Comparisons

Comparisons
Comparable and Comparator
Nuts and Bolts
Sets
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A set is a collection in which all elements are unique—an
element is either in the set, or it isn’t
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Java provides a Set interface and some implementations,
including HashSet and TreeSet
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Since sets cannot contain two or more equal elements, equality must be
properly defined
A HashSet is very fast, and keeps elements in an order defined
by a hash function
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In mathematics, the elements of a set are in no particular order
Therefore, there must be a properly defined hash code function
A TreeSet keeps elements in “sorted” order—smaller elements
before larger ones
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Therefore, there must be a way to compare elements
2
Nuts and bolts
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Four methods underlie many of Java’s important Collection
types: equals, compare and compareTo, and hashCode
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To put your own objects into a Collection, you need to ensure that these
methods are defined properly
Any collection with some sort of membership test uses equals (which,
in many cases, defaults to ==)
Any collection that depends on sorting requires larger/equal/smaller
comparisons (compare or compareTo)
Any collection that depends on hashing requires both equality testing and
hash codes (equals and hashCode)
Any time you implement hashCode, you must also implement equals
Some of Java’s classes, such as String, already define all of
these properly for you

For your own objects, you have to do it yourself
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Comparing our own objects

The Object class provides public boolean
equals(Object obj) and public int
hashCode() methods
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For objects that we define, the inherited equals and
hashCode methods use the object’s address in
memory
We can override these methods
If we override equals, we should override hashCode
If we override hashCode, we must override equals
The Object class does not provide any methods
for “less” or “greater”—however,
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There is a Comparable interface in java.lang
There is a Comparator interface in java.util
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Outline of a Student class
public class Student implements Comparable {
public Student(String name, int score) {...}
public int compareTo(Object o)
throws ClassCastException {...}
public static void main(String args[]) {...}
}
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Constructor for Student
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This is the same for both methods—nothing new
here
public Student(String name, int score) {
this.name = name;
this.score = score;
}
We will be sorting students according to their
score
This example will use sets, but that’s irrelevant—
comparisons happen between two objects,
whatever kind of collection they may or may not
be in
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The main method, version 1
public static void main(String args[]) {
TreeSet<Student> set = new TreeSet<Student>();
set.add(new
set.add(new
set.add(new
set.add(new
set.add(new
}
Student("Ann", 87));
Student("Bob", 83));
Student("Cat", 99));
Student("Dan", 25));
Student("Eve", 76));
Iterator<Student> iter = set.iterator();
while (iter.hasNext()) {
Student s = iter.next();
System.out.println(s.name + " " + s.score);
}
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Using the TreeSet

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In the main method we have the line
TreeSet set = new TreeSet();
Later we use an iterator to print out the values in order,
and get the following result:
Dan
Eve
Bob
Ann
Cat

25
76
83
87
99
How did the iterator know that it should sort Students
by score, rather than, say, by name?
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Implementing Comparable<T>
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public class Student implements Comparable
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This means it must implement the method
public int compareTo(Object o)
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Notice that the parameter is an Object
In order to implement this interface, our parameter must also be
an Object, even if that’s not what we want
public int compareTo(Object o) throws ClassCastException {
if (o instanceof Student)
return score - ((Student)o).score;
else
throw new ClassCastException("Not a Student!");
}
A ClassCastException should be thrown if we are given a nonStudent parameter
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An improved method
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Since casting an arbitrary Object to a Student may throw a
classCastException for us, we don’t need to throw it
explicitly:
public int compareTo(Object o) throws ClassCastException {
return score - ((Student)o).score;
}
Moreover, since classCastException is a subclass of
RuntimeException, we don’t even need to declare that we
might throw one:
public int compareTo(Object o) {
return score - ((Student)o).score;
}
10
Using a separate Comparator

In the program we just finished, Student implemented
Comparable
Therefore, it had a compareTo method
 We could sort students only by their score
 If we wanted to sort students another way, such as by
name, we are out of luck
Now we will put the comparison method in a separate class
that implements Comparator instead of Comparable
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This is more flexible (you can use a different Comparator to sort
Students by name or by score), but it’s also clumsier
Comparator is in java.util, not java.lang
Comparable requires a definition of compareTo but Comparator
requires a definition of compare
Comparator also (sort of) requires equals
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Outline of StudentComparator
import java.util.*;
public class StudentComparator
implements Comparator<Student> {
public int compare(Student s1, Student s2) {...}
}
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public boolean equals(Object o1) {...}
Note: When we are using this Comparator, we don’t need
the compareTo method in the Student class
Because of generics, our compare method can take
Student arguments instead of just Object arguments
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The compare method
public int compare(Student s1, Student s2) {
return s1.score – s2.score;
}
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This differs from compareTo(Object o) in
Comparable in these ways:
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The name is different
It takes both objects as parameters, not just one
We have to either use generics, or check the type of both
objects
If our parameters are Objects, they have to be cast to
Students
13
The someComparator.equals method
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Ignore this method!
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This method is not used to compare two Students—it is
used to compare two Comparators
Even though it’s part of the Comparator interface, you
don’t actually need to override it
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Implementing an interface requires you to have a definition for
every method in the interface--so how can this be an exception?
Because you do have a definition, inherited from Object !
In fact, it’s always safe to ignore this method
The purpose is efficiency—you can replace one
Comparator with an equal but faster one
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The main method
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The main method is just like before, except that
instead of
TreeSet<Student> set = new TreeSet<Student>();
We have
Comparator<Student> comp = new StudentComparator();
TreeSet<Student> set = new TreeSet<Student>(comp);
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When to use each

The Comparable interface is simpler and less work

Your class implements Comparable
 You provide a public int compareTo(Object o) method
 Use no argument in your TreeSet or TreeMap constructor
 You will use the same comparison method every time
The Comparator interface is more flexible but slightly
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more work
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Create as many different classes that implement Comparator
as you like
You can sort the TreeSet or TreeMap differently with each
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Construct TreeSet or TreeMap using the comparator you want
For example, sort Students by score or by name
16
Sorting differently
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Suppose you have students sorted by score, in a TreeSet you
call studentsByScore
Now you want to sort them again, this time by name
Comparator<Student> myStudentNameComparator =
new MyStudentNameComparator();
TreeSet studentsByName =
new TreeSet(myStudentNameComparator);
studentsByName.addAll(studentsByScore);
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The End
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