char charValue)
Appends the string representation of the char argument to this string buffer.
StringBuffer append(char[] charArray)
Appends the string representation of the char array argument to this string buffer.
StringBuffer append(char[] charArray, int startIndex, int numChars)
Appends the string representation of a subarray of the char array argument to this string buffer.
StringBuffer append(double doubleValue)
Appends the string representation of the double argument to this string buffer.
StringBuffer append(float floatValue)
Appends the string representation of the float argument to this string buffer.
StringBuffer append(int intValue)
Appends the string representation of the int argument to this string buffer.
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StringBuffer append(long longValue)
Appends the string representation of the long argument to this string buffer.
StringBuffer append(Object obj)
Appends the string representation of the Object argument to this string buffer.
StringBuffer append(String str)
Appends the string to this string buffer.
This method allows data of any of the simple data types to be added to the end of a StringBuffer.
This is achieved by the method being overloaded for all of the simple data types and using the valueOf method. The append method is called on a StringBuffer object with the data to be added as an input parameter.
The append method can be used to append values together. The append method can accept any data type as input and converts the data to a String to be appended to the end of the value in the
StringBuffer.
StringBuffer rb = new StringBuffer(); rb.append("Hello").append(" ").append("World!");
The StringBuffer variable, rb, now holds the value of "Hello World!".
StringBuffer sb = new StringBuffer(); sb.append("B"); sb.append("u"); sb.append("i"); sb.append("l"); sb.append("d"); sb.append("e"); sb.append("r"); sb.append(".com");
System.out.println(sb.toString());
This code creates the string Builder.com and sends it to the standard output, but only one object is created. The same code using String objects would require more than eight objects. Notice that the code takes advantage of the toString method of the StringBuffer class. This method converts the contents to a String object that can be used for output. This allows text to be manipulated accordingly and used for output or data storage.
StringBuffer b = new StringBuffer();
System.out.println(b.append("Hello”).append("World").toString());
One more example
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sb.append("Builder.com is awesome!");
System.out.println(sb.toString()); sb.setLength(11);
System.out.println(sb.toString());
This code sets the capacity and populates the object with a string. The length property is reset so that the text is truncated. The output follows:
Builder.com is awesome!
Builder.com
The delete method
StringBuffer delete(int start, int end)
Removes the characters in a substring of this StringBuffer.
The deleteCharAt method
StringBuffer deleteCharAt(int index)
Removes the character at the specified position in this StringBuffer (shortening the StringBuffer by one character).
//demonstrate delete and deleteCharAt class deleteDemo {
public static void main(String args[]) {
StringBuffer sb = new StringBuffer(“This is a test”);
sb.delete(4,7);
System.out.println(“After delete :” + sb);
sb.deleteCharAt(0);
System.out.println(“After deleteCharAt : ” + sb);
}
}
The following output is produced :
After replace : This was a test
The replace method
public StringBuffer replace(int start, int end, String str)
Replaces the characters in a substring of this StringBuffer with characters in the specified String.
The reverse method
StringBuffer reverse()
The character sequence contained in this string buffer is replaced by the reverse of the sequence.
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The following code and output show it in action:
StringBuffer sb = new StringBuffer(); sb.ensureCapacity(100); sb.append("Builder.com!");
System.out.println(sb.toString()); sb.reverse();
System.out.println(sb.toString());
Output:
Builder.com!
!moc.redliuB
The reverse method is present only in the StringBuffer class. Hence if you want to reverse a
String you can use the StringBuffer for doing so.
Example class ReverseString {
public static String reverseIt(String source) {
StringBuffer dest = new StringBuffer(source); dest.reverse(); return dest.toString();
}
}
The reverseIt()
method accepts an argument of type String called source
which contains the string data to be reversed. The method creates a StringBuffer, dest
, the same size as source
, then loops backwards over all the characters in source
and appends them to dest
thereby reversing the string. Finally, the method converts dest
, a StringBuffer, to a String.
The getChars method
public void getChars(int srcBegin, int srcEnd, char dst[],
int dstBegin)
Characters are copied from this string buffer into the destination character array dst.
The substring method
String substring(int startIndex)
Returns a new String that contains a subsequence of characters currently contained in this
StringBuffer.The substring begins at the specified index and extends to the end of the
StringBuffer.
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String substring(int startIndex, int endIndex)
Returns a new String that contains a subsequence of characters currently contained in this
StringBuffer.
10.3 The StringBuilder class
J2SE5.0 added the
StringBuilder
class, which is a drop-in replacement for
StringBuffer
in cases where thread safety is not an issue. Because
StringBuilder
is not synchronized, it offers faster performance than
StringBuffer
.
In general, you should use
StringBuilder
in preference over
StringBuffer
. In fact, the J2SE
5.0 javac
compiler normally uses
StringBuilder
instead of
StringBuffer
whenever you perform string concatenation as in
System.out.println ("The result is " + result);
All the methods available on
StringBuffer
are also available on
StringBuilder
, so it really is a drop-in replacement. Following is a sample program called
StringsDemo
, which reverses the characters of a string. This program uses both a string and a string builder. public class StringsDemo { public static void main(String[] args) {
String palindrome = "Dot saw I was Tod"; int len = palindrome.length();
StringBuilder dest = new StringBuilder(len); for (int i = (len - 1); i >= 0; i--) { dest.append(palindrome.charAt(i));
}
System.out.println(dest.toString());
}
}
The output from this program is: doT saw I was toD
Questions
1. class MCZ17 { public static void main (String[] args) {
String a = "\n"; // 1
String b = "\r"; // 2
String c = "\u000a"; // 3 \u000a = new line
String d = "\u000d"; // 4 \u000d = return
}
}
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Compile-time errors are generated at which lines? a. 1 b. 2 c. 3 d. 4
2. class MCZ20 {
} public static void main (String[] args) {
}
// Insert code here.
Which of the following lines can be inserted at the specified location without generating a compile-time error? a. String a = 'a'; b. String b = 'abc'; c. String c = '\u0041'; d. String d = '\uabcd'; e. None of the above
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Chapter 11 : I/O
11.1 I/O Streams
To bring in information, a program opens a stream on an information source (a file, memory, a socket) and reads the information sequentially, as shown here
Similarly, a program can send information to an external destination by opening a stream to a destination and writing the information out sequentially, like this:
Reading open a stream while more information
read information close the stream
Writing open a stream while more information
write information close the stream
The java.io package contains a collection of stream classes that support these algorithms for reading and writing. To use these classes, a program needs to import the java.io package. The stream classes are divided into two class hierarchies, based on the data type (either characters or bytes) on which they operate.
I/O Stream Classes
259
This diagram shows most of the members of the java.io package. Rectangles are classes, parallelograms are abstract classes, and ovals are interfaces
Byte Streams
260
To read and write 8-bit bytes, programs should use the byte streams, descendants of InputStream and OutputStream. InputStream and OutputStream provide the API and partial implementation for input streams (streams that read 8-bit bytes) and output streams (streams that write 8-bit bytes). These streams are typically used to read and write binary data such as images and sounds.
Two of the byte stream classes,
ObjectInputStream
and
ObjectOutputStream
, are used for object serialization.
The InputStream class is an abstract base class that provides a minimal programming interface and a partial implementation of input streams in Java. The InputStream class defines a methods for reading bytes or arrays of bytes, marking locations in the stream, skipping bytes of input, finding out the number of bytes that are available for reading, and resetting the current position within the stream. An input stream is automatically opened when you create it. You can explicitly close a stream with the close() method, or let it be closed implicitly when the object is garbage collected.
The OutputStream class is an abstract base class that provides a minimal programming interface and a partial implementation of output streams in Java. OutputStream defines methods for writing bytes or arrays of bytes to the stream and flushing the stream. An output stream is automatically opened when you create it. You can explicitly close an output stream with the close()
method, or let it be closed implicitly when the object is garbage collected.
Character Streams
Reader and Writer are the abstract superclasses for character streams in java.io. Reader provides the API and partial implementation for readers--streams that read 16-bit characters--and
Writer provides the API and partial implementation for writers--streams that write 16-bit characters.
Subclasses of
Reader
and
Writer
implement specialized streams and are divided into two categories: those that read from or write to data sinks
Understanding the I/O Superclasses
Reader
and
InputStream
define similar APIs but for different data types. For example,
Reader contains these methods for reading characters and arrays of characters: int read() int read(char cbuf[]) int read(char cbuf[], int offset, int length)
InputStream
defines the same methods but for reading bytes and arrays of bytes: int read() int read(byte cbuf[]) int read(byte cbuf[], int offset, int length)
Also, both Reader and InputStream provide methods for marking a location in the stream, skipping input, and resetting the current position.
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Writer and OutputStream are similarly parallel. Writer defines these methods for writing characters and arrays of characters: int write(int c) int write(char cbuf[]) int write(char cbuf[], int offset, int length)
And
OutputStream
defines the same methods but for bytes: int write(int c) int write(byte cbuf[]) int write(byte cbuf[], int offset, int length)
All of the streams--readers, writers, input streams, and output streams--are automatically opened when created. You can close any stream explicitly by calling its close
method. Or the garbage collector can implicitly close it, which occurs when the object is no longer referenced.
Stream classes
The following table lists java.io
's streams and describes what they do. Note that many times, java.io
contains character streams and byte streams that perform the same type of I/O but for different data types.
Memory
Type of I/O Byte Streams
ByteArrayInputStream
ByteArrayOutputStream
Character Streams
CharArrayReader
CharArrayWriter
Pipe
File
Concatenation
Object
Serialization
Data
Conversion
Counting
Peeking Ahead
Printing
Buffering
Filtering
Converting between Bytes and
Characters
StringBufferInputStream StringReader
StringWriter
PipedInputStream
PipedOutputStream
FileInputStream
FileOutputStream
SequenceInputStream
ObjectInputStream
ObjectOutputStream
DataInputStream
DataOutputStream
LineNumberInputStream
PushbackInputStream
PrintStream
BufferedInputStream
BufferedOutputStream
FilterInputStream
FilterOutputStream
PipedReader
PipedWriter
FileReader
FileWriter
N/A
N/A
N/A
LineNumberReader
PushbackReader
PrintWriter
BufferedReader
BufferedWriter
FilterReader
FilterWriter
InputStreamReader
OutputStreamWriter
Use of streams
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CharArrayReader, CharArrayWriter, ByteArrayInputStream,
ByteArrayOutputStream
Use these streams to read from and write to memory. You create these streams on an existing array and then use the read and write methods to read from or write to the array.
StringReader, StringWriter, StringBufferInputStream
Use StringReader to read characters from a String in memory. Use StringWriter to write to a
String. StringWriter collects the characters written to it in a StringBuffer, which can then be converted to a String.
StringBufferInputStream
is similar to
StringReader
, except that it reads bytes from a
StringBuffer
.
PipedReader, PipedWriter, PipedInputStream, PipedOutputStream
Implement the input and output components of a pipe. Pipes are used to channel the output from one thread into the input of another.
FileReader, FileWriter, FileInputStream, FileOutputStream
Collectively called file streams, these streams are used to read from or write to a file on the native file system.
SequenceInputStream
Concatenates multiple input streams into one input stream.
ObjectInputStream, ObjectOutputStream
Used to serialize objects.
DataInputStream, DataOutputStream
Read or write primitive data types in a machine-independent format.
LineNumberInputStream, LineNumberReader
Keeps track of line numbers while reading.
PushbackReader, PushbackInputStream
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These input streams each have a pushback buffer. When reading data from a stream, it is sometimes useful to peek at the next few bytes or characters in the stream to decide what to do next.
PrintStream, PrintWriter
Contain convenient printing methods. These are the easiest streams to write to, so you will often see other writable streams wrapped in one of these.
BufferedReader, BufferedWriter, BufferedInputStream,
BufferedOutputStream
Buffer data while reading or writing, thereby reducing the number of accesses required on the original data source. Buffered streams are typically more efficient than similar nonbuffered streams and are often used with other streams.
FilterReader, FilterWriter, FilterInputStream, FilterOutputStream
These abstract classes define the interface for filter streams, which filter data as it's being read or written.
InputStreamReader, OutputStreamWriter
A reader and writer pair that forms the bridge between byte streams and character streams.
An
InputStreamReader
reads bytes from an
InputStream
and converts them to characters, using the default character encoding or a character encoding specified by name.
An
OutputStreamWriter
converts characters to bytes, using the default character encoding or a character encoding specified by name and then writes those bytes to an
OutputStream
.
You can get the name of the default character encoding by calling
System.getProperty("file.encoding")
.
The predefined Streams
The java.lang defines a class called System, which encapsulates several aspects of the run-time environment. System also contains three predefined streams variables, in, out and err. These fields are declared as public static within System.
System.out refers to standard output stream. By default this is the console. System.in refers to the standard input, which is the keyboard by default. System.err refers to the standard error stream, which also is the console by default. However, these streams may be redirected to any compatible I/O device.
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System.in is an object of the type InputStream. System.out and System.err are objects of type
PrintStream. These are byte streams, even though they typically are used to read and write characters from and to the console.
11.2 Reading console input
In Java, console input is accomplished by reading from System.in. To obtain a character-based stream that is attached to the console, you wrap System.in in a BufferedReader object, to create a character stream. BufferedReader supports buffered input stream. The constructor of
BufferedReader used is shown below:
BufferedReader (Reader inputReader)
Here inputReader is the stream that is linked to the instance of BufferedReader that is being created. Reader is an abstract class. InputStreamReader is its concrete sub-class, that converts bytes to characters. To obtain InputStreamReader object, use the following constructor:
InputStreamReader (InputStream inputStream)
Because System.in refers to an object of type InputStream, it can be used for inputStream.
BufferedReader stdin =new BufferedReader( new InputStreamReader(System.in));
The readLine() method is a member of BufferedReader class. Its general form is shown here:
String readLine() throws IOException
An example of reading an entire line of input from console import java.io.*;
}
} public class ConsoleInputDemo {
public static void main(String[] args) {
try {
BufferedReader stdin =new BufferedReader( new InputStreamReader(System.in));
System.out.print("Enter a line:");
System.out.println(stdin.readLine());
} catch(IOException e) {
System.out.println(e);
}
One example that reads from the console and prints the same string and terminates when an empty string is entered.
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import java.io.*; public class Echo {
public static void main(String[] args) {
DataInputStream in = new DataInputStream( new BufferedInputStream(System.in));
String s = null;
try {
while((s = in.readLine()).length() != 0) {
System.out.println(s);
}
// An empty line terminates the program
}
catch(IOException e) {
e.printStackTrace();
}
}
}
An example of reading characters import java.io.*; class BRRead { public static void main(String args[]) throws IOException{ char c;
BufferedReader br = new BufferedReader ( new InputStreamReader(System.in));
System.out.println(“Enter characters , ‘q’ to quit:”);
//read characters do { c = (char) br.read();
System.out.print(c);
} while( c != ‘q’);
}
}
11.3 Writing console output
This example prints a single character on the console. In this example we simply wrap the 16-bit encoded OutputStreamWriter class around the usual
System.out stream. The OutputStreamWriter class only has a few basic write() methods. We can obtain many print() and println() overloaded methods with the PrintWriter class. import java.io.*;
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public class SingleCharConsole16bit {
public static void main(String f[]) {
char c;
String s = “Hello”;
// Wrap the System.out 8-bit stream with the 16-bit
// output stream.
OutputStreamWriter osw = new
OutputStreamWriter(System.out);
try {
for( int i = 0; i< s.length(); i++) {
c = s.charAt(i);
osw.write(c);
}
osw.flush();
}
catch (IOException ioe) {
System.out.println( "IO error:" + ioe );
}
}
}
Another example that prints a String on the console using PrintWriter class. We take advantage of the autoflushing switch in one of the PrintWriter constructors so that we do not have to flush the buffers explicitly ourselves after every print. import java.io.*; public class StringOutput {
public static void main(String f[]) {
PrintWriter pw = new PrintWriter(System.out,true);
String s = “Hello”; try { pw.println(s);
} catch (IOException ioe) {
System.out.println( "IO error:" + ioe );
}
}
}
11.4 System.out.printf()
The java.text.Format
subclasses offer limited formatting capabilities and are rather clumsy compared to the C language printf()
function, which provides a wide range of formatting options combined with output operation.
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To satisfy the demands of programmers for such a facility and to facilitate the porting of C programs to Java, J2SE 5.0 comes with the class java.util.Formatter
, which can both format numerical output into a string and send the string to a file or other destination
Numerical values are formatted according to format specifiers like those for the printf() function in C.
Furthermore, J2SE 5.0 added a printf()
method to the
PrintStream
class. So now you can use
System.out.printf()
to send formatted numerical output to the console. It uses a java.util.Formatter
object internally.
The simplest of the overloaded versions of the method goes as
printf (String format, Object... args)
The "..." indicates the varargs functionality. It allows a method to accept a variable number of arguments. The arguments can be primitives as well as object references, e.g. the wrappers for the primitives.
The format argument is a string in which you embed specifier substrings that indicate how the arguments appear in the output. For example, double pi = Math.PI;
System.out.printf ("pi = %5.3f%n", pi); results in the console output
pi = 3.142
The format string includes the specifier "
%5.3f
" that is applied to the argument. The '
%
' sign signals a specifier. The width value 5 requires at least five characters for the number, the precision value 3 requires three places in the fraction, and the conversion symbol 'f' indicates a decimal representation of a floating-point number.
A specifier needs at least the conversion character, of which there are several besides 'f'. Some of the other conversions include
'd' - decimal integer
'o' - octal integer
'e' - floating-point in scientific notation
There are also special conversions for dates and times. The general form of the specifier includes several optional terms:
%[argument_index$][flags][width][.precision]conversion
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The argument_index indicates to which argument the specifier applies. For example,
%2$ indicates the second argument in the list. A flag indicates an option for the format. For example,
'+' requires that a sign be included and '0' requires padding with zeros. The width indicates the minimum number of characters and the precision is the number of places for the fraction.
There is also one specifier that doesn't correspond to an argument. It is "
%n
" which outputs a line break. A "
\n
" can also be used in some cases, but since "
%n
" always outputs the correct platformspecific line separator, it is portable across platforms whereas"
\n
" is not.
The following program provides several examples of printf()
. public class PrintfDemo
{
/** Illustrate several output formats with printf() **/
public static void main (String args[]) {
double q = 1.0/3.0;
// Print the number with 3 decimal places.
System.out.printf ("1.0/3.0 = %5.3f %n", q);
// Increase the number of decimal places
System.out.printf ("1.0/3.0 = %7.5f %n", q);
// Pad with zeros.
q = 1.0/2.0;
System.out.printf ("1.0/2.0 = %09.3f %n", q);
// Scientific notation
q = 1000.0/3.0;
System.out.printf ("1000/3.0 = %7.2e %n", q);
// More scientific notation
q = 3.0/4567.0;
System.out.printf ("3.0/4567.0 = %7.2e %n", q);
// Negative infinity
q = -1.0/0.0;
System.out.printf ("-1.0/0.0 = %7.2e %n", q);
q = 0.0/0.0;
// NaN
System.out.printf ("0.0/0.0 = %5.2e %n", q);
// Multiple arguments
System.out.printf ("pi = %5.3f, e = %5.4f %n", Math.PI, Math.E);
double r = 1.1;
// User the argument index to put the argument values into
// different locations within th string.
System.out.printf ("C = 2 * %1$5.5f * %2$4.1f, "+
"A = %2$4.1f * %2$4.1f * %1$5.5f %n",
Math.PI, r);
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}
} // class PrintfDemo
Output of this program:
1.0/3.0 = 0.333
1.0/3.0 = 0.33333
1.0/2.0 = 00000.500
1000/3.0 = 3.33e+02
3.0/4567.0 = 6.57e-04
-1.0/0.0 = -Infinity
0.0/0.0 = NaN pi = 3.142, e = 2.7183
C = 2 * 3.14159 * 1.1, A = 1.1 * 1.1 * 3.14159
11.5 File class
A File class present in java.io can be used to obtain or manipulate the information associated with a disk file, such as permissions, time, date and directory path, and to navigate subdirectory hierarchies. A File object can represent either the name of a particular file or the names of a set of files in a directory. We create an instance of the class by giving the name of the file
(or directory). There are several constructors:
The File class has the following constructors
File(String pathname);
// pathname could be file or a directory name
File(String dirPathname, String filename);
File(File directory, String filename);
The simplest is this (using Windows file-syntax):
File f = new File
("c:\\server\\classes\\hansen\\playground\\MyFile.class");
Or
File f = new File
("c:/server/classes/Hansen/playground/MyFile.class");
You may also use a relative file name like this:
File f = new File("hansen\\playground\\MyFile.class");
To use this format you'll have to know the "current user directory", which most often is where the Java Virtual Machine is invoked. If you are in doubt you can get the name of the current user directory like this:
270
String userdir = System.getProperty("user.dir");
Several methods are available for inspecting an instance of the File class.
Some of the important ones are:
Method
boolean exists() boolean isFile() boolean isDirectory()
String[] list()
String getName()
String getPath()
Purpose
does the file exist? is it a file?
… or a directory?
return the names of all files and directories in a directory get the file or directory's name get the file or directory's path
These methods are actually all we need in order to find all files and directories in a given directory--all the way down to the last leaves in the directory tree
The File class is more than just a representation for an existing directory path, file, or group of files. You can also use a File object to create a new directory or an entire directory path. You can also look at the characteristics of files (size, last modification date, read/write), see whether a File object represents a file or a directory, and delete a file. If the file mentioned is not found the program simply exits.
import java.io.*; public class FileExample { public static void main(String[] args) {
File f = new File(args[0]); if(f.exists()) {
System.out.println(f + " exists");
} else { return;
}
System.out.println(
"Absolute path: " + f.getAbsolutePath() +
"\n Can read: " + f.canRead() +
"\n Can write: " + f.canWrite() +
"\n getName: " + f.getName() +
"\n getParent: " + f.getParent() +
"\n getPath: " + f.getPath() +
"\n file size: " + f.length() +
"\n lastModified: " + f.lastModified()); if(f.isFile())
System.out.println("it's a file"); else if(f.isDirectory())
System.out.println("it's a directory");
File rname = new File(args[1]);
271
f.renameTo(rname);
File another = new File(args[2]);
System.out.println("deleting..." + another); another.delete();
File makedir = new File(args[3]); makedir.mkdir();
}
}
This program takes a filename as the first argument checks whether it exists. If yes then prints the characteristics of the file else exits the program. Next it also checks whether the file object is a file or directory. Then it renames the file as the second argument. The third argument specifies the file to be deleted. The fourth argument is the name of the new directory that gets created.
The following program prints the list of all filenames and names of sub directories into the current directory.
// Displays directory listing import java.io.*; public class DirList {
public static void main(String[] args) {
try {
File path = new File("."); // current directory
String[] list = path.list();
for(int i = 0; i < list.length; i++)
System.out.println(list[i]);
}
catch(Exception e) {
e.printStackTrace();
}
}
}
The following program prints the list of only .java files in the current directory import java.io.*; public class DirList {
public static void main(String[] args) {
try {
File path = new File("."); // current directory
String[] list = path.list(new DirFilter(“.java”));
for(int i = 0; i < list.length; i++)
System.out.println(list[i]);
}
catch(Exception e) {
e.printStackTrace();
}
}
}
272
class DirFilter implements FilenameFilter {
String afn;
DirFilter(String afn) { this.afn = afn; }
public boolean accept(File dir, String name) {
// Strip path information:
String f = new File(name).getName(); return f.indexOf(afn) != -1;
}
}
The interface FilenameFilter has only one method boolean accept(File dir, String name);
It says that all this type of object does is provide a method called accept( ).
The whole reason behind the creation of this class is to provide the accept( ) method to the list( ) method so that list( ) can call back accept( ) to determine which file names should be included in the list. Thus, this technique is often referred to as a callback or sometimes a functor (that is,
DirFilter is a functor because its only job is to hold a method). Because list( ) takes a FilenameFilter object as its argument, it means that you can pass an object of any class that implements FilenameFilter to choose (even at runtime) how the list( ) method will behave. The purpose of a callback is to provide flexibility in the behavior of code.
DirFilter shows that just because an interface contains only a set of methods, you’re not restricted to writing only those methods. In this case, the DirFilter constructor is also created.
The accept( ) method must accept a File object representing the directory that a particular file is found in, and a String containing the name of that file.
You might choose to use or ignore either of these arguments, but you will probably at least use the file name. Remember that the list( ) method is calling accept( ) for each of the file names in the directory object to see which one should be included – this is indicated by the boolean result returned by accept( ).
To make sure that what you’re working with is only the name and contains no path information, all you have to do is take the String object and create a
File object out of it, then call getName( ) which strips away all the path information (in a platform-independent way). Then accept( ) uses the String class indexOf( ) method to see if the search string afn appears anywhere in the name of the file. If afn is found within the string, the return value is the starting index of afn, but if it’s not found the return value is -1. Keep in mind that this is a simple string search and does not have regular expression
“wildcard” matching.
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11.6 Using File Streams
File streams are perhaps the easiest streams to understand. The file streams--
FileReader
,
FileWriter
,
FileInputStream
, and
FileOutputStream
--each read or write from a file on the native file system. You can create a file stream from a file name in the form of a string, a
File object, or a
FileDescriptor
object.
An Example of reading from a file using FileReader and printing the file contents on the console.
The filename is passed as a command line argument.
The FileReader class below accepts the filename as the argument and opens it for reading. If the file is not found an exception of type FileNotFoundException is thrown. FileNotFoundException is a checked exception and is a subclass of IOException. The readLine method of the
BufferedReader reads a String till the end of the line. It throws the checked IOException. Hence both the exeptions are handled explicitly by using catch blocks.
import java.io.*; public class FileReadDemo {
public static void main(String[] args) {
try {
BufferedReader in =new BufferedReader(new
FileReader(args[0]));
String s = new String();
String s2=””;
while((s = in.readLine())!= null)
s2 += s + "\n";
in.close();
}
catch(FileNotFoundException e) {
System.out.println(e);
}
catch(IOException e){
System.out.println(e);
}
}
}
The following program uses FileReader and FileWriter to copy the contents of a file named farrago.txt into a file called outagain.txt: import java.io.*; public class Copy {
public static void main(String[] args) throws IOException {
File inputFile = new File("farrago.txt");
File outputFile = new File("outagain.txt");
FileReader in = new FileReader(inputFile);
274
FileWriter out = new FileWriter(outputFile);
int c;
while ((c = in.read()) != -1) out.write(c);
in.close();
out.close();
}
}
This program is very simple. It opens a
FileReader
on farrago.txt
and opens a
FileWriter on outagain.txt
. The program reads characters from the reader as long as there's more input in the input file and writes those characters to the writer. When the input runs out, the program closes both the reader and the writer.
Here is the code that the Copy program uses to create a file reader:
File inputFile = new File("farrago.txt");
FileReader in = new FileReader(inputFile);
This code creates a
File
object that represents the named file on the native file system.
File
is a utility class provided by java.io
. The
Copy
program uses this object only to construct a file reader on a file. However, the program could use inputFile
to get information, such as its full path name, about the file.
After you've run the program, you should find an exact copy of farrago.txt in a file named outagain.txt in the same directory.
Remember that
FileReader
and
FileWriter
read and write 16-bit characters. However, most native file systems are based on 8-bit bytes. These streams encode the characters as they operate according to the default character-encoding scheme. You can find out the default characterencoding by using
System.getProperty("file.encoding")
. To specify an encoding other than the default, you should construct an
OutputStreamWriter
on a
FileOutputStream
and specify the encoding.
A program that prints the line in a file myfile.txt
import java.io.*; class FileOutputDemo
{
public static void main(String args[])
{
FileOutputStream out;
// declare a file output object
PrintStream p; // declare a print stream object
try
{
275
// Create a new file output stream
// connected to "myfile.txt"
out = new FileOutputStream("myfile.txt");
// Connect print stream to the output stream
p = new PrintStream( out , true);
p.println ("This is written to a file");
p.close();
}
catch (Exception e)
{
System.err.println ("Error writing to file");
}
}
}
Here is another version of the program, CopyBytes, which uses FileInputStream and
FileOutputStream instead of FileReader and FileWriter. import java.io.*; public class CopyBytes {
public static void main(String[] args) throws IOException {
File inputFile = new File("farrago.txt");
File outputFile = new File("outagain.txt");
FileInputStream in = new FileInputStream(inputFile);
FileOutputStream out = new FileOutputStream(outputFile);
int c;
while ((c = in.read()) != -1)
out.write(c);
in.close();
out.close();
}
}
An example to read from a file and print it on screen and also print the line numbers in front of each line import java.io.*; class ReadFileDemo { public static void main(String args[]) {
try {
BufferedReader in4 = new BufferedReader(li);
FileReader fin = new FileReader(“file1.txt”);
LineNumberReader li =new LineNumberReader(fin);
PrintWriter out1 = new PrintWriter(System.out, true);
String s=null; while((s = in4.readLine()) != null ) {
276
out1.println("Line " + li.getLineNumber() + s);
}
out1.close();
}
catch(EOFException e) {
System.out.println("End of stream");
}
}
}
An example of storing some content in a file and recovering it.
import java.io.*; public class FileStore {
public static void main(String[] args) {
try {
DataOutputStream out2 = new DataOutputStream( new BufferedOutputStream( new FileOutputStream("Data.txt")));
out2.writeDouble(3.14159);
out2.writeBytes("That was pi");
out2.close();
DataInputStream in5 = new DataInputStream( new BufferedInputStream( new FileInputStream("Data.txt")));
BufferedReader in5br = new BufferedReader( new InputStreamReader(in5));
System.out.println(in5.readDouble());
System.out.println(in5br.readLine());
}
catch(EOFException e) {
System.out.println("End of stream");
}
}
}
Another program that reads from a file using FileInputStream one byte at a time but checks whether the information is available before reading. import java.io.*; public class TestEOF {
public static void main(String[] args) {
try {
DataInputStream in = new DataInputStream( new BufferedInputStream(
new FileInputStream("TestEof.java")));
while(in.available() != 0) {
System.out.print((char)in.readByte());
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}
}
catch (IOException e) {
System.err.println("IOException");
}
}
}
Concatenating Files
The
SequenceInputStream
creates a single input stream from multiple input sources. This example program,
Concatenate
, uses
SequenceInputStream
to implement a concatenation utility that sequentially concatenates files file1.txt and file2.txt together.
This is the controlling class of the Concatenate utility: import java.io.*; public class Concatenate {
public static void main(String[] args) throws IOException {
FileInputStream fin1 = new FileInputStream(“file1.txt”);
FileInputStream fin2 = new FileInputStream(“file2.txt”);
SequenceInputStream s = new SequenceInputStream(fin1,fin2);
int c;
while ((c = s.read()) != -1)
System.out.write(c);
s.close();
}
}
11.7 Scanning text with java.util.Scanner
J2SE 5.0 adds classes and methods that can make every day tasks easier to perform. You will see how the newly added java.util.Scanner class makes it easier to read and parse strings and primitive types using regular expressions.
Before the J2SE 5.0 release, you probably would have written code such as the following
TextReader class to read text from a file: import java.io.*; public class TextReader { public static void main(String[] args) { try {
File file = new File(“TextSample.txt”);
FileReader reader = new FileReader(file);
BufferedReader in = new
BufferedReader(reader);
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String string = null;
while ((string = in.readLine()) != null) {
System.out.println(string);
}
in.close();
}
}
} catch (IOException e) {
e.printStackTrace();
}
The basic approach in classes like this is to create a File object that corresponds to the actual file on the hard drive. The class then creates a FileReader associated with the file and then a
BufferedReader from the FileReader. It then uses the BufferedFile reader to read the file one line at a time.
To view the TextReader class in action, you need to create a document for the class to read and parse. To create the document, save the following two lines of text in a file named
TextSample.txt in the same directory as TextReader:
Here is a small text file that you will
use to test java.util.scanner.
You should see the original file echoed back to you in standard output.
You can simplify the code in TextReader by using java.util.Scanner, a class that parses primitive types and strings:
import java.io.*;
import java.util.Scanner;
public class TextScanner {
public static void main (String args[]) {
try {
File file = new File(“TextSample.txt”);
Scanner scanner = new Scanner(file);
while (scanner.hasNext()) {
System.out.println(scanner.next());
}
scanner.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
}
}
}
You should get the following output:
279
Here
is
a
small
text
file
that
you
will
use
to
test
java.util.scanner.
TextScanner creates a Scanner object from the File. The Scanner breaks the contents of the File into tokens using a delimiter pattern, By default the delimiter pattern is whitespace. TextScanner then calls the hasNext() method in Scanner. This method returns true if another token exists in the Scanner's input, which is the case until it reaches the end of the file. The next() method returns a String that represents the next token. So until it reaches the end of the file, TextScanner prints the String returned by next() on a separate line.
You can change the delimiter that is used to tokenize the input, through the useDelimiter method of Scanner. You can pass in a String or a java.util.regex.Pattern to the method. For example, you can read the input one line at a time by using the newline character (\n) as a delimiter. Here is the revised code for TextScanner that uses a newline character as the delimiter:
try {
Scanner scanner = new Scanner(new File(fileName));
scanner.useDelimiter
(System.getProperty("line.separator"));
while (scanner.hasNext()) {
System.out.println(scanner.next());
}
scanner.close();
}
catch (FileNotFoundException e) {
e.printStackTrace();
}
One more example of using Scanner public class ScannerTest {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter int, double, word, line: "); int a = input.nextInt(); double x = input.nextDouble();
String word = input.next();
// returns a String up to next whitespace
String line = input.nextLine();
// returns rest of line & eats the '\n'
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System.out.println(a + ":" + x + ":" + word + ":" + line);
}
}
Sample output:
Enter int, double, word, line: -12 34.56 Great time had by all!
-12:34.56:Great: time had by all!
11.8 Redirecting Standard I/O
The methods in System class allow you to redirect the standard input, output, and error IO streams using simple static method calls: setIn(InputStream) setOut(PrintStream) setErr(PrintStream)
Redirecting output is especially useful if you suddenly start creating a large amount of output on your screen and it’s scrolling past faster than you can read it. Redirecting input is valuable for a command-line program in which you want to test a particular user-input sequence repeatedly. Here’s a simple example that shows the use of these methods: import java.io.*; class Redirecting {
public static void main(String[] args) {
try {
BufferedInputStream in = new BufferedInputStream( new FileInputStream("Redirecting.java"));
PrintStream out = new PrintStream(new BufferedOutputStream( new FileOutputStream("test.out"),true));
System.setIn(in);
System.setOut(out);
System.setErr(out);
BufferedReader br = new BufferedReader( new InputStreamReader(System.in));
//reads from the file
String s;
while((s = br.readLine()) != null) {
System.out.println(s); // write it to another file
}
out.close(); // close file output stream
}
catch(IOException e) {
e.printStackTrace();
}
}
}
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This program attaches standard input to a file, and redirects standard output and standard error to another file.
11.9 Working with Random Access Files
The input and output streams in this lesson so far have been sequential access streams--streams whose contents must be read or written sequentially. Although such streams are incredibly useful, they are a consequence of a sequential medium, such as paper and magnetic tape. A
random access file, on the other hand, permits nonsequential, or random, access to a file's contents. The
RandomAccessFile
class in the java.io
package implements a random access file.
The
RandomAccessFile
class implements both the
DataInput
and
DataOutput
interfaces and therefore can be used for both reading and writing.
RandomAccessFile
is similar to
FileInputStream
and
FileOutputStream
in that you specify a file on the native file system to open when you create it. You can do this with a file name or a File object. When you create a
RandomAccessFile
, you must indicate whether you will be just reading the file or also writing to it. (You have to be able to read a file in order to write it.) The following code creates a
RandomAccessFile
to read the file named farrago.txt
: new RandomAccessFile("farrago.txt", "r");
And this one opens the same file for both reading and writing: new RandomAccessFile("farrago.txt", "rw");
After the file has been opened, you can use the common read
or write
methods defined in the
DataInput
and
DataOutput
interfaces to perform I/O on the file.
RandomAccessFile supports the notion of a file pointer. The file pointer indicates the current location in the file. When the file is first created, the file pointer is set to 0, indicating the beginning of the file. Calls to the read and write methods adjust the file pointer by the number of bytes read or written.
In addition to the normal file I/O methods that implicitly move the file pointer when the operation occurs,
RandomAccessFile
contains three methods for explicitly manipulating the file pointer.
int skipBytes(int)
--Moves the file pointer forward the specified number of bytes
282
void seek(long)
--Positions the file pointer just before the specified byte
long getFilePointer()
--Returns the current byte location of the file pointer
An example of using RandomAccessFile import java.io.*; class files03{
public static void main(String[] args)
{
File junkFile = new File("junk.txt");
System.out.println("Now open and read the file for random access“);
try{
RandomAccessFile inData =
new RandomAccessFile(junkFile,"rw"); int temp;
System.out.println("Display the entire file as characters.");
while( (temp = inData.read()) != -1) {
System.out.print((char)temp);
}
System.out.println("Now display four bytes interior to the file.");
//Get current location of the file pointer.
long filePointer = inData.getFilePointer();
//Set the file pointer to a location interior to
// the file.
inData.seek(filePointer-8);
//Now read and display four bytes.
for(int cnt = 0; cnt < 4; cnt++)
System.out.print( (char)inData.read() );
System.out.println(“\nNow write four bytes interior to the file."); filePointer = inData.getFilePointer();
inData.seek(filePointer-4); for(int cnt = 0; cnt < 4; cnt++) inData.write('W'+cnt);
System.out.println("Now display the entire file again.");
System.out.println("Note that four bytes have been overwritten.");
//Note that it is necessary to reposition the file
// pointer to the beginning of the file.
inData.seek(0);
while( (temp = inData.read()) != -1)
System.out.print((char)temp);
inData.close();
}
catch(IOException e){}
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System.out.println("\nEnd of program");
}// end main
}//end class files03 definition
11.10 Filter Streams
The java.io
package provides a set of abstract classes that define and partially implement filter
streams. A filter stream filters data as it's being read from or written to the stream. The filter streams are
FilterInputStream
or
FilterOutputStream
,
FilterInputStream
, and
FilterOutputStream
. A filter stream is constructed on another stream (the underlying stream).
The read
method in a readable filter stream reads input from the underlying stream, filters it, and passes on the filtered data to the caller. The write
method in a writable filter stream filters the data and then writes it to the underlying stream. The filtering done by the streams depends on the stream. Some streams buffer the data, some count data as it goes by, and others convert data to another form.
Most filter streams provided by the java.io package are subclasses of FilterInputStream and
FilterOutputStream and are listed here:
DataInputStream
and
DataOutputStream
BufferedInputStream
and
BufferedOutputStream
LineNumberInputStream
PushbackInputStream
PrintStream
(This is an output stream.)
The java.io
package contains only one subclass of
FilterReader: PushbackReader
.
Using Filter Streams
To use a filter input or output stream, attach the filter stream to another input or output stream when you create it. For example, you can attach a filter stream to the standard input stream, as in the following code:
BufferedReader d = new BufferedReader(new DataInputStream(System.in));
String input; while ((input = d.readLine()) != null) {
... //do something interesting here
}
Note that the readLine
method has been deprecated in the
DataInputStream
; therefore we've wrapped it in a
BufferedReader
.
How to Use DataInputStream and DataOutputStream
This page shows you how to use the java.io
DataInputStream
and
DataOutputStream classes. It features an example,
DataIODemo
, that reads and writes tabular data (invoices for
284
merchandise). The tabular data is formatted in columns separated by tabs. The columns contain the sales price, the number of units ordered, and a description of the item. Conceptually, the data looks like this, although it is read and written in binary form and is non-ASCII:
19.99 12 Java T-shirt
9.99 8 Java Mug
DataOutputStream
, like other filtered output streams, must be attached to another
OutputStream
. In this case, it's attached to a
FileOutputStream
that is set up to write to a file named invoice1.txt
:
DataOutputStream out = new DataOutputStream( new FileOutputStream("invoice1.txt"));
Next,
DataIODemo
uses
DataOutputStream
's specialized write
methods to write the invoice data contained within arrays in the program according to the type of data being written: for (int i = 0; i < prices.length; i ++) {
out.writeDouble(prices[i]);
out.writeChar('\t');
out.writeInt(units[i]);
out.writeChar('\t');
out.writeChars(descs[i]);
out.writeChar('\n');
} out.close();
Next,
DataIODemo
opens a
DataInputStream
on the file just written:
DataInputStream in = new DataInputStream(
new FileInputStream("invoice1.txt"));
DataInputStream
also must be attached to another
InputStream
; in this case, a
FileInputStream
set up to read the file just written, invoice1.txt
. Then
DataIODemo
just reads the data back in using
DataInputStream
's specialized read methods. try {
while (true) {
price = in.readDouble();
in.readChar(); //throws out the tab
unit = in.readInt();
in.readChar(); //throws out the tab
char chr;
desc = new StringBuffer(20);
char lineSep =
System.getProperty("line.separator").charAt(0);
while ((chr = in.readChar() != lineSep) {
desc.append(chr);
}
285
System.out.println("You've ordered " + unit +" units of "
+ desc + " at $" + price);
total = total + unit * price;
}
} catch (EOFException e) { }
System.out.println("For a TOTAL of: $" + total); in.close();
When all of the data has been read,
DataIODemo
displays a statement summarizing the order and the total amount owed and then closes the stream.
Note the loop that DataIODemo uses to read the data from the DataInputStream. Normally, when data is read, you see loops like this: while ((input = in.read()) != null) {
. . .
}
The read
method returns a value, null
, which indicates that the end of the file has been reached.
Many of the
DataInputStream
read methods can't do this, because any value that could be returned to indicate the end of file may also be a legitimate value read from the stream. For example, suppose that you want to use
-1
to indicate end of file. Well, you can't, because
-1
is a legitimate value that can be read from the input stream, using readDouble
, readInt
, or one of the other methods that reads numbers. So
DataInputStream s read methods throw an
EOFException
instead. When the
EOFException
occurs, the while (true)
terminates. import java.io.*; public class DataIODemo {
public static void main(String[] args) throws IOException {
// write the data out
DataOutputStream out = new DataOutputStream(new
FileOutputStream("invoice1.txt"));
double[] prices = { 19.99, 9.99, 15.99, 3.99, 4.99 };
int[] units = { 12, 8, 13, 29, 50 };
String[] descs = { "Java T-shirt",
"Java Mug",
"Duke Juggling Dolls",
"Java Pin",
"Java Key Chain" };
for (int i = 0; i < prices.length; i ++) {
out.writeDouble(prices[i]);
out.writeChar('\t');
out.writeInt(units[i]);
out.writeChar('\t');
out.writeChars(descs[i]);
out.writeChar('\n');
286
}
out.close();
// read it in again
DataInputStream in = new DataInputStream(new
FileInputStream("invoice1.txt"));
double price;
int unit;
StringBuffer desc;
double total = 0.0;
try {
while (true) {
price = in.readDouble();
in.readChar(); // throws out the tab
unit = in.readInt();
in.readChar(); // throws out the tab char chr; desc = new StringBuffer(20); char lineSep =
System.getProperty("line.separator").charAt(0); while ((chr = in.readChar()) != lineSep)
desc.append(chr);
System.out.println("You've ordered " +
unit + " units of " +desc + " at $" + price);
total = total + unit * price;
}
}
catch (EOFException e) {
}
System.out.println ("For a TOTAL of: $" + total);
in.close();
}
}
When you run the DataIODemo program you should see the following output:
You've ordered 12 units of Java T-shirt at $19.99
You've ordered 8 units of Java Mug at $9.99
You've ordered 13 units of Duke Juggling Dolls at $15.99
You've ordered 29 units of Java Pin at $3.99
You've ordered 50 units of Java Key Chain at $4.99
For a TOTAL of: $892.8800000000001
11.11 Object Serialization
Two streams in java.io
--
ObjectInputStream
and
ObjectOutputStream
-- are byte streams and work like the other input and output streams. However, they are special in that they can read and write objects.
287
The key to writing an object is to represent its state in a serialized form sufficient to reconstruct the object as it is read. Thus reading and writing objects is a process called object serialization.
Object serialization is essential to building all but the most transient applications. You can use object serialization in the following ways:
•
•
Remote Method Invocation (RMI)--communication between objects via sockets
Lightweight persistence--the archival of an object for use in a later invocation of the same program.
You need to know about object serialization from two points of view. First, you need to know how to serialize objects by writing them to an
ObjectOutputStream
and reading them in again using an
ObjectInputStream
.
Serializing Objects
Reconstructing an object from a stream requires that the object first be written to a stream.
How to Write to an ObjectOutputStream
Writing objects to a stream is a straightforward process. For example, the following gets the current time in milliseconds by constructing a
Date
object and then serializes that object:
FileOutputStream out = new FileOutputStream("theTime");
ObjectOutputStream s = new ObjectOutputStream(out); s.writeObject("Today"); s.writeObject(new Date()); s.flush();
ObjectOutputStream
must be constructed on another stream. This code constructs an
ObjectOutputStream
on a
FileOutputStream
, thereby serializing the object to a file named theTime
. Next, the string
Today
and a
Date
object are written to the stream with the writeObject
method of
ObjectOutputStream
.
Thus, the writeObject method serializes the specified object, traverses its references to other objects recursively, and writes them all. In this way, relationships between objects are maintained.
ObjectOutputStream implements the DataOutput interface that defines many methods for writing primitive data types, such as writeInt, writeFloat, or writeUTF. You can use these methods to write primitive data types to an ObjectOutputStream.
The writeObject method throws a NotSerializableException if it's given an object that is not serializable. An object is serializable only if its class implements the Serializable interface.
How to Read from an ObjectInputStream
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Once you've written objects and primitive data types to a stream, you'll likely want to read them out again and reconstruct the objects. This is also straightforward. Here's code that reads in the
String
and the
Date
objects that were written to the file named theTime
in the previous example:
FileInputStream in = new FileInputStream("theTime");
ObjectInputStream s = new ObjectInputStream(in);
String today = (String)s.readObject();
Date date = (Date)s.readObject();
Like
ObjectOutputStream
,
ObjectInputStream
must be constructed on another stream. In this example, the objects were archived in a file, so the code constructs an
ObjectInputStream
on a
FileInputStream
. Next, the code uses
ObjectInputStream
's readObject
method to read the
String
and the
Date
objects from the file. The objects must be read from the stream in the same order in which they were written. Note that the return value from readObject
is an object that is cast to and assigned to a specific type.
The readObject method deserializes the next object in the stream and traverses its references to other objects recursively to deserialize all objects that are reachable from it. In this way, it maintains the relationships between the objects.
ObjectInputStream stream implements the DataInput interface that defines methods for reading primitive data types. The methods in DataInput parallel those defined in DataOutput for writing primitive data types. They include methods such as readInt, readFloat, and readUTF. Use these methods to read primitive data types from an ObjectInputStream.
Providing Object Serialization for Your Classes
An object is serializable only if its class implements the
Serializable
interface. Thus, if you want to serialize the instances of one of your classes, the class must implement the
Serializable
interface. The good news is that
Serializable
is an empty interface. That is, it doesn't contain any method declarations; its purpose is simply to identify classes whose objects are serializable.
Implementing the Serializable Interface
Here's the complete definition of the
Serializable
interface: package java.io; public interface Serializable {
// there's nothing in here!
};
Making instances of your classes serializable is easy. You just add the implements
Serializable
clause to your class declaration like this: public class MySerializableClass implements Serializable {
...
289
}
You don't have to write any methods. The serialization of instances of this class are handled by the defaultWriteObject
method of
ObjectOutputStream
. This method automatically writes out everything required to reconstruct an instance of the class, including the following:
Class of the object
Class signature
Values of all nontransient
and nonstatic
members, including members that refer to other objects
You can deserialize any instance of the class with the defaultReadObject
method in
ObjectInputStream
.
Protecting Sensitive Information
When developing a class that provides controlled access to resources, you must take care to protect sensitive information and functions. During deserialization, the private state of the object is restored. For example, a file descriptor contains a handle that provides access to an operating system resource. Being able to forge a file descriptor would allow some forms of illegal access, since restoring state is done from a stream. Therefore the serializing runtime must take the conservative approach and not trust the stream to contain only valid representations of objects.
To avoid compromising a class, you must provide either that the sensitive state of an object must not be restored from the stream or that it must be reverified by the class.
Several techniques are available to protect sensitive data in classes. The easiest is to mark fields that contain sensitive data as private transient. transient and static fields are not serialized or deserialized. Marking the field will prevent the state from appearing in the stream and from being restored during deserialization. Since writing and reading (of private fields) cannot be superseded outside of the class, the class's transient fields are safe.
Particularly sensitive classes should not be serialized. To accomplish this, the object should not implement either the Serializable interface.
Some classes may find it beneficial to allow writing and reading but to specifically handle and revalidate the state as it is deserialized. The class should implement writeObject and readObject methods to save and restore only the appropriate state. If access should be denied, throwing a
NotSerializableException will prevent further access.
An example of Serialization : import java.io.*; class MyClass implements Serializable {
String s; int i; double d;
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public MyClass(String s, int i, double d) { this.s = s; this.i = i; this.d = d;
} public String toString() { return “s =” + s + “ , i =” + i + “, d =” + d;
}
} public class SerializationDemo { try {
FileOutputStream fout = new
FileOutputStream(“serial”);
ObjectOutputStream os = new ObjectOutputStream(fout); os.writeObject(“Hello”); os.writeObject(new java.util.Date()); os.writeObject(new MyClass()); os.flush(); os.close();
} catch(Exception e) {
System.out.println(e);
}
//object deserialization try {
FileInputStream fin = new FileInputStream(“serial”);
ObjectInputStream is = new ObjectInputStream(fin);
String z = (String) is.readObject(); java.util.Date date = (java.util.Date) is.readObject();
MyClass obj = (MyClass) is.readObject(); is.close();
System.out.println(s);
System.out.println(date);
System.out.println(obj);
} catch(Exception e) {
System.out.println(e);
}
}
Summary
The System class provides three built in streams in java – in, out and err. All the api classes for performing input/output in java are a part of java.io package. There are two category of I/O classes – Byte stream and Character stream classes. There are separate classes that can be used for reading and writing into files. The File class can be used to get the properties of the file.
291
Chapter 12 : API classes in java.lang package
By default, each Java application/applet has access to the java.lang package. Inside java.lang are classes that represent primitive data types (such as int & char), as well as more complex classes.
It contains classes pertaining to strings, string buffers, threads, and even the System class from which we obtain input and output streams.
12.1 Wrapper classes
Wrappers are used to enclose a simple datatype or a primitive object into an object. This is sometimes necessary because:
•
•
•
•
•
Simple datatypes are not part of the object hierarchy.
Simple datatypes are passed by value and not by reference.
Two methods can't refer to the same instance of a simple type.
Some classes can only use members of another class and not a simple type.
You can store a value of primitive type in a type-wrapper object whenever an object is required.
There are eight wrapper classes – one for each primitive data type.
Primitive Wrapper boolean java.lang.Boolean
byte char
java.lang.Byte
java.lang.Character
double java.lang.Double
float java.lang.Float
int long short
java.lang.Integer
java.lang.Long
java.lang.Short
All numeric wrapper classes are subclasses of the abstract class Number.
The following figure shows the class hierarchy for the Number classes provided by the
Java platform
292
BigInteger and BigDecimal extend the primitive data types to allow for arbitrary-precision numbers (numbers that might not fit into any of the primitive data types). Note that whereas the other classes are in the java.lang package, BigDecimal and BigInteger are in the java.math package.
The Number class provides these methods
byte byteValue();
short shortValue();
int intValue();
long longValue();
float floatValue();
double doubleValue();
The classes define useful variables, such as
MIN_VALUE
and
MAX_VALUE
, that provide minimum and maximum value in the range of the data type.
All wrapper objects are immutable. Once an object is created, the wrapped primitive value cannot be changed. Wrapper classes are final and hence cannot be sub classed.
The Boolean class wrapper allows passing Boolean values (true and false) by reference. It contains the constants TRUE and FALSE, which define true and false Boolean objects.
All the wrapper classes except Character have two constructors -- one that takes the primitive value and another that takes the String representation of the value.
For instance:
Integer i1 = new Integer(50);
Integer i2 = new Integer("50"); double d = 5.0;
Double aD = new Double(d);
All wrapper classes have parseType methods eg parseInt(), parseShort, etc that take a String and parse it into the appropriate type. The six parser methods are parseInt, parseDouble, parseFloat, parseLong, parseByte, and parseShort. They take a String as the argument and convert it to the corresponding primitive. They throw a NumberFormatException if the String is not properly formed. For example, double d = Double.parseDouble("4.23");
It can also take a radix(base) as the second argument:
int i = Integer.parseInt("10011110",2);
All wrapper classes (except Character) define a static method called valueOf(), which returns the wrapper object corresponding to the primitive value represented by the String argument. For example,
293
Integer I = Integer.valueOf("10011110",2);
Float f1 = Float.valueOf("1.5f");
Integer, Short, Byte and Long
The Integer and Long classes also have the static methods toBinaryString(), toOctalString() and toHexString() which take an integer value and convert it to the appropriate String representation.
The toHexString returns a string which is a hex string version of the number. It has a natural partner in the toBinaryString method which returns a string that represents the number in its binary version. The following example code will output the strings 100 followed by 10.
public class NumberFormats{
public static void main(String argv[]){
System.out.println(Integer.toBinaryString(4));
System.out.println(Integer.toHexString(16));
}
}
Integer, and the longer form, Long, represent whole number values. Integers and Longs can be interchanged through the longValue() and intValue() methods, and can also be converted to floats and doubles using the floatValue() and doubleValue().
Integer my_integer = new Integer(256);
Long my_long = my_integer.longValue();
One more example of converting a wrapper object value to primitive data type
Integer i = new Integer(20); byte b = i.byteValue();
Float and Double
The constructors for Float and Double are shown below:
Float (double num)
Float (float num)
Float (String s) throws NumberFormatException
Double (double num)
Double (String s) throws NumberFormatException
Apart from the constants MIN_VALUE and MAX_VALUE these classes also have other constants like POSITIVE_INFINITY, NEGATIVE_INFINITY, and NaN and the following methods to test a value public boolean isNaN() public static boolean isNaN(type value) public boolean isInfinite() public static boolean isInfinite(type value)
294
An example to show the use of isInfinite() and isNaN() class InfNaN { public static void main(String args[]) {
Double d1 = new Double(1/0.);
Double d2 = new Double(0/0.);
System.out.println(d1 + “ : ”+ d1.isInfinite() +
“,” + d1.isNaN());
System.out.println(d2 + “ : ”+ d2.isInfinite() +
“,” + d2.isNaN());
}
}
This program generates the following output:
Infinity : true, false
NaN : false, true
Both classes have methods to convert a value into a bit pattern or vice versa public static int floatToIntBits(float value) public static float intBitsToFloat(int bits) public static long doubleToLongBits(double value) public static double longBitsToDouble(long bits)
Floating point values, and the longer form, double, represent decimal (fractional) values. Floats and doubles can be interchanged through the doubleValue() and floatValue() methods, and can also be converted to integers and longs using the longValue() and intValue() methods. Its important to remember, however, that there will be a loss of precision, as integers and longs cannot retain the fractional component.
Float my_float = new Float(3.14);
Double my_double = new Double (my_float.doubleValue());
// Print out double (3.14)
System.out.println( "Double : " + my_double);
// Print out integer (3)
System.out.println( "Integer: " + my_double.intValue() );
Here's an example, called NumberDemo, that creates two Float objects and one Double object and then uses compareTo and equals to compare them:. public class NumberDemo {
public static void main(String args[]) {
Float floatOne = new Float(14.78f - 13.78f);
Float floatTwo = Float.valueOf("1.0");
Double doubleOne = new Double(1.0);
int difference = floatOne.compareTo(floatTwo);
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if (difference == 0) {
System.out.println("floatOne is equal to floatTwo.");
}
else if (difference < 0) {
System.out.println("floatOne is less than floatTwo.");
}
else if (difference > 0) {
System.out.println("floatOne is greater than floatTwo.");
}
System.out.println("floatOne is " +
((floatOne.equals(doubleOne)) ? "equal" :
"not equal") + " to doubleOne.");
}
}
The output from this program might surprise you a little: floatOne is equal to oneAgain.
floatOne is not equal to doubleOne.
Even though the values contained in floatOne and doubleOne are both numerically equal to 1, they are considered unequal because the objects are of different types. The numeric type-wrapper classes ( Byte, Integer, Double, Float, Long, and Short) each provide a class method named valueOf that converts a string to an object of that type.
Character
The Character class constructor takes a char type element as an argument:
Character c = new Character('A');
To obtain the char value contained in a Character object, call charValue() shown here, char charValue()
The Character class contains a large set of character comparison routines, in the form of static methods.
static boolean isDigit( char c );
static boolean isLetter( char c );
static boolean isLetterOrDigit( char c );
static boolean isLowerCase( char c );
static boolean isUpperCase( char c );
static char toUpperCase( char c );
static char toLowerCase( char c );
A program is given below:
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class IsDemo { public static void main(String args[]) { char a [] = {‘a’, ‘b’, ‘5’, ‘7’, ‘A’, ‘ ’}; for ( int i = 0; i < a.length ; i++) { if(Character.isDigit(a[i]))
System.out.println(a[i]+ “is a digit”); if(Character.isLetter(a[i]))
System.out.println(a[i]+ “is a letter”); if(Character.isWhitespace(a[i]))
System.out.println(a[i]+ “is whitespace”); if(Character.isUpperCase(a[i]))
System.out.println(a[i]+ “is uppercase”); if(Character.isLowerCase(a[i]))
System.out.println(a[i]+ “is lowercase”);
}
}
}
Character contains two methods for returning the numeric value of a character in the various number systems. public static int digit(char ch, int radix) public static int getNumber(char ch) and one method to return the character value of a number public static char forDigit(int digit, int radix)
12.2 Autoboxing/Unboxing of Wrappers
In the previous section on wrapper classes for primitive type values, we discussed how to create an instance of a wrapper from a primitive value and conversely, how to obtain the primitive value held by the wrapper. This involves a a certain amount of clumsy code.
For example, creating a Float object from a float primitive is straightforward: float primitive_float = 3.0f;
Float wrapper_float = new Float (primitive_float);
Going the other direction, however, requires explicitly calling the floatValue() method on the
Float object: float primitive_float = wrapper_float.floatValue();
If you are dealing with a lot of instances of wrappers and conversions, you will thus need to deal with a lot of method invocations. In J2SE 5.0, however, the code to create the wrapper object allows for this much simpler form:
Float wrapper_float = primitive_float;
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Here, the "wrapping" is done automatically! There is no need to explicitly call the Float constructor. This "wrapping" is called "autoboxing" in the sense that the primitive value is automatically "boxed up" into the wrapper object. Autoboxing is available for all the primitive/wrapper types.
Going the other way, from object type to primitive, is just as simple:
Integer wrapper_integer = 5;
// primitive 5 autoboxed into an Integer
int primitive_int = wrapper_integer;
// automatic unboxing Integer into int
These shortcuts simplify coding and reduce errors in J2SE 5.0. For example, they can even be used in loop control and incrementing and decrementing operations.
int MAX = 100; // a primitive int type
Integer counter = 1; // an Integer type
Integer sum = 0; // ditto
while (true) {
sum += counter;
if (counter == MAX) break;
counter++;
}
System.out.println ("counter is now " + counter);
System.out.println ("sum is now " + sum);
There is a lot of hidden autoboxing and unboxing going on in this simple-looking code. First, the
Integer types counter and sum are autoboxed from the primitive values 1 and 0. Then, in the loop, they are unboxed to primitive values so the += operation can be applied and then reboxed to their "native" Integer types.
To do the == comparison counter is unboxed so it can be compared with the int type MAX. If the break does not apply, then counter is unboxed, operated on with ++, and then reboxed.
Autoboxing and unboxing work in a for loop as well:
Integer sum = 0;
for (Integer counter=1; counter < MAX; counter++) {
sum += counter;
}
Note that both of these loops are likely to perform very slowly with all these autoboxing and unboxing operations. An optimizing compiler might be able to avoid some of the autoboxing and unboxing operations, but in general you should do long looping operations with primitive types unless there is a very good reason to use a wrapper type.
Autoboxing and unboxing also work with Boolean and boolean types. For example,
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boolean one = true; // nothing new here
Boolean two = true;
// autoboxing of primitive 'true' to Boolean type
if (one && two) // auto unboxing do_something ();
Before 5.0, the if, while, and do-while statements all expected boolean expressions. Through the use of unboxing, those flow control statements now also accept expressions that evaluate to
Boolean types. Similarly, the old switch statement expects a byte, short, int, or char type in Java
1.4 and below. With the addition of autoboxing in 5.0, switch now also accepts Byte, Short,
Integer, and Character types.
Autoboxing and Overloading
Autoboxing and unboxing can make method overloading interesting. Consider the two overloaded methods shown here
long method1 (long l) { return l+1; }
long method1 (Integer i) { return i+2; }
If you call method1() with a primitive long parameter, then the first method1() is used. If you call method1() with an Integer object parameter, then the second method1() is used. There is nothing new there. But what happens if you call method1() with an int parameter? In J2SE 1.4 and below, the int is promoted to a long and the first method1() is used. With autoboxing, it is conceivable that the int could be boxed into an Integer type and the second method1() used. That might even be what you want to happen - it might make more sense to convert an int to an
Integer than to promote it to a long. While arguably reasonable, that is not what happens. The general rule is, for compatibility reasons, the same behavior that applied in pre-5.0 versions must continue to hold. The reason is that existing code cannot suddenly start behaving differently when compiled and run under 5.0.
12.3 Math class
The Math class provides the important mathematical constants E and PI which are of type double. All methods are of Math class are static. The Math class cannot be instantiated as it has a private constructor. It cannot be subclassed as it is a final class. The Math class also provides many useful math functions as methods.
Group
Methods
Transcendental acos(x), asin(x), atan(x), atan2(x,y), cos(x), sin(x), tan(x)
Exponential
exp(x), log(x), pow(x,y), sqrt(x)
Rounding
abs(x), ceil(x), floor(x), max(x,y), min(x,y), rint(x), round(x)
Miscellaneous IEEEremainder(x,y), random(), toDegrees(x), toRadians(x)
Methods of java.lang.Math class abs
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It strips off the sign of a number and returns it simply as a number. Thus the following will simply print out 99. If the number is not negative you just get back the same number.
System.out.println(Math.abs(-99));
Math.abs(1234.59): 1234.59
Math.abs(-0.0): 0.0
Math.abs(Float.NEGATIVE_INFINITY): Infinity
Math.abs(Float.NaN): NaN
If the value is equal to Integer.MIN_VALUE, in which case, it returns the value as a negative
Math.abs(Integer.MIN_VALUE): -2147483648
ceil
This method returns the next whole number up that is an integer. Thus if you pass ceil(1.1) it will return a value of 2.0. If you change that to ceil(-1.1) the result will be -1.0;
If the argument is NaN or infinity, returns the argument
Math.ceil( 9.01): 10.0
// counts up (away from zero)
Math.ceil(-9.01): -9.0 // counts up (towards zero)
Math.ceil(10): 10.0
Math.ceil(-0.03): -0.0
Math.ceil(Double.NaN): NaN
floor
This method returns the largest (closest to positive infinity) double value that is not greater than the argument and is equal to a mathematical integer.
Here is a short program and its output public class MyMat{ public static void main(String[] argv){
System.out.println(Math.floor(-99.1));
System.out.println(Math.floor(-99));
System.out.println(Math.floor(99));
System.out.println(Math.floor(-.01));
System.out.println(Math.floor(0.1));
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}
}
And the output is
-100.0
-99.0
99.0
-1.0
0.0
If the argument is an integer, returns the argument. If the argument is NaN, infinity, negative or positive zero, returns the argument.
max and min
min() returns the smallest of two values. max() returns the largest of two values.
Math.min(-1.5, 1.5): -1.5
Math.max(-1.5, 1.5): 1.5
Math.min(0.0, -0.0): -0.0
// zeros are not equivalent
Math.min(Float.NaN,Float.POSITIVE_INFINITY) : NaN
The following code illustrates how these methods work public class MaxMin{ public static void main(String argv[]){
System.out.println(Math.max(-1,-10));
System.out.println(Math.max(1,2));
System.out.println(Math.min(1,1));
System.out.println(Math.min(-1,-10));
System.out.println(Math.min(1,2));
}
}
Here is the output
-1
2
1
-10
1
round
public static long round(double a) public static int round(float a)
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Rounds to the nearest integer. So, if the value is more than half way towards the higher integer, the value is rounded up to the next integer. If the number is less than this the next lowest integer is returned. So for example if the input to round is x then :
2.0 <= x < 2.5. then Math.round(x)==2.0
2.5 <= x < 3.0 the Math.round(x)==3.0
If the argument is not a number, returns zero. if the argument is a negative infinity or less than the MIN_VALUE for the type, returns the MIN_VALUE. If the argument is a positive infinity or greater than the MAX_VALUE for the type, returns the MAX_VALUE. If the value is
Float.MAX_VALUE the round method returns Integer.MAX_VALUE
Math.round( 1.5): 2
Math.round(-1.5): -1
Math.round(Float.NaN): 0
Math.round(Float.NEGATIVE_INFINITY): -2147483648
Math.round(Double.POSITIVE_INFINITY): 9223372036854775807
Math.round(Float.MAX_VALUE): 2147483647
(Float.MAX_VALUE is 3.4028235E38)
Here are some samples with output
Output:
System.out.println(Math.round(1.01));
System.out.println(Math.round(-2.1));
System.out.println(Math.round(20));
1
-2
20
rint
It rounds to the closest integer. If integers are equidistant, favours the even integer.
Math.rint( 5.5): 6.0
Math.rint(-5.5): -6.0
Math.rint( 5.49): 5.0
Math.rint(-5.49): -5.0
random
Returns a random number between 0.0 and 1.0.
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Unlike some random number system Java does not appear to offer the ability to pass a seed number to increase the randomness. Often a program will want to produce a random number between say 0 and 10 or 0 and 100. The following code combines math code to produce a random number between 0 and 100.
System.out.println(Math.round(Math.random()*100));
sqrt
This method returns the positive square root of a number. It returns NaN if argument is negative
Math.sqrt(45): 6.708203932499369
Math.sqrt(-45): NaN
IEEEremainder
This method calculates the remainder as defined by IEEE-754. The remainder operator, %, makes values symmetric around zero ie negative and positive values return corresponding remainders
7 % 2.5 : 2.0
-7 % 2.5 : -2.0
Math.IEEEremainder keeps resulting values y units apart
Math.IEEEremainder( 7, 2.5): -0.5
Math.IEEEremainder(-7, 2.5): 0.5
pow
This method returns the first argument raised to the power of the second argument.
Math.pow(2,2) 4.0
log
log() returns the natural logarithm of the argument. If the argument is less than zero, returns
NaN. If the argument is positive infinity, returns positive infinity. If the argument is -0.0 or 0.0, returns negative infinity.
Math.log(10): 2.302585092994046
Math.log(-10): NaN
Math.log(0.0): -Infinity
exp
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exp() returns e to the power of the argument. If the argument is NaN, returns NaN. If the argument is positive infinity, returns positive infinity. If the argument is negative infinity, returns positive zero.
Math.exp(5): 148.4131591025766
Math.exp(Float.NaN): NaN
Math.exp(Float.POSITIVE_INFINITY): Infinity
Math.exp(Float.NEGATIVE_INFINITY): 0.0
An example program using Math class: public class ExponentialDemo {
public static void main(String[] args) {
double x = 11.635;
double y = 2.76;
System.out.println("The value of e is " + Math.E);
System.out.println("exp(" + x + ") is " + Math.exp(x));
System.out.println("log(" + x + ") is " + Math.log(x));
System.out.println("pow(" + x + ", " + y + ") is " +
Math.pow(x, y));
System.out.println("sqrt(" + x + ") is " + Math.sqrt(x));
}
}
Here's the output you'll see when you run ExponentialDemo:
The value of e is 2.71828
exp(11.635) is 112984 log(11.635) is 2.45402
pow(11.635, 2.76) is 874.008
sqrt(11.635) is 3.41101
The trigonometric methods
All results are returned in radians.
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Method double sin(double) double cos(double) double tan(double) double asin(double) double acos(double) double atan(double)
Description
Returns the sine of the specified double value. If the result is NaN or infinity, returns NaN. if the result is negative zero, returns -0.0
Math.sin(90) 0.8939966636005579
Returns the cosine of the specified double value. If the result is NaN or infinity, returns NaN.
Math.cos(90) -0.4480736161291701
Returns the tangent of the specified double value. If the result is NaN or infinity, returns NaN. If the result is negative zero, returns -0.0
Math.tan(90) -1.995200412208242
Returns the arc sine of the specified double value. It returns a value between -PI/2 and PI/2. If the result is NaN or absolute value is greater than 1, returns NaN. If the result is negative zero, returns -0.0.
Math.asin(-0) 0.0
Returns the arc cosine of the specified double value. It returns a value between 0.0 and PI. If the result is NaN or absolute value is greater than 1, returns NaN.
Math.acos(-0)
1.5707963267948966
Returns the arc tangent of the specified double value. It returns a value between -PI/2 and PI/2. If the result is NaN, returns NaN. If the result is negative zero, returns -0.0
Math.atan(90) 1.5596856728972892
double atan2(double)
Converts rectangular coordinates (b, a) to polar (r, theta). double toDegrees(double) double toRadians(double)
Converts the argument to degrees or radians as indicated by the method name.
Math.toRadians(90)
1.5707963267948966
Math.toDegrees(Math.PI/2) 90.0
An example of using trigonometric functions public class TrigonometricDemo {
public static void main(String[] args) {
double degrees = 45.0;
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double radians = Math.toRadians(degrees);
System.out.println("The value of pi is " + Math.PI);
System.out.println("The sine of " + degrees + " is " +
Math.sin(radians));
System.out.println("The cosine of " + degrees + " is " +
Math.cos(radians));
System.out.println("The tangent of " + degrees + " is " +
Math.tan(radians));
System.out.println("The arc sine of " +
Math.asin(radians) + " is " +
Math.toDegrees(Math.asin(Math.sin(radians))) +
“degrees");
System.out.println("The arc cosine of " +
Math.cos(radians)+ " is " +
Math.toDegrees(Math.acos(Math.cos(radians))) + “degrees");
System.out.println("The arc tangent of” +Math.tan(radians)
+ " is " + Math.toDegrees(Math.atan(Math.tan(radians))) +
" degrees");
}
}
The output of this program is as follows:
The value of pi is 3.141592653589793
The sine of 45.0 is 0.8060754911159176
The cosine of 45.0 is -0.5918127259718502
The tangent of 45.0 is -1.3620448762608377
The arc sine of 45.0 is NaN
The arc cosine of 45.0 is NaN
The arc tangent of 45.0 is 1.570408475869457
12.4 System class
The System class provides access to the native operating system's environment through the use of
static methods. As an example System.currentTimeMillis() retrieves the system clock setting (as a long, in milliseconds counted from January 1, 1970). Some of the available methods are: currentTime() freeMemory() totalMemory() exit(int status) exec(String cmd) getOSName() arraycopy(src[], srcpos, dest[], destpos, len)
The
System
class maintains a set of properties, key/value pairs, that define traits or attributes of the current working environment. When the runtime system first starts up, the system properties are initialized to contain information about the runtime environment. including information about the current user, the current version of the Java runtime, and even the character used to separate components of a filename.
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Here is a complete list of the system properties you get when the runtime system first starts up and what they mean:
Key
"file.separator"
Meaning
File separator (for example, "
/
")
"java.class.path"
"java.class.version"
"java.home"
"java.vendor"
"java.vendor.url"
"java.version"
Java classpath
Java class version number
Java installation directory
Java vendor-specific string
Java vendor URL
Java version number
"line.separator"
Line separator
"os.arch"
"os.name"
"os.version"
Operating system architecture
Operating system name
Operating system version
"path.separator"
Path separator (for example, "
:
")
"user.dir"
"user.home"
"user.name"
User's current working directory
User home directory
User account name
Your Java programs can read or write system properties through several methods in the System class. You can use a key to look up one property in the properties list, or you can get the whole set of properties all at once. You can also change the set of system properties completely. For example, you can use the method
System.getProperty(“file.separator”); to get the file separator character of the OS.
An example to get the user name : class UserNameTest { public static void main(String[] args) {
String name;
name = System.getProperty("user.name");
System.out.println(name);
}
}
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The System class also provides very basic io streams for console read, write and error operations.
System.in.read() reads a keystroke and returns an integer value. System.out.println(string) displays a string to the current output device.
Use of some methods of System class
To terminate an application
// No errors
int errorCode = 0;
// An error occurred
errorCode = -1;
// Terminate
System.exit(errorCode);
To compute elapsed time
// Get current time
long start = System.currentTimeMillis();
// Do something ...
// Get elapsed time in milliseconds
long elapsedTimeMillis = System.currentTimeMillis()-start;
// Get elapsed time in seconds
float elapsedTimeSec = elapsedTimeMillis/1000F;
// Get elapsed time in minutes
float elapsedTimeMin = elapsedTimeMillis/(60*1000F);
// Get elapsed time in hours
float elapsedTimeHour = elapsedTimeMillis/(60*60*1000F);
// Get elapsed time in days
float elapsedTimeDay = elapsedTimeMillis/(24*60*60*1000F);
12.5 Runtime class
The Runtime class encapsulates the runtime environment. You cannot instantiate a Runtime object. You can get the reference of the current Runtime object by calling the getRuntime() method. Once you obtain the reference to the current runtime you can call several methods that control the state and behavior of JVM.
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Runtime can be used to get the size of the heap. The heap is the area in memory in which objects are created.
// Get current size of heap in bytes
long heapSize = Runtime.getRuntime().totalMemory();
// Get maximum size of heap in bytes.
//The heap cannot grow beyond this size.
// Any attempt will result in an OutOfMemoryException.
long heapMaxSize = Runtime.getRuntime().maxMemory();
// Get amount of free memory within the heap in bytes.
//This size will increase
// after garbage collection and decrease as new objects
//are created.
long heapFreeSize = Runtime.getRuntime().freeMemory();
To execute an application, you can use the exec method of the Runtime object class ExecDemo { public static void main(String args[]) {
Runtime r = Runtime.getRuntime();
Process p = null; try { p = r.exec(“notepad”);
} catch(Exception e) {
System.out.println(e);
}
}
}
The abstract class Process encapsulates a process- that is a program in execution. You can kill the subprocess created by the exec() method by destroy() method. The waitFor() method causes your program to wait until the subprocess finishes. The exitValue() returns the value returned by the subprocess when it is finished. This is typically 0 if no problems occur. class ExecDemoFini { public static void main(String args[]) {
Runtime r = Runtime.getRuntime();
Process p = null; try { p = r.exec(“notepad”); p.waitFor();
} catch(Exception e) {
System.out.println(e);
}
System.out.println(“Notepad returned :”
+p.exitValue());
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}
}
12.6 Class class
Class encapsulates the run-time state of an object or interface. Objects of type Class are created automatically, when classes are loaded. You cannot explicitly declare a Class object. Generally, you obtain a Class object by calling the getClass() method defined by Object.
The methods defined by Class are often useful in situations where runtime type information about an object is required. An example of using Class is as follows: class X { int a; float b;
} class Y extends X { double c;
} class RTTI { public static void main(String args[]) {
X x = new X();
Y y = new Y();
Class clObj = x.getClass(); //get class reference
System.out.println(“x is the object of type:”+ clObj.getName()); clObj = y.getClass();
System.out.println(“y is the object of type:”
+clObj.getName()); clObj = clObj.getSuperclass();
System.out.println(“y’s superclass is :”+ clObj.getName());
}
}
The output of the program is as follows: x is object of type : X y is obtect of type : Y y’s superclass is X
Summary
You use an instance of one of the
Number
classes—
Byte
,
Double
,
Float
,
Integer
,
Long
, and
Short
—to contain a number of primitive type.
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The
Number
classes include class methods and constants, which are useful in a variety of ways.
The
MIN_VALUE
and
MAX_VALUE
constants contain the smallest and largest values that can be contained by an object of that type. The byteValue
, shortValue
, and similar methods convert one numeric type to another. The valueOf
method converts a string to a number, and the toString
method converts a number to a string.
The
Math
class contains a variety of class methods for performing mathematical functions. This class includes the trigonometric functions, such as computing sine, cosine, and so on.
Math
also includes functions for logarithm calculations, as well as basic arithmetic functions, such as rounding. Finally,
Math
contains a method, random
, for generating random numbers.
Questions
1. class JJF1 {
}
} public static void main (String args[]) {
System.out.print(Byte.MIN_VALUE+",");
System.out.print(Byte.MAX_VALUE);
What is the result of attempting to compile and run the program? a. Prints: 0,255 b. Prints: 0,256 c. Prints: -127,128 d. Prints: -128,127 e. Compile-time error f. Run-time error g. None of the above
2. class JJF2 {
}
} public static void main (String args[]) {
System.out.print(Short.MIN_VALUE+",");
System.out.print(Short.MAX_VALUE);
What is the result of attempting to compile and run the program? a. Prints: -32767,32768 b. Prints: -32768,32767 c. Prints: 0,65535 d. Prints: 0,65536 e. Compile-time error f. Run-time error g. None of the above
3. class JJF3 {
public static void main(String args[]) {
System.out.print(Integer.toBinaryString(Byte.MAX_VALUE)+",");
System.out.print(Integer.toOctalString(Byte.MAX_VALUE)+",");
System.out.print(Integer.toString(Byte.MAX_VALUE)+",");
System.out.print(Integer.toHexString(Byte.MAX_VALUE));
}
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}
What is the result of attempting to compile and run the program? a. Prints: 1111111,177,127,7f b. Prints: 11111111,377,256,ff c. Compile-time error d. Run-time error e. None of the above
4. class JJF4 {
public static void main(String args[]) {
System.out.print(Long.toHexString(Byte.MAX_VALUE)+",");
System.out.print(Long.toHexString(Character.MAX_VALUE)+",");
System.out.print(Long.toHexString(Short.MAX_VALUE));
} }
What is the result of attempting to compile and run the program? a. Prints: f,ff,7f b. Prints: f,ff,ff c. Prints: 7f,ffff,7fff d. Prints: ff,ffff,ffff e. Prints: 7fff,ffffff,7fffff f. Prints: ffff,ffffff,ffffff g. Compile-time error h. Run-time error i. None of the above
5. class JJF5 {
public static void main(String args[]) {
System.out.print(Integer.toHexString(Integer.MIN_VALUE)+",");
System.out.print(Integer.toHexString(Integer.MAX_VALUE));
}}
What is the result of attempting to compile and run the program? a. Prints: 0000,ffff b. Prints: 00000000,ffffffff c. Prints: 7fff,8000 d. Prints: 8000,7fff e. Prints: 7fffffff,80000000 f. Prints: 80000000,7fffffff g. Compile-time error h. Run-time error i. None of the above
6. class Green { public static void main (String args[]) {
int[] i = null; // 1
Cloneable c = i; // 2 i = (int [])c; // 3
}
}
What is the result of attempting to compile and run the program? a. Compile-time error at line 1.
b. Run-time error at line 1.
c. Compile-time error at line 2.
d. Run-time error at line 2.
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e. Compile-time error at line 3.
f. Run-time error at line 3.
g. None of the above
7. class EBH011 { public static void main (String[] args) {
float a = Float.POSITIVE_INFINITY;
double b = Double.POSITIVE_INFINITY;
double c = Double.NaN;
System.out.print((a == b)+","+(c == c)+","+(c != c));
}
}
What is the result of attempting to compile and run the program? a. Prints: false,false,false b. Prints: false,false,true c. Prints: false,true,false d. Prints: false,true,true e. Prints: true,false,false f. Prints: true,false,true g. Prints: true,true,false h. Prints: true,true,true i. Run-time error j. Compile-time error k. None of the above
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Chapter 13 : Utility & Legacy classes
These classes are a part of java.util package. There are five legacy classes in Java – Vector,
Stack, Properties, Hashtable and Dictionary. There is one legacy interface called Enumeration.
13.1 Enumeration interface
An object that implements the Enumeration interface generates a series of elements, one at a time. Successive calls to the nextElement
method return successive elements of the series.
Methods are provided to enumerate through the elements of a vector, the keys of a hashtable, and the values in a hashtable. Enumerations are also used to specify the input streams to a
SequenceInputStream
.
Methods of Enumeration interface
public abstract boolean hasMoreElements()
Tests if this enumeration contains more elements. Returns true
if this enumeration contains more elements; false
otherwise. public abstract Object nextElement()
Returns the next element of this enumeration. Throws NoSuchElementException if no more element exists.
13.2 Vector class
The Vector class provides the capability to implement a growable array of objects. It is similar to the ArrayList in the Collection Framework, but with two differences: Vector is synchronized and it contains many legacy methods that are not part of the collections framework. Vector extends
AbstractList and implements the List interface of the Collections Framework. Here are Vector constructors:
Vector()
Vector(int size)
Vector(int size, int incr)
Vector(Collection c)
The first form creates a default vector, which has an initial size of 10. The second form creates a vector whose initial capacity is specified by size. The third form creates a vector whose initial capacity is specified by size and whose increment is specified by incr. The increment specifies the number of elements to allocate each time that a vector is resized upward. The fourth form creates a vector that contains the elements of Collection c.
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All vectors start with an initial capacity. After the initial capacity is reached the next time that you attempt to store an object in the vector, the vector automatically allocates space for that object plus extra room for additional objects. By allocating more than just the required memory, the vector reduces the number of allocations that must take place. This reduction is important, because allocations are costly in terms of time. The amount of extra space allocated during each reallocation is determined by the increment that you specify when you create the vector. If you don’t specify an increment, the vector’s size is doubled by each allocation cycle.
Vector defines these protected data members: int capacityIncrement; int elementCount;
Object elementData[];
The increment value is stored in capacityIncrement. The number of elements currently in the vector is stored in elementCount. The array that holds the vector is stored in elementData.
You can add an element to the Vector by calling addElement(). To obtain the element at a specific location call elementAt(). To obtain the first element in the vector call firstElement(). To retrieve the last element, call lastElement(). You can obtain the index of an element using indexOf() and lastIndexOf(). To remove an element, call removeElementAt(). The following program uses a Vector to store various types of numeric objects. It demonstrates several of the legacy methods defined by Vector. It also demonstrates the Enumeration interface. import java.util.*; class VectorDemo { public static void main(String args[]) {
//initial size is 3, increment is 2
Vector v = new Vector();
System.out.println(“Initial size :” + v.size());
System.out.println(“Initial capacity :”+v.capacity()); v.addElement(new Integer(1)); v.addElement(new Integer(2)); v.addElement(new Integer(3)); v.addElement(new Integer(4));
System.out.println(“Capacity after 4 additions :” + v.capacity()); v.addElement(new Double(5.45));
System.out.println(“Current capacity:”+v.capapcity()); v.addElement(new Double(6.08)); v.addElement(new Integer(7));
System.out.println(“Current capacity:”+v.capapcity()); v.addElement(new Double(9.4)); v.addElement(new Integer(10));
System.out.println(“Current capacity:”+v.capapcity()); v.addElement(new Integer(11)); v.addElement(new Integer(12));
System.out.println(“First element:”+
(Integer)v.firstElement());
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System.out.println(“Last element:”+
(Integer)v.lastElement()); if(v.contains(new Integer(3)) {
System.out.println(“Vector contains 3”);
}
//enumerate the elements in the vector
Enumeration vEnum = v.elements();
System.out.println(“\nElements in vector:”); while(vEnum.hasMoreElements()) {
System.out.print(vEnum.nextElement() + “ ”);
}
System.out.println();
}
}
The output of this program is shown here:
Initial size: 0
Initial capacity : 0
Capacity after 4 additions: 5
Current Capacity: 5
Current Capacity: 7
Current Capacity: 9
First element: 1
Last element: 12
Vector contains: 3
Elements in vector:
1 2 3 4 5.45 6.08 7 9.4 10 11 12
13.3 Stack class
Java's Stack class extends the Vector class. A Stack represents a Collection of objects that are in
LIFO (Last In First Out Order). The Stack class provides operations that allow testing for zero elements, inspection of it's top most element, removal of it's top most element, and the addition of elements. Some of the methods of Stack are shown below: boolean empty()
Tests if this stack is empty.
Object peek()
Looks at the object at the top of this stack without removing it from the stack.
Object pop()
Removes the object at the top of this stack and returns that object as the value of this function.
Object push(Object item)
Pushes an item onto the top of this stack.
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int search(Object o)
Returns the 1-based position where an object is on this stack.
An example of using Stack import java.util.*; class StackDemo { static void showpush(Stack st, int a) { st.push(new Integer(a));
System.out.println(“push (”+a+ “)”);
System.out.println(“stack :” + st);
} static void showpop(Stack st) {
System.out.println(“pop -> ”);
Integer a = (Integer) st.pop();
System.out.println(a);
System.out.println(“stack : ”+st);
} public static void main(String args[]) {
Stack st = new Stack();
System.out.println(“stack :” +st); showpush(st, 42); showpush(st, 66); showpush(st, 99); showpop(st); showpop(st); showpop(st); try { showpop(st);
} catch(EmptyStackException e) {
System.out.println(“Empty Stack”);
}
}
}
The following the output produced by the program stack : [ ] push (42) stack : [42] push (66) stack : [42, 66] push (99) stack : [42, 66, 99] pop -> 99 stack : [42, 66] pop -> 66 stack : [42]
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pop -> 42 stack : [ ] pop -> empty stack
13.4 Dictionary class
The Dictionary class is an abstract class, which maps keys to values. Any object can be used as a key and/or value. Given a key and value, you can store the value in a Dictionary object. Once the value is stored, you can retrieve it by using its key. Dictionary is classified as obsolete, because it is superceded by Map in the Collections Framework.
The Dictionary class defines some abstract methods. To add a key and a value, the put() method is used. Use get() to retrieve the value of a given key. The keys and values can each be returned as an Enumeration by the keys() and elements() methods, respectively. The size() method returns the number of key/value pairs stored in a dictionary and isEmpty() returns true when the dictionary is empty. You can use the remove() method to delete a key/value pair.
13.5 Hashtable class
A hash table is conceptually a contiguous section of memory with a number of addressable elements, in which data can be quickly inserted, deleted and found. Hash tables represent a sacrifice of memory for the sake of speed - they are certainly not the most memory efficient means of storing data, but they provide very fast lookup times. Hash tables are a common means of organising data, so the designers of the Java programming language have provided a number of classes for easily creating and manipulating instances of hash tables.
Hashtable is the class which provides hash tables in Java. Hashtable inherits directly from
Dictionary and implements the Map, Cloneable and Serializable interfaces.
A hash table can only store objects that override the hashCode() and equals() methods that are defined by the Object.
The Hashtable constructors are shown below:
Hashtable()
Hashtable(int size)
Hashtable(int size, float fillRatio)
Hashtable(Map m)
The first version is the default constructor. The second version creates a hash table that has an initial space specified by size. The third version creates a hash table that has an initial size specified by size and a fill ratio specified by fillRatio. This ratio must be between 0.0 and 1.0, and it determines how full the hash table can be before it is resized upward. Specifically, when the number of elements is greater than the capacity of hashtable multiplied by its fill ratio, the hash table is expanded. If you do not specify a fill ratio, then 0.75 is used. Finally, the fourth version creates a hash table that is initialized with elements in m. The capacity of the hash table is set to twice the number of elements in m. The default load factor of 0.75 is used.
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If you want to allocate more space for your hash table before the load factor reaches the specified value then use the rehash() method like this:
ht.rehash();
An example of using Hashtable import java.util.*; class HTDemo { public static void main(String args[]){
Hashtable balance = new Hashtable();
String str; double bal; balance.put(“John Doe”, new Double(3434.34)); balance.put(“Tom Smith”, new Double(123.22)); balance.put(“Jane Baker”, new Double(1378.00)); balance.put(“Todd Hall”, new Double(99.22)); balance.put(“Ralph Smith”, new Double(-19.08));
//Show all balances in hash table
Enumeration names = balance.keys(); while(names.hasMoreElements()) { str = (String)names.nextElement();
System.out.println(str + “ : ” + balance.get(str));
}
System.out.println();
//Deposit 1,000 into Jane Doe’s account bal = (Double)balance.get(“John Doe”) balance.put(“John Doe”,new Double(bal + 1000));
System.out.println(“John Doe’s new balance :” + balance.get(“John Doe”));
}
}
13.6 Properties class
Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key is a
String and the value is also a String. The Properties class is used by many other Java classes. For example, it is the type of object returned by System.getProperties() when obtaining environmental values.
Properties defines the following instance variables:
Properties default;
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This variable holds a default property list associated with a Properties object. Properties defines these constructors
Properties ()
Properties (Properties propDefault)
The first version creates a Properties object that has no default values. The second creates an object that uses propDefault for its default values. In both cases the property list is empty.
One useful capability of Properties class is that you can specify a default property that will be returned if no value is associated with a certain key. For example, a default value can be specified along with the key in the getProperty() method – such as getProperty(“name”, “default value”). If the “name” value is not found, then “default value” is returned. When you construct a
Properties object, you can pass another instance of Properties to be used as the default properties for the new instance. In this case, if you call getProperty(“foo”) on a given Properties object, and
“foo”, does not exist, Java looks for “foo” in the default Properties object. This allows for arbitrary nesting of levels of default properties.
Methods
Object setProperty(String key, String value)
String getProperty(String key)
String getProperty(String key, String defaultValue)
Enumeration propertyNames() void load(InputStream in) void store(OutputStream out, String header)
To create and put values in a Properties table
This example creates a new Properties table and assigns a string value to four different keys.
Properties props = new Properties(); props.setProperty("recursiveSearch", "true"); props.setProperty("noCopyPattern", "*.$$$"); props.setProperty("maxLevel", "7"); props.setProperty("fileName", "C:\temp\work.html");
To store a Properties table in a file
Use the Properties store(OutputStream, String) method. It will write the properties to the output stream, with a header line from the String parameter. Assuming you have variables and an
OutputStream like:
OutputStream propOut = new FileOutputStream( new File("props.stat")); props.store(propOut, "Macro Processor Properties");
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To load Properties from a file
Use the Properties load(InputStream); method.
You will probably want to get each property value and assign it to an internal variable and perhaps show it in the user interface. For example, props.load(propsIn);
Here propsIn is the input stream.
boolean recursiveSearch =
Boolean.getBoolean(props.getProperty("recursiveSearch"));
String noCopyPattern = props.getProperty("noCopyPattern"); int maxLevel = Integer.parseInt(props.getProperty("maxLevel");
To use default properties
Create a Properties table with all of the default key/value pairs. For example,
Properties defaultProps = new Properties(); defaultProps.setProperty("sourceFile", ""); defaultProps.serProperty("enableScrolling", "false");
Use this default table in the constructor for your regular properties table:
Properties props = new Properties(defaultProps);
13.7 Formatter class
Java 1.5 introduces a new class named java.util.Formatter that allows you to do string formatting similar to the printf function in C. It depends heavily on the varargs feature being introduced in
1.5. The java.util.Formatter
class includes the method
format (String format, Object... args)
The args
parameters will be displayed in the output according to the specifiers in the format string in the first parameter.
The java.util.Formatter
class provides several constructors, each of which includes a parameter for the destination of the formatted output. Destinations include
OutputStream
, an instance of
File
, and any class that implements the new
Appendable
interface.
The program
FormatWriteApp
shows how we can use
Formatter
to send formatted numerical values to the console rather than using the printf()
method.
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import java.io.*; import java.util.*;
/**
* Demonstrate the java.util.Formatter capabilities for
* formatting primitive types.
**/ public class FormatWriteApp
{
public static void main (String arg[]) {
// Send formatted output to the System.out stream.
Formatter formatter =
new Formatter ((OutputStream)System.out);
formatter.format ("Text output with Formatter. %n");
formatter.format ("Primitives converted to strings: %n");
boolean a_boolean = false;
byte a_byte = 114;
short a_short = 1211;
int an_int = 1234567;
long a_long = 987654321;
float a_float = 983.6f;
double a_double = -4.297e-15;
formatter.format ("boolean = %9b %n", a_boolean);
formatter.format ("byte = %9d %n", a_byte);
formatter.format ("short = %9d %n", a_short);
formatter.format ("int = %9d %n", an_int);
formatter.format ("long = %9d %n", a_long);
formatter.format ("float = %9.3f %n", a_float);
formatter.format ("double = %9.2e %n", a_double);
// Need to flush the data out of the buffer.
formatter.flush ();
formatter.close ();
} // main
} // class FormatWriteApp
The output of this program look like:
Text output with Formatter.
Primitives converted to strings: boolean = false byte = 114 short = 1211 int = 1234567 long = 987654321
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float = 983.600
double = -4.30e-15
In the
Formatter
constructor, the
System.out
argument, which references an instance of
PrintStream
, must be cast to
OutputStream
because otherwise there is an ambiguity over which constructor to use.
You can directly obtain the formatted string created by the
Formatter
by invoking the toString()
method. Also, if you simply want a formatted string, such as for a graphical text component, and don't want to send it to an output destination, you can create a
Formatter
with the no-argument constructor. The
Formatter
uses internally a
StringBuilder
, and you can access the string that it creates via the toString()
method.
13.8 Date class
The Date class encapsulates the current date and time. Date class has two constructors
Date()
Date(long milliseconds)
The first constructor initializes the object with the current date and time. The second constructor accepts one argument that equals the number of milliseconds that have elapsed since midnight,
January 1, 1970. Date do not allow you to obtain the individual components of date or time.
Let’s look at a simple example of creating a date using the system clock’s current date and time and returning a long value. This is often referred to as the system time of the host environment of the Java Virtual Machine (JVM).
import java.util.Date; public class DateExample1 {
public static void main(String[] args) {
// Get the system date/time
Date date = new Date();
System.out.println(date.getTime());
}
}
To create a
Date
object for a specific time, pass the number of milliseconds since midnight,
January 1, 1970, Greenwich Meantime to the constructor, like this:
Date midnight_jan2_1970 = new Date(24L*60L*60L*1000L);
You can return the number of milliseconds in the
Date
as a long
, using the getTime()
method.
For example, to time a block of code, you might do this
Date d1 = new Date();
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// timed code goes here
Date d2 = new Date(); long elapsed_time = d2.getTime() - d1.getTime();
System.out.println("That took " + elapsed_time + milliseconds");
You can change a
Date
by passing the new date as a number of milliseconds since midnight,
January 1, 1970, GMT, to the setTime()
method, like this:
Date midnight_jan2_1970 = new Date(); midnight_jan2_1970.setTime(24L*60L*60L*1000L);
The before()
method returns true if this
Date
is before the
Date
argument, false if it's not. For example if (midnight_jan2_1970.before(new Date())) {
The after()
method returns true if this
Date
is after the Date argument, false if it's not. For example if (midnight_jan2_1970.after(new Date())) {
13.9 Calendar class
The abstract class
Calendar
converts a time in milliseconds since midnight, January 1, 1970,
Greenwich Mean Time, (that is a
Date
object), into days, minutes, hours, and seconds according to the local calendar.
The
Calendar
class represents a point in time (a "
Date
"), interpreted appropriately for some locale and time zone. Each
Calendar
instance wraps a long
variable containing the number of milliseconds since the epoch for the represented point in time.
The
Calendar
class follows an unusual idiom for allowing access to the individual fields of the interpreted date instance. Rather than offering a number of dedicated property getters and setters
(such as getMonth()
), it offers only one, which takes an identifier for the requested field as argument: int get( Calendar.MONTH )
The identifiers for the fields are defined in the
Calendar
class as public static final variables. (These identifiers are raw integers, not wrapped into an enumeration abstraction.)
Besides the identifiers (or keys) for the fields, the
Calendar
class defines a number of additional public static final
variables holding the values for the fields. So, to test whether a certain date (represented by the
Calendar
instance calendar
) falls into the first month of the year, one would write code like this: if( calendar.get( Calendar.MONTH ) == Calendar.JANUARY ) {...}
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Note that the months are called JANUARY, FEBRUARY, etc., irrespective of location (as opposed to more neutral names such as MONTH_1, MONTH_2, and so on). There is also a field
UNDECIMBER, representing the 13th month of the year, which is required by some (non-
Gregorian) calendars.
An example using Calendar class import java.util.*; class CalendarDemo { public static void main(String args[ ]) {
//Create a calendar initialized with the current date
// and time in the default locale and timezone
Calendar c = Calendar.getInstance();
System.out.print(“Date :”);
System.out.println(c.get(Calendar.DATE)+”/”+
(c.get(Calendar.MONTH)+1)+”/”+ c.get(Calendar.YEAR));
System.out.print(“Time:”);
System.out.println(c.get(Calendar.HOUR)+”/”+ c.get(Calendar.MINUTE)+”/”+ c.get(Calendar.SECOND));
//set the time and print it c.set(Calendar.HOUR,10); c.set(Calendar.MINUTE,00); c.set(Calendar.SECOND,00);
System.out.print(“Updated Time:”);
System.out.println(c.get(Calendar.HOUR)+”/”+ c.get(Calendar.MINUTE)+”/”+ c.get(Calendar.SECOND));
}
}
13.10 GregorianCalendar class
The class
GregorianCalendar
is the only commonly available subclass of
Calendar
. It provides an implementation of the basic
Calendar
abstraction suitable for the interpretation of dates according to the conventions used commonly in the West. It adds a number of public constructors, as well as some functions specific to Gregorian Calendars, such as isLeapYear()
.
boolean isLeapYear(int year)
The getInstance() method of Calendar returns a GregorianCalendar object. There are several constructors of GregorianCalendar class. The default, initializes the object with the current date and time in the default locale and timezone. Also there are some more like
GregorianCalendar (int year, int month, int dayOfMonth)
GregorianCalendar (int year, int month, int dayOfMonth, int hours, int minutes)
GregorianCalendar (int year, int month, int dayOfMonth,
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int hours, int minutes, int seconds)
All three versions set the day, month and year. Here year specifies the number of years that have elapsed since 1900. The month is specified by month, with zero indicating January. The first version sets the time of midnight. The second version also sets the hours and the minutes. The third version adds seconds.
You can also construct a GregorianCalendar object by specifying either the locale and/or timezone. The following constructors create objects initialized with the current date and time using the specified time zone and/or locale.
GregorianCalendar (Locale locale)
GregorianCalendar (TimeZone timezone)
GregorianCalendar (TimeZone timezone, Locale locale)
Example import java.util.*; class GregorianCalendarDemo { public static void main(String args[]){
GregorianCalendar gc = new GregorianCalendar();
System.out.print(“Date :”);
System.out.println(gc.get(Calendar.DATE)+”/”+ gc.get(Calendar.MONTH)+ ”/”+ gc.get(Calendar.YEAR));
System.out.print(“Time:”);
System.out.println(gc.get(Calendar.HOUR)+”/”+ gc.get(Calendar.MINUTE)+”/”+ gc.get(Calendar.SECOND)); if(gc.isLeapYear(gc.get(Calendar.YEAR))) {
System.out.println(“Current year is Leap year”);
} else {
System.out.println(“Current year is not a Leap year”);
}
}
}
13.11 TimeZone and SimpleTimeZone classes
The
TimeZone
class and its subclasses are auxiliary classes, required by
Calendar
to interpret dates according to the selected time zone. Semantically, a time zone specifies a certain offset to be added to Greenwich Mean Time (GMT) also referred to as Coordinated Universal Time
(UTC) to reach the local time. Clearly, this offset changes when daylight saving time (DST) is in effect. The
TimeZone
abstraction therefore needs to keep track not only of the additional offset to be applied if DST is in effect, but also of the rules that determine when DST is in effect, in order to calculate the local time for any given date and time.
The abstract base class
TimeZone
provides basic methods to handle "raw" (without taking DST into account) and actual offsets (in milliseconds!), but implementation of any functionality
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related to DST rules is left to subclasses, such as
SimpleTimeZone
. The latter class provides several ways to specify rules controlling the beginning and ending of DST, such as a giving an explicit day in a month or a certain weekday following a given date. Each
TimeZone
also has a human-readable, locale-dependent display name. Display names come in two styles:
LONG
and
SHORT
.
Time zones are unambiguously determined by an identifier string. The base class provides the static method
String[] getAvailableIDs()
to obtain all installed "well-known" standard time zones. Also provided are static factory methods, to obtain
TimeZone
instances — either for a specific ID or the default for the current location.
SimpleTimeZone
also provides some public constructors and, surprisingly for an abstract class, so does
TimeZone.
An example of using TimeZone
// Get the current time in Hong Kong
Calendar cal = new
GregorianCalendar(TimeZone.getTimeZone("Hongkong"));
int hour12 = cal.get(Calendar.HOUR); // 0..11
int minutes = cal.get(Calendar.MINUTE); // 0..59
int seconds = cal.get(Calendar.SECOND); // 0..59
boolean am = cal.get(Calendar.AM_PM) == Calendar.AM;
// Get the current hour-of-day at GMT
cal.setTimeZone(TimeZone.getTimeZone("GMT"));
int hour24 = cal.get(Calendar.HOUR_OF_DAY); // 0..23
// Get the current local hour-of-day
cal.setTimeZone(TimeZone.getDefault());
hour24 = cal.get(Calendar.HOUR_OF_DAY); // 0..23
An example to list all timezones
Date today = new Date();
// Get all time zone ids
String[] zoneIds = TimeZone.getAvailableIDs();
// View every time zone for (int i=0; i<zoneIds.length; i++) {
// Get time zone by time zone id
TimeZone tz = TimeZone.getTimeZone(zoneIds[i]);
// Get the display name
String shortName = tz.getDisplayName(tz.inDaylightTime(today),
TimeZone.SHORT);
String longName = tz.getDisplayName(tz.inDaylightTime(today),
TimeZone.LONG);
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// Get the number of hours from GMT
int rawOffset = tz.getRawOffset();
int hour = rawOffset / (60*60*1000);
int min = Math.abs(rawOffset / (60*1000)) % 60;
// Does the time zone have a daylight savings time
//period?
boolean hasDST = tz.useDaylightTime();
// Is the time zone currently in a daylight savings time?
boolean inDST = tz.inDaylightTime(today);
}
13.12 Locale class
In Java, a locale is simply an identifier for a particular combination of language and region. It is not a collection of locale-specific attributes. Instead, each locale-sensitive class maintains its own locale-specific information. With this design, there is no difference in how user and system objects maintain their locale-specific resources. Both use the standard localization mechanism.
Java programs are not assigned a single global locale. All locale-sensitive operations may be explicitly given a locale as an argument. This greatly simplifies multilingual programs. While a global locale is not enforced, a system wide default locale is available for programs that do not wish to manage locales explicitly. A default locale also makes it possible to affect the behavior of the entire presentation with a single choice.
Java locales act as requests for certain behavior from another object. For example, a French
Canadian locale passed to a Calendar object asks that the Calendar behave correctly for the customs of Quebec. It is up to the object accepting the locale to do the right thing. If the object has not been localized for a particular locale, it will try to find a "close" match with a locale for which it has been localized. Thus if a Calendar object was not localized for French Canada, but was localized for the French language in general, it would use the French localization instead.
Locale objects are generally created from a language name and a county name as follows:
// A locale for Great Britain
Locale greatBritain = new Locale("en","GB");
// A locale for the French language
Locale french = new Locale("fr", "");
In the second case above, an empty country string is given to the Locale constructor to signify a locale for the entire French language. Language names are two letter ISO-639 language codes and country names are two letter ISO-3166 country codes. This naming scheme is not enforced by class Locale but is rather a convention used by all Java's International classes.
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In addition, the Locale class contains a number of handy constants for creating Locale objects for commonly used languages and countries. For example, the following specifies the Locale object for Great Britain and can be used in place of new Locale("en","GB"):
Locale.UK
A Locale may also be created with an optional variant name. This allows multiple locales to be created for a single language and country combination. As an example, this feature could be used to create an "FR_FR_HOST" locale which would match the host's behavior for France rather than Java's portable behavior for France.
Locale contains a static getter and setter method for accessing the system's default locale. At start-up time, the default locale is automatically set by the Java runtime to match the host's current locale. If this is not possible, the en_US locale is used.
Locale supports a number of methods to provide user readable names for the locale and its country, language and variant fields. These names can also be localized:
Locale.setDefault( new Locale("en", "US") );
Locale japan = new Locale("ja", "JP");
String nameUS = japan.getDisplayLanguage();
String nameFR = japan.getDisplayLanguage(new Locale("fr","FR"));
In the above example, the first call to getDisplayLanguage() returns the language of the locale japan, localized for the default locale. Thus nameUS would be "Japanese." The second call to getDisplayLanguage() returns the language of the locale japan, localized for the given locale.
Thus nameFR would be "japonais."
13.13 StringTokenizer class
When working with any general-purpose programming language, it's often necessary to break a large string into smaller components. java.util.StringTokenizer is used to break Java String's into smaller components, called tokens.
You can create a StringTokenizer by using any one of the following three constructors:
StringTokenizer(String sInput) - Breaks on white space (" ", "\t", "\n").
StringTokenizer(String sInput, String sDelimiter) - Breaks on sDelimiter.
StringTokenizer(String sInput, String sDelimiter, boolean bReturnTokens) - Breaks on sDelimiter, but if bReturnTokens is set to true, then the delimiter is also returned as a token.
The first constructor doesn't check whether the input string contains substrings. When the string
"hello. Today \"I am \" going to my home town" is tokenized on white space, the result is in tokens hello., Today, "I, am, ", going, instead of hello., Today, "I am ", going.
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The second constructor doesn't check the consecutive appearance of delimiters. When the string
"book, author, publication,,,date published" is tokenized on ",", the StringTokenizer returns four tokens with values book, author, publication, and date published instead of the six values book, author, publication, "", "", and date published, where "" means string of length 0. To get six, you must set the StringTokenizer's bReturnTokens parameter to true.
The feature of setting the parameter to true is important as it gives an idea about the presence of consecutive delimiters.
The third constructor won't work if a token itself is equal (in length and value) to the delimiter and is in a substring. When the string "book, author, publication,\",\",date published" is tokenized
(this string contains , as a token, which is the same as its delimiter) on string ,, the result is book, author, publication, ", ", date published (with six tokens) instead of book, author, publication, ,
(the comma character), date published (with five tokens).
One of the most famous sentences in American history begins with the words "Four score and seven years ago". For our purposes, suppose this sentence is stored in a variable named speech, like this:
String speech = "Four score and seven years ago";
A simple snippet of code to break that sentence into individual words using Java's
StringTokenizer
class would look like this:
String speech = "Four score and seven years ago";
StringTokenizer st = new StringTokenizer(speech); while (st.hasMoreTokens()) {
println(st.nextToken());
}
In this example, the variable speech
is passed into the
StringTokenizer
constructor method.
Because
StringTokenizer
is not given a field separator value as an input parameter, it uses it's default field separator, and assumes that fields within the string are separated by whitespace characters (spaces, tabs, and carriage-return characters). Therefore, each time through the
while
loop a word is printed on a separate line, and the resulting output from this snippet of code looks like this:
Four score and seven years ago
The
while
loop test checks to see if there are any tokens left in the st
object. As long as there are, the println
statement is executed. Once there are no tokens remaining, the println statement is skipped and the
while
loop is exited.
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In this example a text file is used. It shows how to break a record (separated by colon characters) into tokens typically called "fields".
The following two records are from a hypothetical customer file named customer.txt. Each record contains information about a customer, including their first name, last name, and the city and state of their address. Within a record, each field is separated by a colon character.
Homer:Simpson:Springfield:???
Hank:Hill:Arlen:Texas
Because we know that the fields of each record are separated by the colon character, we specify that the colon character should be the field delimiter (or field separator) when we call the
StringTokenizer
constructor, like this:
StringTokenizer st = new StringTokenizer(dbRecord, ":");
After that, it's a simple matter to break the record into it's four fields using the nextToken() method of the
StringTokenizer
class. import java.io.*; import java.util.*; class TokenTest { public static void main (String[] args) {
TokenTest tt = new TokenTest(); tt.dbTest();
}
void dbTest() {
DataInputStream dis = null;
String dbRecord = null;
try {
File f = new File("customer.txt");
FileInputStream fis = new FileInputStream(f);
BufferedInputStream bis = new BufferedInputStream(fis); dis = new DataInputStream(bis);
// read the first record of the database
while ( (dbRecord = dis.readLine()) != null) {
StringTokenizer st = new StringTokenizer(dbRecord,
":");
String fname = st.nextToken();
String lname = st.nextToken();
String city = st.nextToken();
String state = st.nextToken();
System.out.println("First Name: " + fname);
System.out.println("Last Name: " + lname);
System.out.println("City: " + city);
System.out.println("State: " + state + "\n");
}
}
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catch (IOException e) {
// catch io errors from FileInputStream or readLine()
System.out.println("Uh oh, got an IOException error: "
+ e.getMessage());
}
finally {
// if the file opened okay, make sure we close it
if (dis != null) {
try {
dis.close();
}
catch (IOException ioe) {
System.out.println("IOException error trying to close the file: " + e.getMessage());
}
} // end if
} // end finally
} // end dbTest
} // end class
The simple method we've shown here is a powerful way of breaking a String into tokens. If you need a more powerful tokenizer, you might look at the StreamTokenizer class instead. The
StreamTokenizer class can recognize various comment styles of programming languages, and offers a number of control flags that can be set to various states.
13.14 StreamTokenizer class
As the name of the class suggests, a StreamTokenizer object expects its input to come from an
InputStream class. Like the StringTokenizer, this class converts the input stream into chunks that your parsing code can interpret, but that is where the similarity ends.
StreamTokenizer is a table-driven lexical analyzer. This means that every possible input character is assigned a significance, and the scanner uses the significance of the current character to decide what to do. In the implementation of this class, characters are assigned one of three categories. These are:
Whitespace characters -- their lexical significance is limited to separating words
Word characters -- they should be aggregated when they are adjacent to another word character
Ordinary characters -- they should be returned immediately to the parser
Having created an instance of the class streamTokenizer we can use the nextToken method to read tokens from the input stream. Note also that we do not need to know how big the file is in advance in this case, we simply test the current token's type agaist the class integer constant
TT_EOF (this has a value of -1). There a four possible predefined types of token: TT_EOF,
TT_EOL, TT_Number and Word.
import java.io.*;
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import java.util.*; class TokenizerExample5
{ public static void main(String[] args) throws IOException
{
FileReader file = new FileReader("HelloWorld.java");
StreamTokenizer inputStream = new StreamTokenizer(file); int tokenType = 0; int numberOfTokens = -1;
// Process the file and output the number of tokens in the file do {
tokenType = inputStream.nextToken();
outputTtype(tokenType,inputStream);
numberOfTokens++;
} while (tokenType != StreamTokenizer.TT_EOF);
// Output result and close file
System.out.println("Number of tokens = " + numberOfTokens);
}
/* OUTPUT TTYPE: Method to output the ttype of a stream token and its value. */ private static void outputTtype(int ttype, StreamTokenizer inStream) {
switch (ttype) {
case StreamTokenizer.TT_EOF:
System.out.println("TT_EOF"); break;
case StreamTokenizer.TT_EOL:
System.out.println("TT_EOL"); break;
case StreamTokenizer.TT_NUMBER:
System.out.println("TT_NUMBER: nval = " + inStream.nval); break;
case StreamTokenizer.TT_WORD:
System.out.println("TT_WORD: sval = " + inStream.sval); break;
default:
System.out.println("Unknown: nval = " + inStream.nval +" sval = " + inStream.sval);
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break;
}
}
}
13.15 Random class
Java provides two mechanisms for creating random numbers – Math.random() method and
Random class. To use the Random class create an object of this class (giving a seed to the constructor if you wish), then call one of the methods below to get a new random number.
Random constructors
Random r = new Random(); //Default seed comes from system time.
Random r = new Random(long seed); // For reproducible testing
Random methods
The most common methods are those which return a random number. These methods return a uniform distribution of values, except nextGaussian(). In these examples, x is a Random object. int i = r.nextInt(int n) int i = r.nextInt() long l = r.nextLong() float f = r.nextFloat() double d = r.nextDouble() boolean b = r.nextBoolean() double d = r.nextGaussian()
Returns random int >= 0 and < n
Returns random int (full range)
Returns random long (full range)
Returns random float >= 0.0 and < 1.0
Returns random double >=0.0 and < 1.0
Returns random double (true or false)
Returns random number with mean 0.0 and standard deviation 1.0
Example: Generating a number from 1 to 6
Because nextInt(6) returns a number from 0-5, it's necessary to add 1 to scale the number into the range 1-6, static Random randGen = new Random(); int spots = randGen.nextInt(6) + 1;
A program to generate a random double random import java.util.Random; public class RandomNumber
{
public static void main(String[] args)
{
Random generator = new Random();
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double num1 = generator.nextDouble();
System.out.println("A random double number: " + num1);
}
}
13.16 BitSet class
A BitSet class creates a special type of array that holds bit values. The BitSet class represents a set of bits, which is also known as a bitfield. The
BitSet
class implements a bit-vector of an arbitrary size. It automatically grows dynamically. This example demonstrates how to create and use a
BitSet
.
// Create the bitset
BitSet bits = new BitSet();
// Set a bit on
bits.set(2); // 100 = decimal 4
// Retrieving the value of a bit
boolean b = bits.get(0); // false
b = bits.get(2); // true
// Clear a bit
bits.clear(1);
// Setting a range of bits
BitSet bits2 = new BitSet();
bits2.set(1, 4); // 1110
// And'ing two bitsets
bits.and(bits2); // 0100
// Xor'ing two bitsets
bits.xor(bits2); // 1010
// Flip all bits in the bitset
bits.flip(0, bits.length()); // 0101
// Andnot'ing two bitsets
bits.andNot(bits2); // 0001
// Or'ing two bitsets
bits.or(bits2); // 1111
13.17 Timer and TimerTask classes
The
Timer
class in the java.util
package schedules instances of a class called
TimerTask.
TimerTask implements the Runnable interface; thus it can be used to create a thread of execution.
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import java.util.*;
/**
* Simple demo that uses java.util.Timer to schedule a task to execute once 5 seconds have passed.
*/ public class Reminder {
Timer timer;
public Reminder(int seconds) {
timer = new Timer();
timer.schedule(new RemindTask(), seconds*1000);
}
class RemindTask extends TimerTask {
public void run() {
System.out.println("Time's up!");
timer.cancel(); //Terminate the timer thread
}
}
public static void main(String args[]) {
new Reminder(5);
System.out.println("Task scheduled.");
}
}
When you run the example, you first see this:
Task scheduled.
Five seconds later, you see this:
Time's up!
This simple program illustrates the basic parts of implementing and scheduling a task to be executed by a timer thread.
•
•
•
•
Implement a custom subclass of
TimerTask
. The run
method contains the code that performs the task. In this example, the subclass is named
RemindTask
.
Create a thread by instantiating the
Timer
class.
Instantiate the timer task object ( new RemindTask()
).
Schedule the timer task for execution. This example uses the schedule
method, with the timer task as the first argument and the delay in milliseconds (
5000
) as the second argument.
Another way of scheduling a task is to specify the time when the task should execute. For example, the following code schedules a task for execution at 11:01 p.m.:
//Get the Date corresponding to 11:01:00 pm today.
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Calendar calendar = Calendar.getInstance(); calendar.set(Calendar.HOUR_OF_DAY, 23); calendar.set(Calendar.MINUTE, 1); calendar.set(Calendar.SECOND, 0);
Date time = calendar.getTime(); timer = new Timer(); timer.schedule(new RemindTask(), time);
Stopping Timer Threads
By default, a program keeps running as long as its timer threads are running. You can terminate a timer thread in four ways.
•
•
•
•
Invoke cancel
on the timer. You can do this from anywhere in the program, such as from a timer task's run
method.
Make the timer's thread a "daemon" by creating the timer like this: new Timer(true)
. If the only threads left in the program are daemon threads, the program exits.
After all the timer's scheduled tasks have finished executing, remove all references to the
Timer
object. Eventually, the timer's thread will terminate.
Invoke the
System.exit
method, which makes the entire program (and all its threads) exit.
The
Reminder
example uses the first scheme, invoking the cancel method from the timer task's run
method. Making the timer thread a daemon wouldn't work, because the program needs to keep running until the timer's task executes.
Performing a task repeatedly
Here are all the Timer methods you can use to schedule repeated executions of tasks: schedule(TimerTask task, long delay, long period) schedule(TimerTask task, Date time, long period) scheduleAtFixedRate(TimerTask task, long delay, long period) scheduleAtFixedRate(TimerTask task, Date firstTime, long period)
When scheduling a task for repeated execution, you should use one of the schedule methods when smoothness is important and a scheduleAtFixedRate method when time synchronization is more important.
13.18 Observable class
The Observable class is used to create subclasses that other parts of your program can observe.
When an object of such a subclass undergoes a change, observing classes are notified. Observing classes must implement the Observer interface which defines the update() method. The update() method is called when an observer is notified of a change in an observed object.
An object that is being observed must follow two simple rules. First, if it has changed, it must call setChanged(). Secondly, when it is ready to notify observers of this change, it must call
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notifyObservers(). This causes the update() method in the observing object(s) to be called. If the object calls notifyObservers without having previously called setChanged, no action will take place. The observed object must call both setChanged() and notifyObservers() before update() will be called.
Notice that notifyObserevers() has two forms: one that takes an argument and one does not. If you call the notifyObservers() with an argument, this object is passed to Observer’s update() method as its second parameter. Otherwise null is passed to update(). You can use the second parameter for passing any type of object that is appropriate for your application.
The Observer Interface
To observe an observable object, you must implement the Observer interface. This interface defines only the one method shown here: void update (Observable observOb, Object arg)
Here, observOb is the object being observed, and arg is the value passed by notifyObservers().
The update() method is called when a change in the observed object takes place. There can be more than one observers.
An example of using Observer. Here Watcher implements Observer. This class is being monitored by BeingWatched. When the value in the Watcher is changed the notifyObserver passes the changed value to the BeingWatched Object. import java.util.*; class Watcher implements Observer { public void update (Observable obj, Object arg) {
System.out.println(“update() called, value is:”+
(String)arg);
}
}
// This is the class being observed class BeingWatched extends Observable {
String value; void changeValue(String value) { this.value = value; setChanged(); notifyObservers(value);
}
} class ObserverDemo { public static void main(String args []) {
BeingWatched observed = new BeingWatched();
Watcher observing = new Watcher();
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/** Add the observing to the list of observers for observed object. */ observed.addObserver(observing); observed.changeValue(args[0]);
}
}
13.19 Currency class
Java 2 version 1.4 has added the Currency class. This class encapsulates information about a currency. It defines no constructors. The following program demonstrates the use of Currency: import java.util.*; class CurDemo { public static void main (String args []) {
Currency c = Currency.getInstance(Locale.US);
System.out.println(“Symbol :” + c.getSymbol());
System.out.println(“Default fractional digits :”+ c.getDefaultFractionDigits());
}
}
The output is shown here.
Symbol : $
Default fractional digits : 2
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Chapter 14 : Regular Expression Processing
Using regular expressions and the java.util.regex package, you can easily describe, locate and manipulate complex patterns of text.
14.1 What is a Regular Expression?
A regular expression is a series of metacharacters and literals that allow you to describe substrings in text using a pattern. These metacharacters actually form a miniature language in their own right. Consider the following sentence:
My name is Will and I live in williamstown.
How could we find all occurrences of the text 'Will', regardless of whether or not an upper or lowercase 'w' was used? With regular expressions you can describe this requirement by composing a pattern made from a series of metacharacters and literals. Here is such a pattern:
[Ww]ill
This one's pretty straightforward. The interesting part is the [Ww] grouping -- it indicates that any one of the letters enclosed within the brackets (in this case, either an uppercase 'W' or a lowercase 'w') is acceptable. So, this regular expression will match text that begins with an uppercase or lowercase w, and is followed by the literals i, then l, and then another l.
Let's step it up a notch. The above regular expression will actually match 2 occurrences of will -- the name Will and the first 4 characters of text in williamstown. We may only have wanted to search for will and Will, and not for words that simply contain these 4 characters in sequence.
Here's an improved version:
\b[Ww]ill\b
The \b is how we describe a word boundary. A word boundary will match the likes of spaces, tabs, and the beginning and end points of a line. This effectively rules out williamstown as a match because the second l in williamtown is not followed by a word boundary -- it's followed by an i.
Let's examine one more regular expression
(\w+)@(\w+\.)(\w+)(\.\w+)?
The (\w+) grouping (it appears twice -- examine the one at the start) looks for word characters, as denoted by the \w. The + indicates that one or more word characters must appear (not necessarily the same one). This must be followed by a literal @ character. The parentheses are not actually required here, but they do divide the expression into groupings.
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Based on this first portion of our example regex, the (\w+)@ portion, here are a few examples that meet the requirements so far: billy@
joe@
francisfordcoppola@
The (\w+\.) grouping is similar, but expects a period to follow in order to make a match. The period has been escaped using a backslash because the period character is itself a regex metacharacter (a wildcard that matches any character). You must always escape metacharacters in this way if you want to match on their literal meaning.
Let's take a look at a few examples that would meet the requirements so far: billy@webworld.
joe@optus.
francisfordcoppola@myisp.
The (\w+) grouping is identical to the first grouping -- it looks for one or more word characters.
So, as you've no doubt realised already, our regular expression is intended to match email addresses.
A few examples that meet the requirements so far:
The (\.\w+)* grouping should mostly make sense at this point -- we're looking for a period followed by one or more word characters. But what's with the * after the closing parentheses? In the world of regular expressions, we use * to denote that the preceding metacharacter, literal or group can occur zero or more times. As an example, \w\d* would match a word character followed by zero or more digits. In our example, we use parentheses to group together a series of metacharacters, so the * applies to the whole group. So, you can interpret (\.\w+)* as 'match a period followed by one or more word characters, and match that combination zero or more times'.
A few examples that meet the requirements of the complete regular expression:
Java Safe Regular Expressions
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Any backslash delimited metacharacters in java will need to be escaped. This is because the backslash character has its own special meaning in Java. So, our example email address regex would have to be rewritten as follows:
String emailRegEx = "(\\w+)@(\\w+\\.)(\\w+)(\\.\\w+)*";
14.2 Pattern class
This class lets you compile your regular expression -- this effectively optimises it for efficiency and use by multiple target strings (strings which you want to test the compiled regular expression against). Consider the following example:
String emailRegEx = "(\\w+)@(\\w+\\.)(\\w+)(\\.\\w+)*";
// Compile and get a reference to a Pattern object.
Pattern pattern = Pattern.compile(emailRegEx);
Take note that the Pattern object was retrieved via the Pattern class's static compile method -- you cannot instantiate a Pattern object using new. Once you have a Pattern object you can use it to get a reference to a Matcher object.
The Pattern class has a number of flags that you can use as a second argument to its compile() method. For example, you can use Pattern.CASE_INSENSITIVE to tell the regex engine to match ASCII characters regardless of case.
Pattern.MULTILINE is another useful one. You will sometimes want to tell the regex engine that your target string is not a single line of code; rather, it contains several lines that have their own termination characters.
If you need to, you can combine multiple flags by using the java | (vertical bar) operator. For instance, if you wanted to compile a regex with multiline and case insensitivity support, you could do the following:
Pattern.compile(myRegEx, Pattern.CASE_INSENSITIVE |
Pattern.MULTILINE );
14.3 Matcher class
Once you have created a Pattern object, you will use it to create a Matcher. This is done by calling the matcher() factory method defined by Pattern as shown here
Matcher matcher(CharSequence str)
Here str is the character sequence that the pattern will be matched against. This is called the input sequence.
The simplest pattern matching method is matches(), which simply determines whether the character sequence matches the pattern, and not just a subsequence of it.
boolean matches()
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To determine if a subsequence of the input sequence matches the pattern use find(). Each call to find() begins where the previous one left off.
You can obtain a string containing the last matching sequence by calling group(). You can obtain the index within the input sequence of the current match by calling start(). The index one past the end of the current match is obtained by calling end().
The above example is continued below:
String targetString = "You can email me at [email protected] or [email protected] to get more info";
// Get a Matcher based on the target string.
Matcher matcher = pattern.matcher(targetString);
// Find all the matches.
while (matcher.find()) {
System.out.println("Found a match: " + matcher.group());
System.out.println("Start position: " + matcher.start());
System.out.println("End position: " + matcher.end());
}
First up, notice that we used the Pattern class's matcher() method to obtain a Matcher object. The while loop runs conditionally based on the results of the Matcher class's find() method. This method will parse just enough of our target string to make a match, at which point it will return true. Be careful: any attempts to use the matcher before calling find() will result in the unchecked
IllegalStateException being thrown at runtime.
In the body of our while loop we retrieved the matched substring using the Matcher class's group() method. Our while loop executes twice: once for each email address in our target string.
On each occasion, it prints the matched email address, returned by the group() method, and the substring location information. Take a look at the output:
Found a match: [email protected]
Start position: 20
End position: 38
Found a match: [email protected]
Start position: 42
End position: 58
As you can see, it was simply a matter of using the Matcher's start() and end() methods to find out where the matched substrings occurred in the target string.
Understanding Groups
Matcher.group() will retrieve a complete match from the target string. But what if you were also interested in subsections, or 'subgroups' of the matched text? In our email example, it may have been desirable to extract the host name portion of the email address and the username portion.
Have a look at a revised version of our Matcher driven while loop:
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while (matcher.find()) {
System.out.println("Found a match: " + matcher.group(0) +
". The Username is " + matcher.group(1) +
" and the ISP is " + matcher.group(2));
}
As you may recall, groups are represented as a set of parentheses wrapped around a subsection of your pattern. The first group, located using Matcher.group() or, as in the example, the more specific Matcher.group(0), represents the entire match. Further groups can be found using the same group(int index) method. Here is the output for the above example:
Found a match: [email protected].. The Username is g_andy and the ISP is example.
Found a match: [email protected].. The Username is andy and the ISP is example.
As you can see, group(1) retrieves the username portion of the email address and group(2) retrieves the ISP portion. When crafting your own regular expressions it is, of course, up to you how you logically subgroup your patterns. A minor oversight in this example is that the period itself is captured as part of the subgroup returned by group(2)!
Keep in mind that subgroups are indexed from left to right based on the order of their opening parentheses. This is particularly important when you are working with groups that are nested within other groups.
The Matcher class has a number of interesting methods: String replaceAll(String replacementString) and String replaceFirst(String replacementString), in particular, are worth a mention here.
The replaceAll() method takes a replacement string and replaces all matches with it. The replaceFirst() method is very similar but will replace only the first occurrence of a match. Have a look at the following code:
// Matches 'BBC' words that end with a digit.
String thePattern = "bbc\\d";
// Compile regex and switch off case sensitivity.
Pattern pattern = Pattern.compile(thePattern,
Pattern.CASE_INSENSITIVE);
// The target string.
String target = "I like to watch bBC1 and BbC2 - I suppose MTV
is okay too";
// Get the Matcher for the target string.
Matcher matcher = pattern.matcher(target);
// Blot out all references to the BBC.
System.out.println(matcher.replaceAll("xxxx") );
Here' the output:
I like to watch xxxx and xxxx - I suppose MTV is okay too
BackReferences
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Backreferences allow you to access captured subgroups while the regex engine is executing.
Basically, this means that you can refer to a subgroup from an earlier part of a match later on in the pattern. Imagine that you needed to inspect a target string for 3-letter words that started and ended with the same letter -- wow, sos, mum, that kind of thing. Here's a pattern that will do the job:
(\w)(\w)(\1)
In this case, the (\1) group contains a backreference to the first match made in the pattern.
Basically, the third parenthesised group will only match when the character at this position is the same as the character in the first parenthesised group. Of course, you would simply substitute \1 with \2 if you wanted to backreference the second group. It's simple, but in many cases, tremendously useful.
The Matcher object's replacement methods (and the String class's counterparts) also support a notation for doing backreferences in the replacement string. It works in the same way, but uses a dollar sign instead of a backslash. So, matcher.replaceAll("$2") would replace all matches in a target string with the value matched by the second subgroup of the regular expression.
14.4 String Class RegEx Methods
The Java String class has been updated to take advantage of regular expressions. There are 5 such methods available.
You can use the boolean matches(String regex) method to quickly determine if a string exactly matches a particular pattern. The appropriately named String replaceFirst(String regex, String replacement) and String replaceAll(String regex, String replacement) methods allow you to do quick and dirty text replacements. And finally, the String[] split(String regEx) and String[] split(String regEx, int limit) methods let you split a string into substrings based on a regular expression. These last two methods are, in concept, similar to the java.util.StringTokenizer, only much more powerful.
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Chapter 15 : API classes in java.text
15.1 DateFormat class
While Calendar and related classes handle the locale-specific interpretation of dates, the
DateFormat classes assist with the transformation of dates to and from human-readable strings.
When representing points in time, an additional localization issue arises: not only the language, but also the date format is locale-dependent (U.S.: Month/Day/Year, Germany: Day.Month.Year, etc.). The DateFormat utility tries to manage these differences for the application programmer.
The abstract base class DateFormat does not require (and does not permit) the definition of arbitrary, programmer-defined date formats. Instead, it defines four different format styles:
SHORT, MEDIUM, LONG, and FULL (in increasing order of verbosity). Given a locale and a style, the programmer can rely on the class to use an appropriate date format.
The abstract base class DateFormat does not define static methods for formatting (date to text) or parsing (text to date). Instead, it defines several static factory methods to obtain instances (of concrete subclasses) initialized for a given locale and a chosen style. Since the standard formats always include both date and time, additional factory methods are available to obtain instances treating only the time or date part. static final DateFormat getDateInstance() static final DateFormat getDateInstance(int style) static final DateFormat getDateInstance(int style, Locale locale)
The String format(Date) and Date parse(String) methods then perform the transformation. Note that concrete subclasses may choose to break this idiom.
The commands
DateFormat df = DateFormat.getDateTimeInstance();
String now = df.format( new Date() );
System.out.println(now); print the current date and time (as returned by "new Date()"), formatted according the conventions of the default locale. The output will look different in different countries.
The Calendar object used internally to interpret dates is accessible and can be modified, as are the employed TimeZone and NumberFormat objects. However, the locale and style can no longer be changed once the DateFormat has been instantiated.
Also available are (abstract) methods for piece-wise parsing or formatting, taking an additional
ParsePosition or FieldPosition argument, respectively. There are two versions for each of these methods. One takes or returns a Date instance and the other takes or returns a general Object, to allow handling of alternatives to Date in subclasses. The class defines several public static
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variables with names ending in _FIELD to identify the various possible fields for use with
FieldPosition
In the following example the getTime() method of GregorianCalendar returns a Date corresponding to the GregorianCalendar object. You can put the whole process of creating a
GregorianCalendar object, converting it to a Date, and getting and outputting the corresponding
String in the following program: import java.util.*; import java.text.*;
}
} public class Flight {
public static void main(String[] args) {
GregorianCalendar firstFlight = new GregorianCalendar
(1903, Calendar.DECEMBER, 17);
Date d = firstFlight.getTime();
DateFormat df = DateFormat.getDateInstance( DateFormat.LONG
String s = df.format(d);
System.out.println("First flight was " + s);
);
One more example that specifies the date style and locale import java.text.*; import java.util.*; public class DateFormatDemo { public static void main (String args[]) {
Date date = new Date();
DateFormat df =
DateFormat.getDateInstance(DateFormat.SHORT,
Locale.JAPAN);
System.out.println(“Japan :”+df.format(date)); df = DateFormat.getDateInstance(DateFormat.MEDIUM,
Locale.KOREA);
System.out.println(“Korea :”+df.format(date)); df = DateFormat.getDateInstance(DateFormat.LONG,
Locale.UK);
System.out.println(“UK :”+ df.format(date)); df = DateFormat.getDateInstance(DateFormat.FULL,
Locale.US);
System.out.println(“US :”+ df.format(date));
}
}
Sample output is shown below:
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Japan : 02/05/08
Korea: 2002-05-08
UK: 08 May 2002
US: Wednesday, May 8, 2002
The getTimeInstance() method returns an instance of DateFormat that can format time information. It is available in these versions: static final DateFormat getTimeInstance() static final DateFormat getTimeInstance(int style) static final DateFormat getTimeInstance(int style, Locale locale)
The style argument is one of the following values: DEFAULT, SHORT, MEDIUM, LONG or
FULL. These are int constants which cause different details about the time to be presented. The argument locale is one of the static references of Locale. If the style and/or locale is not specified defaults are used. An example of using it is shown below: import java.util.*; import java.text.*; public class TimeFormatDemo { public static void main(String args[]) {
Date date = new Date();
DateFormat df = DateFormat.getTimeInstance(
DateFormat.SHORT,Locale.JAPAN);
System.out.println(“Japan :”+df.format(date)); df = DateFormat.getTimeInstance(DateFormat.LONG,Locale.UK);
System.out.println(“UK :”+df.format(date)); df = DateFormat.getTimeInstance(DateFormat.FULL,
Locale.CANADA);
System.out.println(“Canada :”+df.format(date));
}
}
Sample output from the program is shown here:
Japan : 20:25
UK : 20:25:14 CDT
Canada : 8:25:14 o’clock PM CDT
The DateFormat class also has a getDateTimeInstance() method that can format both date and time information.
15.2 SimpleDateFormat class
The only commonly available concrete subclass of DateFormat is SimpleDateFormat. It provides all of the aforementioned functionality, additionally allowing the definition of arbitrary dateformatting patterns. The pattern can be specified as an argument to the constructors of this class or set explicitly.
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The constructor normally takes a formatting string made from the following symbols:
: y z s w h k m
Char Meaning Char a d
AM or PM
Day of month
D
E
Meaning
Day of year
Day of week
Hour (1-12)
Hour (1-24)
Minute
F Day of week in month
G Era (AD or BC)
H Hour in Day (0-23)
Second K Hour in Day (0-11)
Week of year M Month
Year
Timezone
Separator
S
W
/
Millisecond
Week of month
Escape character
SimpleDateFormat(String formatString)
In most cases, the number of times a symbol is repeated determines how that data is presented.
Text information is displayed in an abbreviated form if the pattern letter is repeated less than four times. Otherwise unabbreviated form is used. For numbers, the number of times a pattern letter is repeated determines how many digits are presented. M or MM causes the month to be displayed as one or two digits. Three or more repetitions of M cause the month to be displayed as a text string.
One example of using SimpleDateFormat import java.text.*; import java.util.*; public class test
{
public static void main (String args[])
{
Date date = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy MMM dd hh:mm");
String rptDate = sdf.format(date);
System.out.println(rptDate+"\n");
}
}
One more example to format and parse it back.
// Create a formatter with the following pattern:
//Hour(0-23):Minute:Second
SimpleDateFormat formatter = new SimpleDateFormat( "HH:mm:ss" );
Date now = new Date();
String logEntry = formatter.format(now);
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// To read the string back in try {
Date sometime = formatter.parse( logEntry );
} catch ( ParseException exc ) {
exc.printStackTrace();
}
Note the ParseException that needs to be caught. It is thrown when the beginning of the input string cannot be parsed.
15.3 DateFormatSymbols class
The format
method of the
SimpleDateFormat
class returns a
String
composed of digits and symbols. For example, in the
String
"Friday, April 10, 1998," the symbols are "Friday" and
"April." If the symbols encapsulated in
SimpleDateFormat
don't meet your needs, you can change them with the
DateFormatSymbols
. You can change symbols that represent names for months, days of the week, and time zones, among others. The following table lists the
DateFormatSymbols
methods that allow you to modify the symbols:
DateFormatSymbol Methods
Setter Method Example of a Symbol the Method Modifies setAmPmStrings
PM setEras
AD setMonths
December setShortMonths Dec setShortWeekdays setWeekdays setZoneStrings
Tue
Tuesday
PST
The following example invokes setShortWeekdays to change the short names of the days of the week from lowercase to uppercase characters. The full source code for this example is in
DateFormatSymbolsDemo. The first element in the array argument of setShortWeekdays is a null String. Therefore the array is one-based rather than zero-based. The SimpleDateFormat constructor accepts the modified DateFormatSymbols object as an argument. Here is the source code:
DateFormatSymbols symbols = new DateFormatSymbols( new Locale("en","US"));
String [] defaultDays = symbols.getShortWeekdays(); for (int i = 0; i < defaultDays.length; i++) {
System.out.print(defaultDays[i] + " ");
}
System.out.println();
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String[] capitalDays = {“", "SUN", "MON", "TUE", "WED", "THU",
"FRI", "SAT"}; symbols.setShortWeekdays(capitalDays);
String [] modifiedDays = symbols.getShortWeekdays(); for (int i = 0; i < modifiedDays.length; i++) {
System.out.print(modifiedDays[i] + " ");
}
System.out.println();
SimpleDateFormat formatter = new SimpleDateFormat("E", symbols);
Date today = new Date();
String result = formatter.format(today);
System.out.println(result);
The preceding code generates this output:
Sun Mon Tue Wed Thu Fri Sat
SUN MON TUE WED THU FRI SAT
WED
15.4 NumberFormat class
Using Java's NumberFormat class, you can automatically format numbers according to the local conventions in use where your application is run. If your application is run in Germany, for instance, your number will automatically use the comma as the decimal separator and the period as the grouping separator--just the opposite of the custom in the US.
The first step is to create a NumberFormat object. You have your choice of three built-in formatting styles: a generic number formatter, a currency formatter, and a percentage formatter.
You don't use the new operator to construct your NumberFormat objects; instead you use very specific methods called factory methods.
The three factory methods you use to create built-in format objects are the getNumberInstance(), getCurrencyInstance(), and getPercentInstance(). Here's an example:
NumberFormat nf, cf, pf; nf = NumberFormat.getNumberInstance(); cf = NumberFormat.getCurrencyInstance(); pf = NumberFormat.getPercentInstance();
To use your NumberFormat object, you simply call its format() method, passing either a double primitive value or a Double object. For example,
NumberFormat fmt = NumberFormat.getCurrencyInstance();
System.out.println (“Total amount: "+fmt.format(amount));
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You can use the
NumberFormat
methods to format primitive-type numbers, such as double
, and their corresponding wrapper objects, such as
Double
.
The following code example formats a
Double
according to
Locale
. Invoking the getNumberInstance
method returns a locale-specific instance of
NumberFormat
. The format method accepts the
Double
as an argument and returns the formatted number in a
String
.
Double amount = new Double(345987.246);
NumberFormat numberFormatter =
NumberFormat.getNumberInstance(currentLocale);
String amountOut = numberFormatter.format(amount);
System.out.println(amountOut + " " + currentLocale.toString());
The output from this example shows how the format of the same number varies with
Locale
:
345 987,246
345.987,246
345,987.246
fr_FR
de_DE
en_US
If you're writing business applications, you'll probably need to format and to display currencies.
You format currencies in the same manner as numbers, except that you call getCurrencyInstance
to create a formatter. When you invoke the format method, it returns a
String that includes the formatted number and the appropriate currency sign.
This code example shows how to format currency in a locale-specific manner:
Double currency = new Double(9876543.21);
NumberFormat currencyFormatter =
NumberFormat.getCurrencyInstance(currentLocale);
String currencyOut = currencyFormatter.format(currency);
System.out.println(currencyOut + " " + currentLocale.toString());
The output generated by the preceding lines of code is as follows:
9 876 543,21 F fr_FR
9.876.543,21 DM de_DE
$9,876,543.21
en_US
At first glance this output may look wrong to you, because the numeric values are all the same.
Of course, 9 876 543,21 F is not equivalent to 9.876.543,21 DM. However, bear in mind that the
NumberFormat
class is unaware of exchange rates. The methods belonging to the
NumberFormat class format currencies but do not convert them.
You can also use the methods of the
NumberFormat
class to format percentages. To get the locale-specific formatter, invoke the getPercentInstance
method. With this formatter, a decimal fraction such as 0.75 is displayed as 75%. The following code sample shows how to format a percentage.
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Double percent = new Double(0.75);
NumberFormat percentFormatter =
NumberFormat.getPercentInstance(currentLocale);
String percentOut = percentFormatter.format(percent);
15.5 DecimalFormat class
You can use the
DecimalFormat
class to format decimal numbers into locale-specific strings.
This class allows you to control the display of leading and trailing zeros, prefixes and suffixes, grouping (thousands) separators, and the decimal separator. If you want to change formatting symbols, such as the decimal separator, you can use the
DecimalFormatSymbols in conjunction with the
DecimalFormat
class. These classes offer a great deal of flexibility in the formatting of numbers, but they can make your code more complex.
Applications that require highly customized number formatting and parsing may create custom
DecimalFormat class objects by passing a suitable pattern to the DecimalFormat() constructor method. The applyPattern() method can be used to change this pattern. A
DecimalFormatSymbols object can be optionally specified when creating a DecimalFormat object. If one is not specified, a DecimalFormatSymbols object suitable for the default locale is used. Decimal format patterns consists of a string of characters from the following table. For example: "$#,##0.00;($#,##0.00)"
'
,
.
-
;
%
Char
0
#
Interpretation
A digit // leading zeros show as 0
A digit // leading zeros show as absent
The locale-specific decimal separator
The locale-specific grouping separator (comma)
The locale-specific negative prefix
Shows value as a percentage
Separates a positive number format (on left) from an optional negative number format (on right)
Escapes a reserved character so it appears literally in the output
You specify the formatting properties of
DecimalFormat
with a pattern
String
. The pattern determines what the formatted number looks like.
The example that follows creates a formatter by passing a pattern String to the DecimalFormat constructor. The format method accepts a double value as an argument and returns the formatted number in a String:
DecimalFormat myFormatter = new DecimalFormat(pattern);
String output = myFormatter.format(value);
System.out.println(value + " " + pattern + " " + output);
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The output for the preceding lines of code is described in the following table. The value
is the number, a double
, that is to be formatted. The pattern
is the
String
that specifies the formatting properties. The output
, which is a
String
, represents the formatted number.
123456.78
9
value
123456.78
9
123.78
12345.67
Output from DecimalFormatDemo Program
pattern
###,###.###
###.##
000000.000
$###,###.###
12345.67 \ u00A5###,###.###
123,456.78
9
output
Explanation
The pound sign (#) denotes a digit, the comma is a placeholder for the grouping separator, and the period is a placeholder for the decimal separator.
123456.79 The value
has three digits to the right of the decimal point, but the pattern has only two. The format
method handles this by rounding up.
000123.780 The pattern
specifies leading and trailing zeros, because the 0 character is used instead of the pound sign (#).
$12,345.67 The first character in the pattern
is the dollar sign ($). Note that it immediately precedes the leftmost digit in the formatted output
.
¥12,345.67 The pattern
specifies the currency sign for Japanese yen (¥) with the
Unicode value 00A5.
Here is an example of how DecimalFormat can be used: import java.text.*; public class test
{
public static void main (String args[])
{
int numb = 3; String rptNumb;
DecimalFormat df = new DecimalFormat("000");
rptNumb = df.format(numb); System.out.println(rptNumb+"\n");
}
}
The preceding example created a
DecimalFormat
object for the default
Locale
. If you want a
DecimalFormat
object for a nondefault
Locale
, you instantiate a
NumberFormat
and then cast it to
DecimalFormat
. Here's an example:
NumberFormat nf = NumberFormat.getNumberInstance(loc);
DecimalFormat df = (DecimalFormat)nf; df.applyPattern(pattern);
String output = df.format(value);
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System.out.println(pattern + " " + output + " " + loc.toString());
Running the previous code example results in the output that follows. The formatted number, which is in the second column, varies with
Locale
:
###,###.### 123,456.789 en_US
###,###.### 123.456,789 de_DE
###,###.### 123 456,789 fr_FR
So far the formatting patterns discussed here follow the conventions of U.S. English. For example, in the pattern ###,###.## the comma is the thousands-separator and the period represents the decimal point. This convention is fine, provided that your end users aren't exposed to it. However, some applications, such as spreadsheets and report generators, allow the end users to define their own formatting patterns. For these applications the formatting patterns specified by the end users should use localized notation. In these cases you'll want to invoke the applyLocalizedPattern
method on the
DecimalFormat
object.
You can use the DecimalFormatSymbols class to change the symbols that appear in the formatted numbers produced by the format
method. These symbols include the decimal separator, the grouping separator, the minus sign, and the percent sign, among others.
The next example demonstrates the DecimalFormatSymbols class by applying a strange format to a number. The unusual format is the result of the calls to the setDecimalSeparator, setGroupingSeparator, and setGroupingSize methods.
DecimalFormatSymbols unusualSymbols =
new DecimalFormatSymbols(currentLocale); unusualSymbols.setDecimalSeparator('|'); unusualSymbols.setGroupingSeparator('^');
String strange = "#,##0.###";
DecimalFormat weirdFormatter =
new DecimalFormat(strange, unusualSymbols); weirdFormatter.setGroupingSize(4);
String bizarre = weirdFormatter.format(12345.678);
System.out.println(bizarre);
When run, this example prints the number in a bizarre format:
1^2345|678
15.6 Format class
The class
Format
provides several overloaded
Format.print(..)
static methods to output a
single number or string with formatting descriptions similar to the printf
function in C .
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For example,
double q =10.0/3.0;
Format.print("0.4f", q); will result in an output of
3.3333
To send formatted outputs directly to a string, the class provides several overloaded format(arg)
methods, where arg
is a number type. An instance of the format must first be created with the desired formatting description.
This can be done in a convenient single line approach by appending the method invocation to the instantiation operation as shown here: double q =10.0/3.0;
String str = new Format("0.3e").format(q) which results in the string variable str referencing "
3.333e+000
".
The following program shows the use of Format class. import java.text.*; class FormatDemo { public static void main(String args[]) { double q = 1.0/3.0;
// Create an instance of the format pattern
// and then create the string with the method format
String qValStr = new Format ("%0.3f").format (q);
System.out.println ("1.0/3.0 = " + qValStr);
// Can change the format pattern:
qValStr = new Format ("%0.5f").format (q);
System.out.println ("1.0/3.0 = " + qValStr);
// The # symbol indicates trailing blanks
q = 1.0/2.0;
qValStr = new Format ("%0.5g").format (q);
System.out.println ("1.0/2.0 = " + qValStr);
//
q = 1000.0/3.0;
qValStr = new Format ("%0.2e").format (q);
System.out.println ("1000.0/3.0 = " + qValStr);
//
q = 3.0/4567.0;
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qValStr = new Format ("%0.2e").format (q);
System.out.println ("3.0/4567.0 = " + qValStr);
// Negative infinity
q = -1.0/0.0;
qValStr = new Format ("%0.3e").format (q);
System.out.println ("-1.0/0.0 = " + qValStr);
}
}
// NaN
q = 0.0/0.0;
qValStr = new Format ("%0.3e").format (q);
System.out.println ("0.0/0.0 = " + qValStr);
Output of this program:
1.0/3.0 = 0.333
1.0/3.0 = 0.33333
1.0/2.0 = 0.5
1000.0/3.0 = 3.33e+002
3.0/4567.0 = 6.57e-004
-1.0/0.0 = -Inf
0.0/0.0 = NaN
Summary
To format a number to display to an end user, you use the
NumberFormat
class in the java.text package. When using
NumberFormat
, you can get a default format for decimal numbers, percentages, or currency. Or, you can design a custom format using patterns.
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Chapter 16 : Collections Framework and Generics
16.1 What is a Collection?
A collection (sometimes called a container) is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve and manipulate data, and to transmit data from one method to another. Collections typically represent data items that form a natural group.
A Java collection is a flexible data structure that can hold heterogeneous objects where the elements may have any reference type. It is your responsibility, however, to keep track of what types of objects your collections contain. As an example, consider adding an int to a collection; since you cannot have collections of primitive data types you must convert the int to the corresponding reference type (i.e. Integer) before storing it in the collection. Now, when the element is extracted from the collection an Object is returned that must be cast to an Integer in order to ensure type safety. All this makes Java programs unnecessarily hard to read and maintain, and are more likely to fail with runtime errors.
16.2 Generics
The motivation for adding generics to the Java programming language stems from the lack of information about a collection's element type, the need for developers to keep track of what type of elements collections contain, and the need for casts all over the place. Using generics, a collection is no longer treated as a list of Object references, but you would be able to differentiate between a collection of references to Integers and collection of references to Bytes.
A collection with a generic type has a type parameter that specifies the element type to be stored in the collection.
Generics allow you to define a class or interface once and instantiate it with a variety of types. A
generic (also known as a parametrized type) is a type that has one or more type parameters. To use a generic you supply an actual type argument for each type parameter and in doing so constrain the generic type to act only on the argument types.
Defining and Using Generic Types
To define a generic, you include type parameters following the type name. The type parameters are a comma separated list of identifiers delimited by angle brackets. By convention, type parameters are uppercase letters. The type parameters then appear in the type's methods, either as the type of arguments in the method's parameter list or as the type of its return value.
GenSample.java contains a very simple generic class called BasicGeneric, and a second class called GenSample that calls it. class BasicGeneric <A>
{
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private A data;
public BasicGeneric(A data)
{ this.data = data;
}
public A getData()
{ return data;
}
} public class GenSample
{
public String test01(String input)
{
String data01 = input;
BasicGeneric<String> basicGeneric = new
BasicGeneric<String>(data01);
String data02 = basicGeneric.getData(); return data02;
}
public int test02(int input)
{
Integer data01 = new Integer(input);
BasicGeneric <Integer> basicGeneric = new
BasicGeneric<Integer>(data01);
Integer data02 = basicGeneric.getData(); return data02;
}
public static void main(String [] args)
{
GenSample sample = new GenSample();
System.out.println(sample.test01("This generic
data"));
System.out.println(sample.test02(12));
}
}
Here you can see the brackets that surround the capital letter A: <A>. This syntax specifies that the class is a generic type. Notice also that the class declares a variable of type A – data. This syntax can be confusing at first glance. But it is quite sensible when you begin to understand generics. BasicGeneric does not work with any specific type. As a result, we don't assign a type to A. It is a generic type. But when you declare an instance of this class, you must specify the type with which you want to work:
BasicGeneric<String> basicGeneric
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The method getData() returns a value of type A, which is to say that it will return a generic type.
But that does not mean that the function will not have a type at run time, or even at compile time.
After you declare an instance of BasicGeneric, you will have specified the type of A. After that,
BasicGeneric will act as if it were declared from the very beginning to work with that specific type, and that type only.
16.3 What Is a Collections Framework?
A collections framework is a unified architecture for representing and manipulating collections.
The collections framework is a part of java.util package. All collections frameworks contain three things:
•
•
•
Interfaces: abstract data types representing collections. Interfaces allow collections to be
manipulated independently of the details of their representation. In object-oriented languages like Java, these interfaces generally form a hierarchy.
Implementations: concrete implementations of the collection interfaces. In essence,
these are reusable data structures.
Algorithms: methods that perform useful computations, like searching and sorting, on
objects that implement collection interfaces. These algorithms are said to be polymorphic because the same method can be used on many different implementations of the appropriate collections interface. In essence, algorithms are reusable functionality.
Benefits of Collection Framework
•
•
•
•
It reduces programming effort: By providing useful data structures and algorithms, a
collections framework frees you to concentrate on the important parts of your program, rather than the low-level plumbing required to make it work. By facilitating interoperability among unrelated APIs, the collections framework frees you from writing oodles of adapter objects or conversion code to connect APIs.
It increases program speed and quality: The collections framework does this primarily
by providing high-performance, high-quality implementations of useful data structures and algorithms. Also, because the various implementations of each interface are interchangeable, programs can be easily tuned by switching collection implementations.
Finally, because you're freed from the drudgery of writing your own data structures, you'll have more time to devote to improving the quality and performance of the rest of the program.
It allows interoperability among unrelated APIs: The collections interfaces will
become the "lingua franca" by which APIs pass collections back and forth. If my network administration API furnishes a
Collection
of node names, and your GUI toolkit expects a Collection of column headings, our APIs will interoperate seamlessly even though they were written independently.
It reduces the effort to learn and use new APIs: Many APIs naturally take collections
on input and output. In the past, each such API had a little "sub-API" devoted to manipulating its collections. There was little consistency among these ad-hoc collections sub-APIs, so you had to learn each one from scratch and it was easy to make mistakes
360
•
• when using them. With the advent of standard collections interfaces, the problem goes away.
It reduces effort to design new APIs: This is the flip-side of the previous advantage:
designers and implementers don't have to reinvent the wheel each time they create an API that relies on collections. They just use the standard collections interfaces.
It fosters software reuse: New data structures that conform to the standard collection
interfaces are by nature reusable. The same goes for new algorithms that operate on objects that implement these interfaces.
16.4 Collection Interfaces
The core collection interfaces are the interfaces used to manipulate collections, and to pass them from one method to another. The basic purpose of these interfaces is to allow collections to be manipulated independently of the details of their representation. The core collection interfaces are the heart and soul of the collections framework. The core collections interfaces are shown below:
Note that all of the core collection interfaces are generic. For example, the declaration of the
Collection interface is: public interface Collection<E> ...
The <E> syntax tells you that the interface is generic. When you declare a <code>
Collection instance you can and should specify the type of object contained in the collection. Specifying the type allows the compiler to verify (at compile time) that the type of object you put into the collection is correct, thus reducing errors at runtime.
Collection
The Collection interface is the root of the collection hierarchy. The Collection interface is the least common denominator that all collections implement, and is used to pass collections around and to manipulate them when maximum generality is desired. Some types of collections allow duplicate elements, and others do not. Some are ordered and others unordered. The Java platform doesn't provide any direct implementations of this interface but provides implementations of more specific subinterfaces, such as Set and List.
Set
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A Set is a collection that cannot contain duplicate elements. As you might expect, this interface models the mathematical set abstraction. The
Set
interface extends
Collection and contains no methods other than those inherited from
Collection.
It is used to represent sets like the cards comprising a poker hand, the courses making up a student's schedule, or the processes running on a machine.
List
A List is an ordered collection (sometimes called a sequence). Lists can contain duplicate elements. The user of a
List
generally has precise control over where in the
List
each element is inserted. The user can access elements by their integer index (position).
Queue
A collection used to hold multiple elements prior to processing. Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations. Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering. Whatever the ordering used, the head of the queue is that element that would be removed by a call to remove or poll. In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every Queue implementation must specify its ordering properties.
Map
A Map is an object that maps keys to values. Maps cannot contain duplicate keys: Each key can map to at most one value.
SortedSet
A
SortedSet
is a
Set
that maintains its elements in ascending order.
SortedMap
A SortedMap is a
Map
that maintains its mappings in ascending key order. It is the
Map
analogue of
SortedSet
. The
SortedMap
interface is used for apps like dictionaries and telephone directories.
16.5 The Collection Interface
The Collection interface is used to pass around collections of objects where maximum generality is desired. For example, by convention all general-purpose collection implementations have a constructor that takes a Collection argument. This constructor, known as a conversion constructor, initializes the new collection to contain all the elements in the specified Collection, whatever the given collection's subinterface or implementation type. In other words, it allows you to convert the type of the collection.
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Suppose, for example, that you have a Collection<String> c, which may be a List, a Set, or another kind of Collection. The following idiom creates a new ArrayList (an implementation of the List interface), initially containing all the elements in c:
List<String> list = new ArrayList<String>(c);
The interface does about what you'd expect, given that a Collection represents a group of objects.
The interface has methods to tell you how many elements are in the collection (size, isEmpty), to check whether a given object is in the collection (contains), to add and remove an element from the collection (add, remove), and to provide an iterator over the collection (iterator).
The add method is defined generally enough so that it makes sense for collections that allow duplicates as well as those that don't. It guarantees that the Collection will contain the specified element after the call completes, and returns true if the Collection changes as a result of the call.
Similarly, the remove method is defined to remove a single instance of the specified element from the Collection, assuming that it contains the element to start with, and to return true if the
Collection was modified as a result.
The following program shows a simple use of ArrayList. An arraylist is created , and then objects of type String are added to it. The list is then displayed. Some of the elements are removed and the list is displayed again. import java.util.*; class ArrayListDemo { public static void main(String args[]) {
//create an array list
ArrayList al = new ArrayList();
System.out.println(“Initial size of al :”+al.size());
//add elements to arraylist al.add(“C”); al.add(“A”); al.add(“E”); al.add(“B”); al.add(“D”); al.add(“F”);
System.out.println(“Size of al after additions:” + al.size());
//display the array list
System.out.println(“Contents of al:”+al);
//Remove elements from arraylist al.remove(“F”); al.remove(“E”);
System.out.println(“Size of al after deletions :”
+al.size());
System.out.println(“Contents of al : ” + al);
}
}
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The output of this program is shown here:
Initial size of al : 0
Size of al after additions : 6
Contents of al : [C,A,E,B,D,F]
Size of al after deletions : 4
Contents of al : [C,A,B,D]
Traversing Collections
There are two ways to traverse collections: with the for-each construct and using iterators.
For-Each Construct
The for-each construct allows you to concisely traverse a collection or array using a for loop The for Statement. The following code uses the for-each construct to print out each element of a collection on a separate line: for (Object o : collection)
System.out.println(o);
The following code shows an example import java.util.*; class ArrayListDemo2 { public static void main(String args[]) {
ArrayList al = new ArrayList(); al.add(new Integer(10)); al.add(“ABC”); al.add(new Float(1.0)); for(Object o : al) {
System.out.println(o);
}
}
}
Iterators
An Iterator is an object that enables you to traverse through a collection, and to remove elements from the collection selectively, if desired. You get an Iterator for a collection by calling its iterator method.
The hasNext method returns true if the iteration has more elements, and the next method returns the next element in the iteration. The remove method removes from the underlying Collection the last element that was returned by next. The remove method may be called only once per call to next and throws an exception if this rule is violated.
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Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.
Use an iterator instead of the for-each construct when:
•
•
You need to remove the current element. The for-each construct hides the iterator, so you cannot call remove. Therefore, the for-each construct is not usable for filtering.
•
You need to replace elements in a list or array as you traverse it.
You need to iterate over multiple collections in parallel.
An example that shows the use of Iterator:
//Demonstrate Iterators import java.util.*; class IteratorDemo { public static void main(String args[]) {
ArrayList al = new ArrayList();
//add elements to arraylist al.add(“A”); al.add(“B”); al.add(“C”); al.add(“D”); al.add(“E”);
//use Iterator to display contents of al
Iterator itr = al.iterator(); while(itr.hasNext()) {
Object element = itr.next();
System.out.print(element+ “ ”); if(((String)element).equals(“E”)) { itr.remove();
}
}
System.out.println();
System.out.println(al);
}
}
The output of this program is as follows :
A B C D E
[A,B,C,D]
Collection Interface Bulk Operations
The bulk operations perform an operation on an entire
Collection
. You could implement these shorthand operations using the basic operations, though in most cases such implementations would be less efficient. The bulk operations are:
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•
• containsAll
: Returns true
if the target
Collection
contains all of the elements in the specified
Collection
.
• addAll
: Adds all the elements in the specified
Collection
to the target
Collection
.
• removeAll
: Removes from the target
Collection
all its elements that are also contained in the specified
Collection
.
• retainAll
: Removes from the target
Collection
all its elements that are not also contained in the specified
Collection
. That is, it retains in the target
Collection
only those elements that are also contained in the specified
Collection
. clear
: Removes all elements from the
Collection
.
The addAll
, removeAll
, and retainAll
methods all return true
if the target
Collection
was modified in the process of executing the operation.
Collection Interface Array Operations
The toArray
methods are provided as a bridge between collections and older APIs that expect arrays on input. The array operations allow the contents of a
Collection
to be translated into an array. The simple form with no arguments creates a new array of
Object
. The more complex form allows the caller to provide an array or to choose the runtime type of the output array.
For example, suppose that c is a Collection. The following snippet dumps the contents of c into a newly allocated array of Object whose length is identical to the number of elements in c:
Object[] a = c.toArray();
Suppose that c
is known to contain only strings (perhaps because c
is of type
Collection<String>
). The following snippet dumps the contents of c
into a newly allocated array of
String
whose length is identical to the number of elements in c
:
String[] a = c.toArray(new String[0]);
An example of converting an ArrayList into an array.
import java.util.*; class ArrayListToArray { public static void main(String args[]){
ArrayList al = new ArrayList();
//add elements to array list al.add(new Integer(1)); al.add(new Integer(2)); al.add(new Integer(3)); al.add(new Integer(4));
System.out.println(“Contents of al:”+al);
//get array
Object [] ia = al.toArray(); int sum=0;
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//sum of array elements for(int i=0; i < ia.length ; i++) { sum += (Integer)ia;
}
System.out.println(“Sum is:”+sum);
}
}
The output of the program is shown below:
Contents of al: [1,2,3,4]
Sum is: 10
16.6 The Set Interface
A
Set
is a
Collection
that cannot contain duplicate elements. It models the mathematical set abstraction. The
Set
interface contains only methods inherited from
Collection
, and adds the restriction that duplicate elements are prohibited.
Set
also adds a stronger contract on the behavior of the equals
and hashCode
operations, allowing
Set
instances to be compared meaningfully even if their implementation types differ. Two
Set
instances are equal if they contain the same elements.
The Java platform contains three general-purpose Set implementations: HashSet, TreeSet, and
LinkedHashSet HashSet, which stores its elements in a hash table, is the best-performing implementation but it makes no guarantees concerning the order of iteration. TreeSet, which stores its elements in a red-black tree, orders its elements based on their values, but is substantially slower than HashSet. LinkedHashSet, which is implemented as a hash table with a linked list running through it, orders its elements based on the order in which they were inserted into the set (insertion-order). LinkedHashSet spares its clients from the unspecified, generally chaotic ordering provided by HashSet, at a cost that is only slightly higher.
Here's a simple but useful Set idiom. Suppose you have a Collection, c, and you want to create another Collection containing the same elements but with all duplicates eliminated. The following one-liner does the trick:
Collection<Type> noDups = new HashSet<Type>(c);
It works by creating a Set, which by definition, cannot contain duplicate,) initially containing all the elements in c
Here is a minor variant of this idiom that preserves the order of the original collection while removing duplicate element:
Collection<Type> noDups = new HashSet<Type>(c);
Here is a generic method that encapsulates the above idiom, returning a set of the same generic type as the one passed in:
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public static <E> Set<E> removeDups(Collection<E> c) { return new LinkedHashSet<E>(c);
}
Set Interface Basic Operations
The size operation returns the number of elements in the Set (its cardinality). The isEmpty method does exactly what you think it does. The add method adds the specified element to the
Set if it's not already present, and returns a Boolean indicating whether the element was added.
Similarly, the remove method removes the specified element from the Set if it's present and returns a Boolean indicating whether the element was present. The iterator method returns an
Iterator over the Set.
Here's a program that takes the words in its argument list and prints out any duplicate words, the number of distinct words, and a list of the words with duplicates eliminated: import java.util.*; public class FindDups {
public static void main(String args[]) {
Set<String> s = new HashSet<String>();
for (String a : args) {
if (!s.add(a)) {
System.out.println("Duplicate: " + a);
}
}
System.out.println(s.size()+" distinct words:" + s);
}
}
Now let's run the program: java FindDups i came i saw i left
The following output is produced:
Duplicate: i
Duplicate: i
4 distinct words: [i, left, saw, came]
Note that the code always refers to the collection by its interface type (Set), rather than by its implementation type (HashSet). This is a strongly recommended programming practice, as it gives you the flexibility to change implementations merely by changing the constructor. If either the variables used to store a collection or the parameters used to pass it around are declared to be of the collection's implementation type rather than its interface type, all such variables and parameters must be changed in order to change the collection's implementation type. If the program uses any nonstandard operations that are present in the original implementation type but
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not in the new one, the program will fail. Referring to collections only by their interface prevents you from using any nonstandard operations.
The implementation type of the Set in the preceding example is HashSet, which makes no guarantees as to the order of the elements in the Set. If you want the program to print the word list in alphabetical order, merely change the set's implementation type from HashSet to TreeSet.
Making this trivial one-line change causes the command line in the previous example to generate the following output: java FindDups i came i saw i left
Duplicate word: i
Duplicate word: i
4 distinct words: [came, i, left, saw]
Set Interface Bulk Operations
The bulk operations are particularly well suited to Sets; when applied to sets, they perform standard set-algebraic operations. Suppose s1 and s2 are Sets. Here's what the bulk operations do:
• s1.containsAll(s2)
: Returns true
if s2
is a subset of s1
. ( s2
is a subset of s1
if set s1 contains all the elements in s2
.)
• s1.addAll(s2)
: Transforms s1
into the union of s1
and s2
. (The union of two sets is the set containing all the elements contained in either set.)
• s1.retainAll(s2)
: Transforms s1
into the intersection of s1
and s2
. (The intersection of two sets is the set containing only the elements that are common to both sets.)
• s1.removeAll(s2)
: Transforms s1
into the (asymmetric) set difference of s1
and s2
.
(For example, the set difference of s1
- s2
is the set containing all the elements found in s1
but not in s2
.)
To calculate the union, intersection, or set difference of two sets nondestructively (without modifying either set), the caller must copy one set before calling the appropriate bulk operation.
The resulting idioms follow:
Set<Type> union = new HashSet<Type>(s1); union.addAll(s2);
Set<Type> intersection = new HashSet<Type>(s1); intersection.retainAll(s2);
Set<Type> difference = new HashSet<Type>(s1); difference.removeAll(s2);
The implementation type of the result Set in the preceding idioms is HashSet, which is, as already mentioned, the best all-around Set implementation in the Java platform. However, any general-purpose Set implementation could be substituted.
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Let's revisit the FindDups program. Suppose that you want to know which words in the argument list occur only once and which occur more than once but that you do not want any duplicates printed out repeatedly. This effect can be achieved by generating two sets, one containing every word in the argument list and the other containing only the duplicates. The words that occur only once are the set difference of these two sets, which we know how to compute. Here's how the resulting program looks: import java.util.*; public class FindDups2 {
public static void main(String args[]) {
Set<String> uniques = new HashSet<String>();
Set<String> dups = new HashSet<String>();
for (String a : args) {
if (!uniques.add(a)) {
dups.add(a);
}
}
// Destructive set-difference
uniques.removeAll(dups);
System.out.println("Unique words: " + uniques);
System.out.println("Duplicate words: " + dups);
}
}
When run with the same same argument list used earlier (i came i saw i left), the program yields the output:
Unique words: [left, saw, came]
Duplicate words: [i]
A less common set-algebraic operation is the symmetric set difference: the set of elements contained in either of two specified sets but not in both. The following code calculates the symmetric set difference of two sets nondestructively:
Set<Type> symmetricDiff = new HashSet<Type>(s1); symmetricDiff.addAll(s2);
Set<Type> tmp = new HashSet<Type>(s1); tmp.retainAll(s2)); symmetricDiff.removeAll(tmp);
Set Interface Array Operations
The array operations don't do anything special for
Sets
beyond what they do for any other
Collection
.
Set Implementations
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HashSet extends AbstractSet and implements the Set interface. It creates a collection that uses a hash table for storage. An example of using HashSet. import java.util.*; class HashSetDemo { public static void main(String args[]) {
//create a hash set
HashSet hs = new HashSet(); hs.add(“A”); hs.add(“B”); hs.add(“C”); hs.add(“D”); hs.add(“E”);
System.out.println(hs);
}
}
The LinkedHashSet extends HashSet and maintains a linked list of entries in the set in the order in which they were inserted.
TreeSet stores objects in ascending order. Access and retrieval times are quite fast which makes
TreeSet an excellent choice when storing large amounts of sorted information that must be found quickly. An example of TreeSet is shown below: import java.util.*; class TreeSetDemo { public static void main(String args[]) {
TreeSet ts = new TreeSet(); ts.add(“C”); ts.add(“A”); ts.add(“B”); ts.add(“E”); ts.add(“F”); ts.add(“D”);
System.out.println(ts);
}
}
The output from this program is shown here:
[A,B,C,D,E,F]
16.7 The List Interface
A
List
is an ordered
Collection
(sometimes called a sequence). Lists may contain duplicate elements. In addition to the operations inherited from
Collection
, the
List
interface includes operations for the following:
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•
Positional Access: Manipulate elements based on their numerical position in the list.
•
Search: Search for a specified object in the list and return its numerical position.
•
List Iteration: Extend
Iterator
semantics to take advantage of the list's sequential nature.
•
Range-view: Perform arbitrary range operations on the list.
The Java platform contains two general-purpose
List
implementations.
ArrayList
, which is generally the better-performing implementation, and
LinkedList
which offers better performance under certain circumstances. Also,
Vector
has been retrofitted to implement
List
.
Comparison to Vector
If you've used
Vector
, you're already familiar with the general flavor of
List
.
List
fixes several minor API deficiencies in
Vector
. Commonly used
Vector
operations such as elementAt
and setElementAt
, have been given much shorter names. When you consider that these two operations are the
List
analogue of square brackets for arrays, it becomes apparent that shorter names are highly desirable. Consider the following assignment statement: a[i] = a[j].times(a[k]);
The
Vector
equivalent is: v.setElementAt(v.elementAt(j).times(v.elementAt(k)), i);
The
List
equivalent is: v.set(i, v.get(j).times(v.get(k)));
You may already have noticed that the set method, which replaces the Vector method setElementAt, reverses the order of the arguments so that they match the corresponding array operation. Consider this assignment statement: gift[5] = "golden rings";
The
Vector
equivalent is: gift.setElementAt("golden rings", 5);
The
List
equivalent is: gift.set(5, "golden rings");
For consistency's sake, the method add(int, E), which replaces insertElementAt(Object, int), also reverses the order of the arguments.
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The various range operations in Vector (indexOf, lastIndexOf(setSize) have been replaced by a single range-view operation (subList), which is far more powerful and consistent.
Collection Operations
The operations inherited from
Collection
all do about what you'd expect them to do, assuming you're already familiar with them from
Collection
. The remove
operation always removes the
first occurrence of the specified element from the list. The add
and addAll
operations always append the new element(s) to the end of the list. Thus, the following idiom concatenates one list to another: list1.addAll(list2);
Here's a non-destructive form of this idiom, which produces a third
List
consisting of the second list appended to the first:
List<Type> list3 = new ArrayList<Type>(list1); list3.addAll(list2);
Note that the idiom, in its non-destructive form, takes advantage of ArrayList's standard conversion constructor.
Like the Set interface, List strengthens the requirements on the equals and hashCode methods so that two List objects can be compared for logical equality without regard to their implementation classes. Two List objects are equal if they contain the same elements in the same order.
Positional Access and Search Operations
The basic positional access operations ( get
, set
, add
and remove
) behave just like their longernamed counterparts in
Vector
( elementAt
, setElementAt
, insertElementAt
and removeElementAt
) with one noteworthy exception. The set
and remove
operations return the old value that is being overwritten or removed; the
Vector
counterparts ( setElementAt
and removeElementAt
) return nothing ( void
). The search operations indexOf
and lastIndexOf behave exactly like the identically named operations in
Vector
.
The addAll operation inserts all of the elements of the specified Collection starting at the specified position. The elements are inserted in the order they are returned by the specified
Collection's iterator. This call is the positional access analogue of Collection's addAll operation.
Here's a little method to swap two indexed values in a List: public static <E> void swap(List<E> a, int i, int j) {
E tmp = a.get(i);
a.set(i, a.get(j));
a.set(j, tmp);
}
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Of course there's one big difference. This is a polymorphic algorithm: It swaps two elements in any List, regardless of its implementation type. Here's another polymorphic algorithm that uses the swap method above: public static void shuffle(List<?> list, Random rnd) {
for (int i = list.size(); i > 1; i--)
swap(list, i - 1, rnd.nextInt(i));
}
This algorithm, which is included in the Java platform's
Collections
class, randomly permutes the specified
List
using the specified source of randomness. It's a bit subtle: It runs up the list from the bottom, repeatedly swapping a randomly selected element into the current position.
Unlike most naive attempts at shuffling, it's fair (all permutations occur with equal likelihood, assuming an unbiased source of randomness) and fast (requiring exactly list.size()-1
swaps).
The following program uses this algorithm to print the words in its argument list in random order: import java.util.*; public class Shuffle {
public static void main(String args[]) {
List<String> list = new ArrayList<String>();
for (String a : args) {
list.add(a);
}
Collections.shuffle(list, new Random());
System.out.println(list);
}
}
We can make this program even shorter and faster. The Arrays class has a static factory method called asList that allows an array to be viewed as a List. This method does not copy the array.
Changes in the
List
write through to the array, and vice-versa. The resulting
List
is not a general-purpose
List
implementation, in that it doesn't implement the (optional) add
and remove operations: Arrays are not resizable. Taking advantage of
Arrays.asList
and calling the library version of shuffle
that uses a default source of randomness, you get the following tiny program
, whose behavior is identical to the previous program: import java.util.*; public class Shuffle {
public static void main(String args[]) {
List<String> list = Arrays.asList(args);
Collections.shuffle(list);
System.out.println(list);
}
}
An example that demonstrates the use of various algorithms
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import java.util.*; class AlgorithmDemo { public static void main(String args[]) {
ArrayList al = new ArrayList(); al.add(new Integer(-8)); al.add(new Integer(20)); al.add(new Integer(-20)); al.add(new Integer(8));
//create a reverse order Comparator
Comparator r = Collections.reverseOrder();
//Sort list using comparator
Collections.sort(al,r);
//Get iterator
Iterator itr = al.iterator();
System.out.println(“List sorted in reverse:”); while(itr.hasNext()) {
System.out.print(li.next()+ “ ”);
}
System.out.println();
Collections.shuffle(al);
//display randomized list itr = al.iterator();
System.out.println(“List shuffled:”); while(itr.hasNext()) {
System.out.print(li.next()+ “ ”);
}
System.out.println();
System.out.println(“Minimum:”+Collections.min(al));
System.out.println(“Maximum:”+Collections.max(al));
}
}
An example that shows the use of Arrays class import java.util.*; class ArraysDemo { public static void main(String args[]) {
//allocate and initialize array int array[] =new int[10]; for(int i=0; i< 10; i++) { array[i] = -3 * i;
}
//display, sort and display
System.out.print(“Original contents:”); display(array);
Arrays.sort(array);
System.out.print(“Sorted:”); display(array);
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//fill and display
Arrays.fill(array,2,6,-1);
System.out.print(“After fill:”); display(array);
//sort and display
Arrays.sort(array);
System.out.print(“After sorting again:”); display(array);
//binary search for –9
System.out.print(“The value –9 is at location:”); int index = Arrays.binarySearch(array,-9);
System.out.print(index);
} static void display(int array[]) { for(int i= 0 ;i<array.length;i++) {
System.out.print(array[i]+ “ ");
}
System.out.println();
}
}
The output is as follows:
Original contents : 0 –3 –6 –9 –12 –15 –18 –21 –24 –27
Sorted : -27 –24 –21 –18 –15 –12 –9 –6 –3 0
After fill: -27 –24 –1 –1 –1 –1 –9 –6 –3 0
After sorting again: -27 –24 –9 –6 –3 –1 –1 –1 –1 0
The value –9 is at location 2
Iterators
The
Iterator
returned by
List
's iterator
operation returns the elements of the list in proper sequence.
List
also provides a richer iterator, called a
ListIterator
, that allows you to traverse the list in either direction, modify the list during iteration, and obtain the current position of the iterator.
The three methods that
ListIterator
inherits from
Iterator
( hasNext
, next
, and remove
) do exactly the same thing in both interfaces. The hasPrevious
and the previous
operations are exact analogues of hasNext
and next
. The former operations refer to the element before the
(implicit) cursor, whereas the latter refer to the element after the cursor. The previous
operation moves the cursor backwards, whereas next
moves it forwards.
Here's the standard idiom for iterating backwards through a list: for (ListIterator<Type> i = list.listIterator(list.size());
i.hasPrevious(); ) {
Type t = i.previous();
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...
}
Note the argument to listIterator in the preceding idiom. The List interface has two forms of the listIterator method. The form with no arguments returns a ListIterator positioned at the beginning of the list; the form with an int argument returns a ListIterator positioned at the specified index.
The index refers to the element that would be returned by an initial call to next. An initial call to previous
would return the element whose index was index-1
. In a list of length n
, there are n+1 valid values for index
, from
0
to n
, inclusive.
Intuitively speaking, the cursor is always between two elements, the one that would be returned by a call to previous and the one that would be returned by a call to next. The n+1 valid index values correspond to the n+1 gaps between elements, from the gap before the first element to the gap after the last one. The figure below shows the five possible cursor positions in a list containing four elements.
Calls to next
and previous
can be intermixed, but you have to be a bit careful. The first call to previous
returns the same element as the last call to next
. Similarly, the first call to next
after a sequence of calls to previous
returns the same element as the last call to previous
.
It should come as no surprise that the nextIndex method returns the index of the element that would be returned by a subsequent call to next, and previousIndex returns the index of the element that would be returned by a subsequent call to previous. These calls are typically used either to report the position where something was found or to record the position of the
ListIterator so that another ListIterator with identical position can be created.
It should also come as no surprise that the number returned by nextIndex is always one greater than the number returned by previousIndex. This implies the behavior of the two boundary cases: a call to previousIndex when the cursor is before the initial element returns -1, and a call to nextIndex when the cursor is after the final element returns list.size(). To make all of this concrete, here's a possible implementation of List.indexOf: public int indexOf(E o) {
for (ListIterator<E> i = listIterator(); i.hasNext(); ) {
if (o==null ? i.next()==null : o.equals(i.next())) {
return i.previousIndex();
}
}
return -1; // Object not found
}
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Note that the indexOf method returns i.previousIndex() though it is traversing the list in the forward direction. The reason is that i.nextIndex() would return the index of the element that we are about to examine, and we want to return the index of the element that we just examined.
The Iterator interface provides the remove operation to remove from the Collection the last element returned by next. For ListIterator, this operation removes the last element returned by next
or previous
. The
ListIterator
interface provides two additional operations to modify the list: set
and add
. The set
method overwrites the last element returned by next
or previous with the specified element. The following polymorphic algorithm uses set
to replace all occurrences of one specified value with another: public static <E> void replace(List<E> s, E val, E newVal) {
for (ListIterator<E> i = s.listIterator(); i.hasNext(); ) {
if (val==null ? i.next()==null : val.equals(i.next())) {
i.set(newVal);
}
}
}
The only bit of trickiness in this example is the equality test between val
and i.next
. We have to special-case an val
value of null
in order to prevent a
NullPointerException
.
The add method inserts a new element into the list, immediately before the current cursor position. This method is illustrated in the following polymorphic algorithm to replace all occurrences of a specified value with the sequence of values contained in the specified list: public static <E> void replace(List<E> s, E val,
List<E> newVals) {
for (ListIterator<E> i = s.listIterator(); i.hasNext();){
if (val==null ? i.next()==null : val.equals(i.next())) {
i.remove();
for (E e : newVals)
i.add(e);
}
}
}
Range-View Operation
The range-view operation, subList(int fromIndex, int toIndex)
, returns a
List
view of the portion of this list whose indices range from fromIndex
, inclusive, to toIndex
, exclusive.
This half-open range mirrors the typical for
loop: for (int i = fromIndex; i < toIndex; i++) {
...
}
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As the term view implies, the returned
List
is backed by the
List
on which subList
was called, so changes in the former
List
are reflected in the latter.
This method eliminates the need for explicit range operations (of the sort that commonly exist for arrays). Any operation that expects a List can be used as a range operation by passing a subList view instead of a whole List. For example, the following idiom removes a range of elements from a list: list.subList(fromIndex, toIndex).clear();
Similar idioms may be constructed to search for an element in a range: int i = list.subList(fromIndex, toIndex).indexOf(o); int j = list.subList(fromIndex, toIndex).lastIndexOf(o);
Note that the above idioms return the index of the found element in the subList, not the index in the backing List.
Any polymorphic algorithm that operates on a List, such as the replace and shuffle examples above, works with the List returned by subList.
Here's a polymorphic algorithm whose implementation uses subList to deal a hand from a deck.
That is to say, it returns a new List (the "hand") containing the specified number of elements taken from the end of the specified List (the "deck"). The elements returned in the hand are removed from the deck. public static <E> List<E> dealHand(List<E> deck, int n) {
int deckSize = deck.size();
List<E> handView = deck.subList(deckSize - n, deckSize);
List<E> hand = new ArrayList<E>(handView);
handView.clear();
return hand;
}
Note that this algorithm removes the hand from the end of the deck. For many common List implementations, such as ArrayList, the performance of removing elements from the end of the list is substantially better than that of removing elements from the beginning.
Here's a program using the dealHand method in combination with Collections.shuffle to generate hands from a normal 52-card deck. The program takes two command line arguments: the number of hands to deal and the number of cards in each hand. import java.util.*; class Deal {
public static void main(String[] args) {
int numHands = Integer.parseInt(args[0]);
int cardsPerHand = Integer.parseInt(args[1]);
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// Make a normal 52-card deck
String[] suit = new String[]
{"spades", "hearts", "diamonds", "clubs"};
String[] rank = new String[]
{"ace","2","3","4","5","6","7","8",
"9","10","jack","queen","king"};
List<String> deck = new ArrayList<String>();
for (int i = 0; i <suit.length; i++) {
for (int j = 0; j <rank.length; j++) {
deck.add(rank[j] + " of " + suit[i]);
}
}
Collections.shuffle(deck);
for (int i=0; i<numHands; i++) {
System.out.println(dealHand(deck, cardsPerHand));
}
}
}
Running the program produces the following output: java Deal 4 5
[8 of hearts, jack of spades, 3 of spades, 4 of spades,
king of diamonds]
[4 of diamonds, ace of clubs, 6 of clubs, jack of hearts,
queen of hearts]
[7 of spades, 5 of spades, 2 of diamonds, queen of diamonds,
9 of clubs]
[8 of spades, 6 of diamonds, ace of spades, 3 of hearts,
ace of hearts]
Although the subList operation is extremely powerful, some care must be exercised when using it. The semantics of the List returned by subList become undefined if elements are added to or removed from the backing List in any way other than via the returned List. Thus, it's highly recommended that you use the List returned by subList only as a transient object: to perform one or a sequence of range operations on the backing List. The longer you use the sublist instance, the greater the probability that you'll compromise it by modifying the backing List directly or through another sublist object. Note that it is legal to modify a sublist of a sublist and to continue using the original sublist (though not concurrently).
List Algorithms
Most of the polymorphic algorithms in the Collections class apply specifically to List. Having all these algorithms at your disposal makes it very easy to manipulate lists. Here's a summary of these algorithms.
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sort
: Sorts a List using a merge sort algorithm, which provides a fast, stable sort. (A
stable sort is one that does not reorder equal elements.)
shuffle
: Randomly permutes the elements in a
List
.
reverse
: Reverses the order of the elements in a
List
.
rotate
: Rotates all of the elements in a
List
by a specified distance.
swap
: Swaps the elements at specified positions in in a
List
.
replaceAll
: Replaces all occurrences of one specified value with another.
fill
: Overwrites every element in a
List
with the specified value.
copy
: Copies the source
List
into the destination List.
binarySearch
: Searches for an element in an ordered
List
using the binary search algorithm.
indexOfSubList
: Returns the index of the first sublist of one
List
that is equal to another.
lastIndexOfSubList
: Returns the index of the last sublist of one
List
that is equal to another.
16.8 The Queue Interface
A Queue is a collection for holding elements prior to processing. Besides basic Collection operations, queues provide additional insertion, removal, and inspection operations.
Each
Queue
method exists in two forms: one throws an exception if the operation fails, the other returns a special value (either null
or false
, depending on the operation). The regular structure of the interface is illustrated in the following table:
Queue Interface Structure
Throws exception Returns special value
Insert add(e) offer(e)
Remove remove()
Examine element() poll() peek()
Queues typically, but not necessarily, order elements in a FIFO (first-in-first-out) manner.
Among the exceptions are priority queues, which order elements according to a their values.
Whatever ordering is used, the head of the queue is the element that would be removed by a call to remove
or poll
. In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every
Queue
implementation must specify its ordering properties.
The add method, which Queue inherits from Collection, inserts an element unless it would violate the queue's capacity restrictions, in which case it throws IllegalStateExcepion. The offer method, which is intended solely for use on bounded queues, differs from add only in that it indicates failure to insert an element by returning false.
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The remove and poll methods both remove and return the head of the queue. Exactly which element gets removed is a function of the queue's ordering policy. The remove and poll methods differ in their behavior only when the queue is empty. Under these circumstances, remove throws
NoSuchElementException, while poll returns null.
The element and peek methods return, but do not remove, the head of the queue. They differ from one another in precisely the same fashion as remove and poll: if the queue is empty, element throws NoSuchElementException while peek returns false.
Queue implementations generally do not allow insertion of null elements. The LinkedList implementation (which was retrofitted to implement Queue) is an exception. For historical reasons, it permits null elements, but you should refrain from taking advantage of this, as null is used as a special return value by the poll and peek methods.
Queue implementations generally do not define element-based versions of the equals and hashCode methods but instead inherit the identity-based versions from Object.
In the following example program, a queue is used to implement a countdown timer. The queue is preloaded with all the integer values from a number specified on the command line to zero, in descending order. Then the values are removed from the queue and printed at one second intervals. The program is artificial in that it would be more natural to do the same thing without using a queue, but it illustrates the use of a queue to store elements prior to subsequent processing: import java.util.*; public class Countdown {
public static void main(String[] args) throws
InterruptedException {
int time = Integer.parseInt(args[0]);
Queue<Integer> queue = new LinkedList<Integer>();
for (int i = time; i >= 0; i--)
queue.add(i);
while(!queue.isEmpty()) {
System.out.println(queue.remove());
Thread.sleep(1000);
}
}
}
In the following example, a priority queue is used to sort a collection of elements. Again this program is artificial, in that there is no reason to use it in favor of the sort method provided in
Collections, but it illustrates the behavior of priority queues: static <E> List<E> heapSort(Collection<E> c) {
Queue<E> queue = new PriorityQueue<E>(c);
List<E> result = new ArrayList<E>();
while (!queue.isEmpty())
result.add(queue.remove());
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return result;
}
The following program illustrates several methods supported by LinkedList.
import java.util.*; class LinkedListDemo { public static void main(String args[]) {
//create a linked list
LinkedList ll = new LinkedList();
//add elements to linked list ll.add(“A”); ll.add(“B”); ll.add(“C”); ll.add(“D”); ll.addLast(“E”); ll.addFirst(“F”); ll.add(1, “A2”);
System.out.println(“Original contents of ll:”+ll);
//remove elements from linked list ll.remove(“F”); ll.remove(2);
System.out.println(“Contents of ll after deletion:”
+ ll);
//remove first and last elements ll.removeFirst(); ll.removeLast();
System.out.println(“ll after deleting first and last:”+ ll);
//get and set value
Object val = ll.get(1); ll.set(1,(String)val+ “changed”);
System.out.println(“ll after change:” + ll);
}
}
The output of this program is shown here
Original contents of ll : [F,A,B,C,D,E]
Contents of ll after deletion: [A,B,D,E] ll after deleting first and last: [B,D] ll after change: [B,Dchanged]
16.9 The Map Interface
A
Map
is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value.
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The Java platform contains three general-purpose
Map
implementations:
HashMap
,
TreeMap
, and
LinkedHashMap
. Their behavior and performance are precisely analogous to
HashMap
,
TreeMap
, and
LinkedHashMap
. Also,
Hashtable
was retrofitted to implement
Map
.
Comparison to Hashtable
If you've used
Hashtable
, you're already familiar with the general flavor of
Map
. (Of course
Map is an interface, while
Hashtable
is a concrete implementation.) Here are the major differences:
•
Map
provides
Collection
views instead of direct support for iteration via
Enumeration objects.
Collection
views greatly enhance the expressiveness of the interface.
•
Map
allows you to iterate over keys, values, or key-value pairs;
Hashtable
does not provide the third option.
•
Map
provides a safe way to remove entries in the midst of iteration;
Hashtable
did not.
Finally,
Map
fixes a minor deficiency in the
Hashtable
interface.
Hashtable
has a method called contains
, which returns true
if the
Hashtable
contains a given value. Given its name, you'd expect this method to return true
if the
Hashtable
contained a given key, as the key is the primary access mechanism for a
Hashtable
. The Map interface eliminates this source of confusion by renaming the method containsValue
. Also, this improves the consistency of the interface: containsValue
parallels containsKey
.
Map Interface Basic Operations
The basic operations (put, get, containsKey, containsValue, size, and isEmpty) behave exactly like their counterparts in Hashtable. Here's a program to generate a frequency table of the words found in its argument list. The frequency table maps each word to the number of times it occurs in the argument list. import java.util.*; public class Freq {
public static void main(String args[]) {
Map<String, Integer> m = new HashMap<String, Integer>();
// Initialize frequency table from command line
for (String a : args) {
Integer freq = m.get(a);
m.put(a, (freq == null ? 1 : freq + 1));
}
System.out.println(m.size() + " distinct words:");
System.out.println(m);
}
}
The only thing tricky about this program is the second argument of the put statement. That argument is a conditional expression that has the effect of setting the frequency to one if the
384
word has never been seen before or one more than its current value if the word has already been seen. Try running this program with the command: java Freq if it is to be it is up to me to delegate
The program yields the following output:
8 distinct words:
{to=3, delegate=1, be=1, it=2, up=1, if=1, me=1, is=2}
Suppose you'd prefer to see the frequency table in alphabetical order. All you have to do is change the implementation type of the
Map
from
HashMap
to
TreeMap
. Making this fourcharacter change causes the program to generate the following output from the same command line:
8 distinct words:
{be=1, delegate=1, if=1, is=2, it=2, me=1, to=3, up=1}
Similarly, you could make the program print the frequency table in the order the words first appear on the command line simply by changing the implementation type of the map to
LinkedHashMap
. Doing so results in the following output:
8 distinct words:
{if=1, it=2, is=2, to=3, be=1, up=1, me=1, delegate=1}
This flexibility provides a potent illustration of the power of an interface-based framework.
Like the Set and List interfaces, Map strengthens the requirements on the equals and hashCode methods so that two Map objects can be compared for logical equality without regard to their implementation types. Two Map instances are equal if they represent the same key-value mappings.
By convention, all Map implementations provide constructors that take a Map object and initialize the new Map to contain all the key-value mappings in the specified Map. This standard
Map conversion constructor is entirely analogous to the standard Collection constructor: It allows the caller to create a Map of a desired implementation type that initially contains all of the mappings in another Map, regardless of the other Map's implementation type. For example, suppose you have a Map, named m. The following one-liner creates a new HashMap initially containing all of the same key-value mappings as m:
Map<K, V> copy = new HashMap<K, V>(m);
Map Interface Bulk Operations
The clear operation does exactly what you think it does: it removes all the mappings from the
Map. The putAll operation is the Map analogue of the Collection interface's addAll operation. In addition to its obvious use of dumping one Map into another, it has a second, more subtle use.
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Suppose a Map is used to represent a collection of attribute-value pairs; the putAll operation, in combination with the Map conversion constructor, provides a neat way to implement attribute map creation with default values. Here's a static factory method demonstrating this technique: static <K, V> Map<K, V> newAttributeMap(
Map<K, V>defaults, Map<K, V> overrides) {
Map<K, V> result = new HashMap<K, V>(defaults);
result.putAll(overrides);
return result;
}
Collection Views
The Collection view methods allow a Map to be viewed as a Collection in these ways:
keySet
: the
Set
of keys contained in the
Map
.
values
: The
Collection
of values contained in the
Map
. This
Collection
is not a
Set
, as multiple keys can map to the same value.
The Collection views provide the only means to iterate over a Map. Here's an example illustrating the standard idiom for iterating over the keys in a Map with a for-each construct: for (KeyType key : m.keySet())
System.out.println(key); and with an iterator:
// Filter a map based on some property of its keys for (Iterator<Type> i=m.keySet().iterator(); i.hasNext(); )
if (i.next().isBogus())
i.remove();
At first, many people worry that these idioms may be slow because the Map has to create a new
Collection instance each time a Collection view operation is called. There's no reason that a Map can't always return the same object each time it is asked for a given Collection view. This is precisely what all the Map implementations in java.util do.
With all three Collection views, calling an Iterator's remove operation removes the associated entry from the backing Map, assuming that the backing map supports element removal to begin with.
The Collection views support element removal in all its many forms: the remove, removeAll, retainAll, and clear operations, as well as the Iterator.remove operation. (Yet again, this assumes that the backing Map supports element removal.)
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The Collection views do not support element addition under any circumstances. It would make no sense for the keySet and values views, and it's unnecessary for the entrySet view, as the backing Map's put and putAll provide the same functionality.
Fancy Uses of Collection Views: Map Algebra
When applied to the Collection views, the bulk operations (containsAll, removeAll and retainAll) are a surprisingly potent tool. Suppose that you want to know whether two Map objects contain mappings for all the same keys: if (m1.keySet().equals(m2.keySet())) {
...
}
Suppose you have a map that represents a collection of attribute-value pairs, and two sets representing required attributes and permissible attributes. (The permissible attributes include the required attributes.) The following snippet determines whether the attribute map conforms to these constraints and prints a detailed error message if it doesn't: static <K, V> boolean validate(Map<K, V> attrMap,
Set<K> requiredAttrs, Set<K>permittedAttrs) {
boolean valid = true;
Set<K> attrs = attrMap.keySet();
if(!attrs.containsAll(requiredAttrs)) {
Set<K> missing = new HashSet<K>(requiredAttrs);
missing.removeAll(attrs);
System.out.println("Missing attributes: " + missing);
valid = false;
}
if (!permittedAttrs.containsAll(attrs)) {
Set<K> illegal = new HashSet<K>(attrs);
illegal.removeAll(permittedAttrs);
System.out.println("Illegal attributes: " + illegal);
valid = false;
}
return valid;
}
Suppose that you want to know all the keys common to two
Map
objects:
Set<KeyType>commonKeys = new HashSet<KeyType>(m1.keySet()); commonKeys.retainAll(m2.keySet);
A similar idiom gets you the common values.
All the idioms presented thus far have been nondestructive; that is, don't modify the backing
Map. Here are a few that do. Suppose that you want to remove all the key-value pairs that one
Map has in common with another:
387
m1.entrySet().removeAll(m2.entrySet());
Suppose you want to remove from one Map all the keys that have mappings in another: m1.keySet().removeAll(m2.keySet());
What happens when you start mixing keys and values in the same bulk operation? Suppose that you have a Map, managers, that maps each employee in a company to the employee's manager.
We'll be deliberately vague about the types of the key and the value objects. It doesn't matter, so long as they're the same. Now suppose you want to know who all the "individual contributors"
(or nonmanagers) are. The following snippet tells you exactly what you want to know:
Set<Employee> individualContributors =
new HashSet<Employee>(managers.keySet()); individualContributors.removeAll(managers.values());
Suppose that you want to fire all the employees who report directly to some manager, Simon:
Employee simon = ... ; managers.values().removeAll(Collections.singleton(simon));
Note that this idiom makes use of Collections.singleton, a static factory method that returns an immutable Set with the single, specified element.
Once you've done this, you may have a bunch of employees whose managers no longer work for the company (if any of Simon's direct-reports were themselves managers). The following code tells you all of the employees whose manager no longer works for the company:
Map<Employee, Employee> m =
new HashMap<Employee, Employee>(managers); m.values().removeAll(managers.keySet());
Set<Employee> slackers = m.keySet();
This example is a bit tricky. First, it makes a temporary copy of the Map, and it removes from the temporary copy all entries whose (manager) value is a key in the original Map. Remember that the original Map has an entry for each employee. Thus, the remaining entries in the temporary Map comprise all the entries from the original
Map
whose (manager) values are no longer employees. The keys in the temporary copy, then, represent precisely the employees that we're looking for.
16.10 Object Ordering
A
List l
may be sorted as follows:
Collections.sort(l);
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If the list consists of String elements, it will be sorted into alphabetical order. If it consists of
Date elements, it will be sorted into chronological order. How does this happen? String and Date both implement the Comparable interface. The Comparable interfaces provides a natural ordering for a class, which allows objects of that class to be sorted automatically. The following table summarizes some of the more important Java platform classes that implement Comparable:
Classes Implementing
Comparable
Class
Character
Byte, Long, Integer, Short,
Double, Float, BigInteger,
BigDecimal
Boolean
File
String
Date
CollationKey
Natural Ordering
UnSigned numerical
Signed numerical
Boolean.FALSE < Boolean.TRUE
System-dependent lexicographic on path name
Lexicographic
Chronological
Locale-specific lexicographic
If you try to sort a list whose elements do not implement Comparable, Collections.sort(list) will throw a ClassCastException . Similarly, if you try to sort a list whose elements cannot be compared to one another, Collections.sort will throw a ClassCastException. Elements that can be compared to one another are called mutually comparable. Although elements of different types be mutually comparable, none of classes listed here permit interclass comparison.
This is all you really need to know about the Comparable interface if you just want to sort lists of comparable elements or to create sorted collections of them.
Writing Your Own Comparable Types
The
Comparable
interface consists of a single method: public int compareTo(T o);
The compareTo method compares the receiving object with the specified object and returns a negative integer, zero, or a positive integer as the receiving object is less than, equal to, or greater than the specified object. If the specified object cannot be compared to the receiving object, the method throws a ClassCastException.
The following class representing a person's name implements Comparable: import java.util.*; public final class Name implements Comparable<Name> {
private final String firstName, lastName;
389
public Name(String firstName, String lastName) {
if (firstName == null || lastName == null) {
throw new NullPointerException();
}
this.firstName = firstName;
this.lastName = lastName;
}
public String firstName() { return firstName; }
public String lastName() { return lastName; }
public boolean equals(Object o) {
if (!(o instanceof Name))
return false;
Name n = (Name) o;
return n.firstName.equals(firstName) &&
n.lastName.equals(lastName);
}
public int hashCode() {
return 31*firstName.hashCode() + lastName.hashCode();
}
public String toString() {
return firstName + " " + lastName;
}
public int compareTo(Name n) {
int lastCmp = lastName.compareTo(n.lastName);
return (lastCmp != 0 ? lastCmp :
firstName.compareTo(n.firstName));
}
}
To keep the example short, the class is somewhat limited: It doesn't support middle names, it demands both a first and a last name, and it is not internationalized in any way. Nonetheless, it illustrates several important points:
•
Name
objects are immutable. All other things being equal, immutable types are the way to go, especially for objects that will be used as elements in
Set s, or as keys in
Map s. These collections will break if you modify their elements or keys while they're in the collection.
•
•
•
The constructor checks its arguments for null
. This ensures that all
Name
objects are well formed, so that none of the other methods will ever throw a
NullPointerException
.
The hashCode
method is redefined. This is essential for any class that redefines the equals
method. (Equal objects must have equal hash codes.)
The equals
method returns false
if the specified object is null
, or of an inappropriate type. The compareTo
method throws a runtime exception under these circumstances.
Both of these behaviors are required by the general contracts of the respective methods.
390
•
The toString
method has been redefined to print the
Name
in human-readable form. This is always a good idea, especially for objects that are going to get put into collections. The various collection types' toString
methods depend on the toString
methods of their elements, keys and values.
Since this section is about element ordering, let's talk a bit more about
Name
's compareTo method. It implements the standard name-ordering algorithm, where last names take precedence over first names. This is exactly what you want in a natural ordering. It would be very confusing if the natural ordering were unnatural!
Take a look at how compareTo is implemented, because it's quite typical. First, you compare the most significant part of the object (in this case, the last name). Often, you can just use the natural ordering of the part's type. In this case, the part is a String, and the natural (lexicographic) ordering is exactly what's called for. If the comparison results in anything other than zero, which represents equality, you're done: you just return the result. If the most significant parts are equal, you go on to compare the next-most-significant parts. In this case, there are only two parts: first name and last name. If there were more parts, you'd proceed in the obvious fashion, comparing parts until you found two that weren't equal or you were comparing the least-significant parts, at which point you'd return the result of the comparison.
Just to show that it all works, here's a program that builds a list of names and sorts them: import java.util.*; public static void main(String[] args) {
Name nameArray[] = {
new Name("John", "Lennon"),
new Name("Karl", "Marx"),
new Name("Groucho", "Marx"),
new Name("Oscar", "Grouch")
};
List<Name> names = Arrays.asList(nameArray);
Collections.sort(names);
System.out.println(names);
}
}
If you run this program, here's what it prints:
[Oscar Grouch, John Lennon, Groucho Marx, Karl Marx]
There are four restrictions on the behavior of the compareTo method, which we won't go over now because they're fairly technical and boring and are better left in the API documentation. It's really important that all classes that implement Comparable obey these restrictions, so read the documentation for Comparable if you're writing a class that implements it. Attempting to sort a list of objects that violate these restrictions has undefined behavior. Technically speaking, these restrictions ensure that the natural ordering is a total order on the objects of a class that implements it; this is necessary to ensure that sorting is well-defined.
391
Comparators
What if you want to sort some objects in an order other than their natural order? Or what if you want to sort some objects that don't implement Comparable? To do either of these things, you'll need to provide a Comparator, an object that encapsulates an ordering. Like the Comparable interface, the Comparator interface consists of a single method: int compare(T o1, T o2);
The compare method compares its two arguments, returning a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second. If either of the arguments has an inappropriate type for the Co mparator
, the compare
method throws a
ClassCastException
.
Much of what was said about Comparable applies to Comparator as well. Writing a compare method is nearly identical to writing a compareTo method, except that the former gets both objects passed in as arguments. The compare method has to obey the same four technical restrictions as Comparable's compareTo method, for the same reason: a Comparator must induce a total order on the objects it compares.
Suppose that you have a class called Employee: public class Employee implements Comparable<Employee> {
public Name name() { ... }
public int number() { ... }
public Date hireDate() { ... }
...
}
Let's assume that the natural ordering of Employee instances is Name ordering (as defined in the previous example) on employee name. Unfortunately, the boss has asked us for a list of employees in order of seniority. This means that we have to do some work, but not much. Here's a program that will produce the required list: import java.util.*; class EmpSort {
static final Comparator<Employee> SENIORITY_ORDER =
new Comparator<Employee>() {
public int compare(Employee e1, Employee e2) {
return e2.hireDate().compareTo(e1.hireDate());
}
};
// Employee Database
static final Collection<Employee> employees = ... ;
public static void main(String[] args) {
392
List<Employee>e = new ArrayList<Employee>(employees);
Collections.sort(e, SENIORITY_ORDER);
System.out.println(e);
}
}
The Comparator in the program is reasonably straightforward. It relies on the natural ordering of
Date applied to the values returned by the hireDate accessor method. Note that the Comparator passes the hire date of its second argument to its first, rather than vice versa. The reason is that the employee who was hired most recently is least senior: sorting in order of hire date would put the list in reverse seniority order. Another technique that people sometimes use to achieve this effect is to maintain the argument order but to negate the result of the comparison:
//Don't do this!!
return -r1.hireDate().compareTo(r2.hireDate());
You should always use the former technique in favor of the latter, as the latter is not guaranteed to work! The reason for this is that the compareTo method can return any negative int if its argument is less than the object on which it is invoked. There is one negative int that remains negative when negated. Strange as it may seem,
-Integer.MIN_VALUE == Integer.MIN_VALUE
The Comparator in the preceding program works fine for sorting a List, but it does have one deficiency: it cannot be used to order a sorted collection, such as
TreeSet
, because it generates an ordering that is not compatible with equals. This means that this comparator equates objects that the equals
method does not. In particular, any two employees who were hired on the same date will compare as equal. When you're sorting a
List
, this doesn't matter, but when you're using the
Comparator
to order a sorted collection, it's fatal. If you use this
Comparator
to insert multiple employees hired on the same date into a
TreeSet
, only the first one will be added to the set. The second will be seen as a duplicate element and will be ignored.
To fix this problem, simply tweak the Comparator so that it produces an ordering that is compatible with equals. In other words, tweak it so that the only elements that are seen as equal when using compare are those that are also seen as equal when compared using equals. The way to do this is to do a two-part comparison (as we did for Name), where the first part is the one that we're interested in (in this case, the hire date), and the second part is an attribute that uniquely identifies the object. In this case, the employee number is the obvious attribute. Here's the
Comparator that results: static final Comparator<Employee> SENIORITY_ORDER =
new Comparator<Employee>() {
public int compare(Employee e1, Employee e2) {
int dateCmp = e2.hireDate().compareTo(e1.hireDate());
if (dateCmp != 0)
return dateCmp;
return (e1.number() < e2.number() ? -1 :
(e1.number() == e2.number() ? 0 : 1));
393
}
};
One last note: You might be tempted to replace the final return statement in the Comparator with the simpler: return r1.empNumber() - r2.empNumber();
Don't do it unless you're absolutely sure that no one will ever have a negative employee number!
This trick does not work in general, as the signed integer type is not big enough to represent the difference of two arbitrary signed integers. If i is a large positive integer and j is a large negative integer, i - j will overflow and will return a negative integer. The resulting comparator violates one of the four technical restrictions that we keep talking about (transitivity) and produces horrible, subtle bugs. This is not a purely theoretical concern; people get burned by it.
An example of using Comparator: import java.util.*;
//A reverse comparator for Strings class MyComp implements Comparator { public int compare(Object a, Object b) {
String astr,bstr; astr = (String)a; bstr = (String)b;
//reverse comparison return bstr.compareTo(astr);
}
//no need to override equals
} class CompDemo { public static void main(String args[]) {
//create a tree set
TreeSet ts = new TreeSet(new MyComp());
//add elements to tree set ts.add(“C”); ts.add(“A”); ts.add(“B”); ts.add(“E”); ts.add(“F”); ts.add(“D”);
//Get an iterator
Iterator i = ts.iterator();
//display elements while(i.hasNext()) {
Object element = i.next();
System.out.println(element + “ ”);
}
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System.out.println();
}
}
As the following output shows, the tree is now sorted in the reverse order
F E D C B A
16.11 The SortedSet Interface
A SortedSet is a Set that maintains its elements in ascending order, sorted according to the elements' natural order or according to a Comparator provided at SortedSet creation time. In addition to the normal Set operations, the SortedSet interface provides operations for:
•
•
•
Range view: Allows arbitrary range operations on the sorted set.
Endpoints: Returns the first or last element in the sorted set.
Comparator access: Returns the
Set Operations
Comparator
, if any, used to sort the set.
The operations that
SortedSet
inherits from
Set
behave identically on sorted sets and normal sets with two exceptions:
• order.
•
The
Iterator
returned by the iterator
operation traverses the sorted set in
The array returned by toArray
contains the sorted set's elements in order.
Although the interface doesn't guarantee it, the toString
method of the Java platform's
SortedSet
implementations returns a string containing all the elements of the sorted set, in order.
Standard Constructors
By convention, all general-purpose Collection implementations provide a standard conversion constructor that takes a Collection; SortedSet implementations are no exception. In TreeSet, this constructor creates an instance that sorts its elements according to their natural order. It would have been better to check dynamically if the specified collection were a SortedSet instance, and if so, to sort the new TreeSet according to the same criterion (comparator or natural ordering).
Because TreeSet took the approach that it did, it also provides a constructor that takes a
SortedSet and returns a new TreeSet containing the same elements sorted according to the same criterion. Note that it is the compile-time type of the argument, not its runtime type, that determines which of these two constructors is invoked (and whether the sorting criterion is preserved).
SortedSet implementations also provide by convention a constructor that takes a Comparator and returns an empty set sorted according to the specified Comparator. If null is passed to this constructor, it returns a set that sorts its elements according to their natural order.
395
Range-View Operations
The range-view operations are somewhat analogous to those provided by the List interface, but there is one big difference. Range views of a sorted set remain valid even if the backing sorted set is modified directly. This is feasible because the endpoints of a range view of a sorted set are absolute points in the element space rather than specific elements in the backing collection, as is the case for lists. A range view of a sorted set is really just a window onto whatever portion of the set lies in the designated part of the element space. Changes to the range view write back to the backing sorted set, and vice versa. Thus, it's okay to use range views on sorted sets for long periods of time, unlike range views on lists.
Sorted sets provide three range-view operations. The first, subSet, takes two endpoints, like subList. Rather than indices, the endpoints are objects and must be comparable to the elements in the sorted set, using the set's Comparator or the natural ordering of its elements, whichever the set uses to order itself. Like subList, the range is half open, including its low endpoint but excluding the high one.
Thus, the following one of code tells you how many words between "doorbell" and "pickle," including "doorbell" but excluding "pickle," are contained in a SortedSet of strings called dictionary: int count = dictionary.subSet("doorbell", "pickle").size();
Similarly, the following one-liner removes all the elements beginning with the letter " f
": dictionary.subSet("f", "g").clear();
A similar trick can be used to print a table telling you how many words begin with each letter: for (char ch = 'a'; ch <= 'z'; ) {
String from = String.valueOf(ch++);
String to = String.valueOf(ch);
System.out.println(from + ": " +
dictionary.subSet(from, to).size());
}
Suppose that you want to view a closed interval, which contains both of its endpoints, instead of an open interval. If the element type allows for the calculation of the successor of a given value in the element space, merely request the subSet from lowEndpoint to successor(highEndpoint).
Although it isn't entirely obvious, the successor of a string s in String's natural ordering is s +
"\0"
(that is, s
with a null character appended).
Thus, the following one-liner tells you how many words between "doorbell" and "pickle," including "doorbell" and "pickle," are contained in the dictionary: count = dictionary.subSet("doorbell", "pickle\0").size();
396
A similar technique can be used to view an open interval, which contains neither endpoint. The open-interval view from lowEndpoint to highEndpoint is the half-open interval from successor(lowEndpoint) to highEndpoint
. To calculate the number of words between "doorbell" and "pickle," excluding both: count = dictionary.subSet("doorbell\0",
"pickle").size();
The
SortedSet
interface contains two more range-view operations, headSet
and tailSet
, both of which take a single
Object
argument. The former returns a view of the initial portion of the backing
SortedSet
, up to but not including the specified object. The latter returns a view of the final portion of the backing
SortedSet
, beginning with the specified object and continuing to the end of the backing
SortedSet
. Thus, the following code allows you to view the dictionary as two disjoint "volumes" (a — m and n — z):
SortedSet<String> volume1 = dictionary.headSet("n");
SortedSet<String>> volume2 = dictionary.tailSet("n");
Endpoint Operations
The
SortedSet
interface contains operations to return the first and last elements in the sorted set, called (not surprisingly) first
and last
. In addition to their obvious uses, last
allows a workaround for a deficiency in the
SortedSet
interface. One thing you'd like to do with a
SortedSet
is to go into the interior of the set and iterate forward or backward. It's easy enough to go forward from the interior: Just get a tailSet
and iterate over it. Unfortunately, there's no easy way to go backwards.
The following idiom obtains the first element that is less than a specified object o in the elementspace:
Object predecessor = ss.headSet(o).last();
This is a fine way to go one element backward from a point in the interior of a sorted set. It could be applied repeatedly to iterate backward, but this is very inefficient, requiring a lookup for each element returned.
Comparator Accessor
The SortedSet interface contains an accessor method called comparator that returns the
Comparator used to sort the set, or null if the set is sorted according to the natural order of its elements. This method is provided so that sorted sets can be copied into new sorted sets with the same ordering. It is used by the SortedSet constructor.
397
16.12 The SortedMap Interface
A
SortedMap
is a
Map
that maintains its entries in ascending order, sorted according to the keys' natural order, or according to a
Comparator
provided at
SortedMap
creation time. The
Map interface provides operations for the normal
Map
operations and for:
•
•
•
Range view: Performs arbitrary range operations on the sorted map.
Endpoints: Returns the first or the last key in the sorted map.
Comparator access: Returns the
Comparator
, if any, used to sort the map.
Map Operations
The operations that
SortedMap
inherits from
Map
behave identically on sorted maps and normal maps with two exceptions:
•
•
The
Iterator
returned by the iterator
operation on any of the sorted map's
Collection
views traverse the collections in order.
The arrays returned by the
Collection
views' toArray
operations contain the keys, values, or entries in order.
Although it isn't guaranteed by the interface, the toString
method of the
Collection
views in all the Java platform's
SortedMap
implementations returns a string containing all the elements of the view, in order.
Standard Constructors
By convention, all general-purpose
Map
implementations provide a standard conversion constructor that takes a
Map
;
SortedMap
implementations are no exception. In
TreeMap
, this constructor creates an instance that orders its entries according to their keys' natural order. It would have been better to check dynamically if the specified
Map
instance were a
SortedMap
, and if so, to sort the new map according to the same criterion (comparator or natural ordering).
Because
TreeMap
took the approach that it did, it also provides a constructor that takes a
SortedMap
and returns a new
TreeMap
containing the same mappings as the given
SortedMap
, sorted according to the same criterion. Note that it is the compile-time type of the argument, not its runtime type, that determines whether the
SortedMap
constructor is invoked in preference to the ordinary map
constructor.
SortedMap implementations also provide by convention a constructor that takes a Comparator and returns an empty map sorted according to the specified Comparator. If null is passed to this constructor, it returns a set that sorts its mappings according to their keys' natural order.
16.13 Relationships Among Generics
You might expect that a
Stack<Object>
is a supertype of a
Stack<String>
, because
Object
is a supertype of
String
. In fact, no such relationship exists for instantiations of generic types. The
398
lack of a super-subtype relationship among instantiations of a generic type when the type arguments possess a super-subtype relationship can make programming polymorphic methods challenging.
Suppose you would like to write a method that prints out a collection of objects, regardless of the type of objects contained in the collection: public void printAll(Collection<Object> c) {
for (Object o : c) {
System.out.println(o);
}
}
You might choose to create a list of strings and use this method to print all the strings:
List<String> list = new ArrayList<String>();
...
printall(list); //error
If you try this you will notice that the last statement produces a compilation error. Since
ArrayList<String>
is not subtype of
Collection<Object>
it cannot be passed as argument to the print method even though the two types are instantiations of the same generic type with type arguments related by inheritance. On the other hand, instantiations of generic types related by inheritance for the same type argument are compatible: public void printAll(Collection<Object> c) {
for (Object o : c) {
System.out.println(o);
}
}
List<Object> list = new ArrayList<Object>();
...
printall(list); //this works
List<Object>
is compatible with
Collection<Object>
because the two types are instantiations of a generic supertype and its subtype and the instantiations are for the same type argument, namely
Object
.
16.14 Wildcard Types
To get around the puzzle posed by the first version of the printAll
method you can specify that the argument to printAll
is a collection whose element type matches anything, written as
Collection<?>
: public void printAll(Collection<?> c) {
for (Object o : c) {
System.out.println(o);
}
}
399
The
?
type is known as a wildcard type. You can always extract objects from this collection because the returned type is always guaranteed to be
Object
. However, you cannot add objects to this collection, because
?
stands for some unknown type and it's not possible to know if the type of the object you want to add is a subtype of the unknown type. The only exception is null
, which is a member of every type.
You can also constrain (or bound) the wildcard by a type. Bounded wildcards are useful when you have partial knowledge about the type argument. For example, suppose you have a class hierarchy consisting of a geometric shape (Shape) and its subtypes (Circle, Rectangle, and so on). The drawing program that references these objects invokes a method drawAll to draw a collection of these shapes: public void drawAll(Collection<Shapes> shapes) {
for (Shape s: shapes) {
s.draw();
}
}
Since we have seen that it is not legal to pass in a subtype of
Shape
(for example,
Circle
) as the type argument for the generic collection passed to drawAll
, this method has limited usefulness: for example, it cannot be called with
Collection<Circle>
. To enable passing a subtype of
Shape
as the type argument, you could express the type parameter of the shape collection as a wildcard. However, since we know that the type argument will be some type of shape, the wildcard should be bounded by the superclass
Shape
as follows: void drawAll(Collection<? extends Shapes> shapes) { ... }
This allows drawAll
to accept collections of any subclass of
Shape
.
In summary, a wildcard with an upper bound is specified as <? extends Type> and stands for all types that are subtypes of Type. It is also possible to constrain a wildcard with a lower bound. A wildcard with a lower bound is specified as <? super Type> and stands for all types that are supertypes of Type. Note that just as it is not possible to add an object to a collection of unknown type, it is also not legal to add an object to a collection of an unknown type that has a bound.
16.15 Defining and Using Generic Methods
Not only types can be parameterized; methods can be parameterized too. Static and non-static methods as well as constructors can have type parameters.
The syntax for declaring method type parameters is the same as the syntax for generics. The type parameter section is delimited by angle brackets and appears before the method's return type. For example the following Collections class method fills a List of type <? super T> with objects of type T: static <T> void fill(List<? super T> list, T obj)
400
Generic methods allow you to use type parameters to express dependencies among the types of one or more arguments to a method or its return type (or both). The type parameters of generic methods generally are independent of any class or interface-level type parameters.
One difference between generic types and generic methods is that generic methods are invoked like regular methods. The type parameters are inferred from the invocation context, as in this invocation of the fill method: public static void main(String[] args) {
List<String> list = new ArrayList<String>(10);
for (int i = 0; i < 10; i++) {
list.add("");
}
String filler = args[0];
Collections.fill(list, filler);
...
}
401
Answers
Chapter 2
No.
1
2
3
4
5
6 a g l d f h 4 6 8 b d a e
1
Answer
None of the above
None of the above
'\u0000' to '\uffff' 0 to
65535
1 5
Remark
The escape sequences are as follows: '\b'
(backspace), '\f' (formfeed), '\n' (newline), '\r'
(carriage return), '\t' (horizontal tab), '\\'
(backslash), '\"' (double quote), '\'' (single quote).
Yes, you must memorize the escape sequences!
Just remember "big farms need red tractors".
The Java Language Specification requires that the main
method must accept a single argument that is an array of components of type
String
.
In each of the three class declarations, the single argument is indeed an array of components of type
String
. Please note that the square brackets within an array declaration may appear as part of the type or part of the declarator (i.e. array name).
All of these are keywords of the Pascal programming language, but none are Java keywords.
The first letter of an identifier can be any
Unicode character that is a Java letter. The first letter can not be a number. The dollar sign $ and underscore _ are considered Java letters.
A char
is a 16 bit unsigned value; so none of the char
values are negative and the minimum value is zero. The maximum value is 2
16
- 1.
An array creation expression must have either a dimension expression or an initializer. If both are present, then a compile-time error is generated. Similarly, if neither is present, then a compile-time error is generated. If only the dimension expression is present, then an array with the specified dimension is created with all elements set to the default values. If only the initializer is present, then an array will be created that has the required dimensions to accommodate the values specified in the initializer. Java avoids the possibility of an incompatible dimension expression and initializer by not allowing both to appear in the
402
No.
7
Answer a Prints: 3,4,8
Remark same array creation expression. A compile-time error is generated by the array creation expression for a1, because it needs either a dimension expression or an initializer. A compile-time error is generated at 5, because either the dimension expression or the initializer must be removed.
An array variable a1
is declared, and the declaration contains the initializer
{{1,2,3},
{4,5,6},{7,8,9,10}}
. The initializer creates an array containing three components, and each is a reference to a subarray of type int[]
. Each subarray contains components of type int
, so the elements of the array referenced by a1
are of type int
. The array access expression, a1[0][2]
= a1[1st subarray][third component]
= 3.
Chapter 3
No.
1
2
3
4
5 f j f d e
Answer
Compile-time error
Compile-time error
None of the above
Prints: true,true
Prints: 10
Remark
Variables declared inside of a block or method are called local variables; they are not automatically initialized. The compiler will generate an error as a result of the attempt to access the local variables before a value has been assigned.
Both operands of the conditional and operator and the conditional or operator must be of type boolean
.
All of the declarations are legal. The first three (
061
,
'\61'
,
'\061'
) are declared in octal format. The fourth (
0x0031
) is declared as a hexadecimal literal. The fifth (
'\u0031'
) is a
Unicode escape sequence.
The sign of an integral numeric type is changed by inverting all of the bits and by adding one.
If the left-hand operand of the shift operator is of type byte
, short
, or char
then the left operand is promoted to a 32 bit int
and all four bytes are shifted. If the promoted type of the left-hand operand is of type int
, then the shift
403
No.
6
7
8
Answer c Prints: 5 b Prints: 2,true b Prints: FFT
Remark distance is always within the range of 0 to 31, inclusive; and is specified by the least significant 5 bits of the right-hand operand. In this case, the shift distance is 33, and the five least significant bits are 00001; so the shift distance is one bit. Note: If the type of the left hand operand is long, then the least significant six bits of the right hand operand are used.
The two statements, int a=1
followed by a
+= ++a + a++
, can be rewritten as the single statement, a=(int)((1)+(++a + a++))
.
Further evaluation produces a=(int)
((1)+(2 + 2))
. Generally speaking, a compound assignment expression of the form
E1 op= E2
can be rewritten as
E1=(T)
((E1)op(E2))
where T is the type of E1.
Suppose the left operand were divided by the right operand. The remainder operator returns the remainder of the division operation. For integral types, the identity,
(y ==
((y/x)*x+(y%x)))
, is always true.
The expression used to assign variable b1
is equivalent to the expression used to assign variable b2
. The results demonstrate that the conditional operator (?:) groups from right-toleft.
Chapter 4
No.
1 c 3
Answer Remark
The length
member of the array type is an attribute. A compile-time error is generated as a result of the attempt to access length
as though it were a method.
2 b Prints: 147258369
The array variable a1
is declared with the initializer,
{{1,2,3},{4,5,6},{7,8,9}}. The array access expression, a1[0][1]
= a1[first subarray][second element]
= 2. If the argument of the print statement had been a1[i][j]
then the output would have been
123456789. The tricky feature of this question is the reversal of i
and j
to produce the deceptive array access expression, a1[j][i]
. The output is 147258369.
3 c Prints: v w x x y z z Cases one and three have no break
statement, so the
404
No.
Answer
4 c Prints: 61433
Remark next case
is also executed and x
and z
are printed twice.
On the first pass through the loop, the value of x
is 6, so 5 is subtracted from x
. On the second pass, the value of x
is
1, so 3 is added to x
. On the third pass, the value of x
is 4, so 1 is subtracted from x
. On the fourth pass, the value of x
is 3, so the variable, success
, is incremented from zero to one. On the final pass, the value of x
is 3 and the variable, success
, is incremented to the value, 2. The boolean
expression of the do
loop is now false
, so control passes out of the loop.
Chapter 5
No.
1 d e f
2
3
4
5 e f e c c
Answer Remark
An attempt to run
GRC7
from the command line results in an error at runtime. An attempt to run
GRC8
from the
The JLS requires the main
method to be declared static
. In this example, each of the command line results in an error at runtime. An attempt to run
GRC9
from the three main
methods are not declared
The result is an error at run-time. static
. command line results in an error at runtime.
An attempt to run
GRC2
from the command line fails.
An attempt to run
GRC3
from the command line fails.
The JLS requires the main
method to be declared public
. The main
methods of
GRC2 and
GRC3
are not declared public and can not be invoked from the command line.
Compile-time error at
The local variable y has not been initialized so line 5. the attempt to access the variable results in a compile-time error.
Prints: 0null
The numeric sum of variables a
, b
, c
, d
and e
is zero. The zero is converted to a
String
and concatenated with s
.
Compile-time error Local variables are not initialized automatically, and must be initialized explicitly before
405
No.
6
7
8
9 b a c d
Answer
Prints:
4
BCD
Prints: float,float
Prints: Bird,Cat
Remark attempting to access the value. The local variable i3 will not be initialized if i1 is less than or equal to zero; so the result is a compile-time error.
The index for the first element of an array is zero so the first argument printed by this program is the second argument on the command line following the name of the class.
A method invocation conversion can widen an argument of type float
to match a method parameter of type double
, so any argument that can be passed to m(float i)
can also be passed to m(double i)
without generating a compile-time type error. For that reason, we can say that m(float i)
is more specific than m(double i)
. Since both methods are applicable, the more specific of the two, m(float i)
, is chosen over the less specific, m(double i)
. The arguments of the method invocation expressions, m(a1)
and m(b1)
, are of types int
and long
respectively. A method invocation conversion can widen an argument of type int
or long
to match either of the two method parameter types float
or double
; so both methods, m(float i)
and m(double i)
, are applicable to the two method invocation expressions. Since both methods are applicable, the more specific of the two, m(float i)
is chosen rather than the less specific, m(double i)
.
Although the reference variable r2
is assigned the value of reference variable r1
in method m1
, the reference pet2
remains unchanged in the main
method. Object references are passed by value: the invoked method gets a copy of the object reference. The reference parameter r1
can be used to modify the state of the instance referenced by pet1
, but r2
can not be used to force pet2
to reference a different instance.
The assignment expression, a = c + a
, requires an explicit cast to type int
. If one of the two operands of a numeric expression is of type long
and if the other operand is of type int
, short
, char
or byte
; then it will be promoted
406
No.
10
11
12 b f g h d
Answer
Prints: 1,3
6 7 8
Remark to type long
, and the result of the expression will be of type long
. (Note: The rule does not apply to the shift operator.) The type long
result can not be assigned to a variable of type int without an explicit cast.
Although the reference parameters i1
and i2
are reassigned inside of m1
, the change has no impact outside of m1
. Array references are passed by value: the invoked method gets a copy of the array reference.
The compiler will implicitly do a narrowing conversion for an assignment statement if the right hand operand is a compile time constant of type byte
, short
, char
, or int
and the value falls within the range of the variable on the left and if the variable is of type byte
, short
, or char
.
Compile-time error at There is a compile-time error at 2. The char
2 type variable c2
is not a compile-time constant, so it can not be assigned to type byte
without an explicit cast. The method parameter c2
is declared final, so the value of c2
can not be changed within method m2
. The value of method parameter c2
is set at run time to the value of the argument that is provided when m2
is invoked at line 3. For that reason, the method parameter c2 is not a compile-time constant. In method m2
, the statement, " return c2;
", is a return statement with an expression, c2
. A compiletime error occurs if the type of the expression is not assignable to the declared result type of the method. The declared result type of method m2
is byte
. The return
statement attempts to return the value of the char
type variable c2
. If a char
value is a compile-time constant, and if the value falls within the range of type byte
, then the char
value is assignable to type byte
. In method m2
, variable c2
is not a compile-time constant, because the value of c2
is not known at compile time. Instead, the value of c2
is assigned at run time to the value of the argument. Since the char
type variable c2
is not a compile-time constant, the value of variable c2
is not
407
No.
Answer Remark assignable to the return type of method m2 without an explicit cast. While the declaration of method m2
produces a compile-time error, the declaration of method m1
does not. The local variable c1
is declared final and the value is set at compile time; so c1
is a compile-time constant. The value \u0001 falls within the range of type byte
; so the value of the compile-time constant c1
is assignable to the return type of method m1
without an explicit cast.
Chapter 6
No.
1
2 j
Answer Remark
Compile-time error
The type of the reference color2
is
Red
. Since
Red
is not a subclass or a superclass of
Blue
, the expression color2 instanceof Blue
is rejected at compile-time. Please note:
The expression, x instanceof T
, produces a compile-time error whenever the cast expression
(T)x
produces a compiletime error. If the program had been able to compile and run, the expression color1 instanceof Color
would evaluate to true
at run-time. The reference color1
refers to an instance of type
Red
. Since
Red
is a subclass of
Color
, the expression color1 instanceof Color
would evaluate to true at runtime. The expression, color1 instanceof Blue
would evaluate to false
at run-time. The reference, color1
, is of type
Color
. Since
Color
is a superclass of
Blue
, the expression, color1 instanceof Blue
, is accepted at compile-time. The type of the object instance referenced by color1
is
Red
. Since
Red
is not
Blue
or a subclass of
Blue
, the expression, color1 instanceof Blue
, would be false
at run-time. b c d e final private protected public
A field is a class member. A static field is sometimes called a class variable. A non-static field is sometimes called an instance variable. A variable declaration that is immediately contained by a block such as a method body is called a local variable. The access modifiers, private
, protected
and public
, can be applied to a field. A final more than once. The abstract
modifier may be applied to methods but not to fields
field can not have its value assigned
408
No.
3 b d
4 b
Answer
The compiler attempts to create a default constructor for class B.
Compile-time error at 2.
Remark
If no constructor is declared explicitly, then the compiler will implicitly create a default constructor that accepts no parameters, has no throws
clause, and invokes its superclass constructor.
Since class
A
has an explicitly declared constructor, the compiler will not create an implicit default constructor. Class
B
does not have an explicit constructor declaration, so the compiler attempts to create a default constructor. Since class
A
does not have a noparameter constructor, the attempt by class
B
to invoke the no parameter constructor of
A
would fail. As a result, a compiler error is generated at marker 2.
A local class can be declared abstract
.
An anonymous class can not be extended; therefore, an anonymous class can not be declared abstract
. A local class can be abstract
. An abstract
class can not be instantiated.
If a class declaration contains an abstract
method, then the class must also be declared abstract
. A class can be declared abstract
even if it does not contain an abstract
method.
An abstract
class can never be declared final
.
5 c d e
3 4 5
If a class
C
is declared as a member of an enclosing class then
C may be declared using no access modifier or any of the three access modifiers, private
, protected
or public
.
However, if class
C
is not a local class, anonymous class or a member of an enclosing class or interface; then
C
may be declared with the public
modifier or with package access (i.e. no modifier). The other two access modifiers, private
and protected
, are not applicable to any class that is not a member class. The class declaration,
Class Basics4 {}
, generates a compile-time error, because all of the letters of the reserved word class
must be lower case.
6 b Prints:
A11
The declaration
A11[] a1 = new A11[1]
declares a variable a1
that references an array that contains one component of type
A11
. The declaration
A11[][] a2 = new A11[2]
[]
declares a variable a2
that references an array that contains two components of type
A11[]
. In other words, the array referenced by a2
contains two reference variables, and each is able to reference a subarray. The initial value of the subarray references is null
. The size of the subarrays has not been specified. The declaration
A11[][][] a3 = new A11[3]
[][]
declares a variable a3
that references an array that contains three components of type
A11[][]
. In other words, the array referenced by a3
contains three reference variables, and each is able to reference a subarray. The initial value of each subarray reference is null
. The dimensions of the subarrays have not been specified. At line 5, a reference to the array
409
No.
Answer Remark referenced by a1
is assigned to each of the two components of the array referenced by a2
, a2[0] = a2[1] = a1
. At line 6, a reference to the array referenced by a2
is assigned to each of the three components of the array referenced by a3
, a3[0] = a3[1] = a3[2] = a2
. In other words, after line 6, each component of the array referenced by a3
is a reference to the array referenced by a2
, and each element of the array referenced by a2
is a reference to the array referenced by a1
, and the array referenced by a1
contains a reference to an instance of class
A11
. Every element of the multi-dimensional array referenced by a3
contains a reference to a single instance of class
A11
. The print method invokes the toString
method on the instance, and produces the output,
A11
.
Chapter 7
No.
1
Answer e None of the above
2 d
Compile-time error at
3.
Remark
Line 4 does not generate a compile-time error. The reference named base
actually refers to an instance of type
Sub
, so the reference may be cast to type
Sub
.
Both class
A
and
B
are declared in the same package, so class
B
has access to the public
, protected
, and package access methods of class
A
.
Chapter 8
No.
Answer
1 c
Compile-time error at
3.
Remark
The compiler creates a constructor for class
C implicitly. The implicitly created constructor accepts no parameters and has no throws
clause. The constructors for class
B
and class
C
both invoke the constructor for
A
. The constructor for class
A declares
Exception
in the throws
clause. Since the constructors for
B
and
C
invoke the constructor for
A
implicitly, both
B
and
C
must declare
Exception
in their throws
clause. A compiletime error is generated at marker 3, because the default constructor does not declare
Exception
in the throws
clause.
410
No.
2
3
4 b e
Answer d Prints: true,true b Prints: 0,0,1,1,0,1
Remark
Both
Error
and
Exception
are subclasses of
Throwable
.
The nested catch
clause is able to catch a
Level2Exception
or any subclass of it. The switch
statement throws a
Level1Exception that can not be caught by the nested catch
clause; so the nested finally
block is executed as control passes to the first of the two outer catch
clauses.
The outer finally
block is executed as control passes out of the try
statement.
With assertions enabled it prints
210210 followed by an
AssertionError message. With assertions disabled it prints
210210-1
If, under normal operating circumstances, the default label of a switch statement should not be reached, then an assert statement can be placed after the default label to verify that an unexpected condition has not not occurred.
Chapter 10
No.
1
2 c d e
Answer
3 4
None of the above
Remark
The compiler interprets
\u000a
as a line terminator. The escape sequence
\n
should be used instead. Similarly,
\u000d
is interpreted as a line terminator. The escape sequence
\r
should be used instead.
String
literals are declared using double quotes, but all of the declarations here use single quotes.
Chapter 12
No.
Answer
1 d Prints: -128,127
2 b Prints: -32768,32767
Remark
A byte
is an 8 bit signed value; so the minimum byte
value is -(2
7
) and the maximum value is (2
7
-
1).
A short
is a 16 bit signed value; so the minimum short
value is -(2
15
) and the maximum value is (2
15
- 1).
411
No.
Answer
3 a Prints: 1111111,177,127,7f
4 c Prints: 7f,ffff,7fff
5 f Prints: 80000000,7fffffff
6 g None of the above
7 f Prints: true,false,true
Remark
A byte is an 8 bit signed value. The left most bit is the sign bit. The sign bit is set to zero for positive numbers and is set to one for negative numbers.
The most positive byte value is represented as a sign bit that is set to zero and all of the other bits set to one. The Integer.toBinaryString method does not print leading zeros; so only seven bits are printed. An eight bit binary value is represented as three octal digits. The first of the three digits represents the left most two bits of the binary value. In this case, the left most two bits are zero and one. The second octal digit represents the next three bits of the binary value. The last of the octal digits represents the right most three bits of binary value. Note that the Integer.toOctalString method does not print a leading zero as is required for an octal literal value. An eight bit binary value is represented as two hexadecimal digits. The left hex digit represents the left most four bits of the binary value. The right hex digit represents the right most four bits of the binary value.
A byte
is an 8 bit signed value. A char
is a 16 bit unsigned value. A short
is a 16 bit signed value. The left most bit of a signed value is the sign bit. The sign bit is zero for positive numbers and one for negative numbers. The maximum byte
value in hexadecimal format is
7f
and in decimal format is
127
. The minimum byte
value in hexadecimal format is
80
and in decimal format is
-128
. The byte
value of decimal
-1
is ff
in hexadecimal.
An int
is a 32 bit signed value. The left most bit is the sign bit. The sign bit is zero for positive numbers and one for negative numbers.
The null
literal is converted to an int
array type with the value null
. All array types implement the
Cloneable
interface, so any array reference can be assigned to a reference of type
Cloneable
. The int
array object referenced by the
Cloneable
reference, c
, can be assigned to a reference of the int
array type, int[]
.
The positive infinity of type float
is promoted to the positive infinity of type double
. NaN is
412
No.
Answer Remark not equal to anything including itself.
413
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