Refactoring 1

Refactoring 1
Refactoring
14-Apr-16
Refactoring
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Refactoring is:
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Refactoring is not just any old restructuring
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restructuring (rearranging) code...
...in a series of small, semantics-preserving transformations (i.e. the
code keeps working)...
...in order to make the code easier to maintain and modify
You need to keep the code working
You need small steps that preserve semantics
You need to have unit tests to prove the code works
There are numerous well-known refactoring techniques
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You should be at least somewhat familiar with these before inventing
your own
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When to refactor
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You should refactor:
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Any time that you see a better way to do things
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You can do so without breaking the code
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“Better” means making the code easier to understand and to modify
in the future
Unit tests are essential for this
You should not refactor:
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Stable code (code that won’t ever need to change)
Someone else’s code
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Unless you’ve inherited it (and now it’s yours)
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Design vs. coding
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“Design” is the process of determining, in detail, what
the finished product will be and how it will be put
together
“Coding” is following the plan
In traditional engineering (building bridges), design is
perhaps 15% of the total effort
In software engineering, design is 85-90% of the total
effort
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By comparison, coding is cheap
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The refactoring environment
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Traditional software engineering is modeled after traditional
engineering practices (= design first, then code)
Assumptions:
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“Agile” software engineering is based on different assumptions:
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The desired end product can be determined in advance
Workers of a given type (plumbers, electricians, etc.) are interchangeable
Requirements (and therefore design) change as users become acquainted
with the software
Programmers are professionals with varying skills and knowledge
Programmers are in the best position for making design decisions
Refactoring is fundamental to agile programming
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Refactoring is sometimes necessary in a traditional process, when the
design is found to be flawed
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A personal view
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In my opinion,
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Design, because it is a lot more creative than simple
coding, is also a lot more fun
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Most small to medium-sized projects could benefit from
an agile programming approach
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Admittedly, “more fun” is not necessarily “better”
...but it does help you retain good programmers
We don’t yet know about large projects
Most programming methodologies attempt to turn
everyone into a mediocre programmer
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Sadly, this is probably an improvement in general
These methodologies work less well when you have some very
good programmers
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Back to refactoring
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When should you refactor?
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Any time you find that you can improve the design of
existing code
You detect a “bad smell” (an indication that something is
wrong) in the code
When can you refactor?
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You should be in a supportive environment (agile
programming team, or doing your own work)
You should have an adequate set of unit tests
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Example 1: switch statements
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switch statements are very rare in properly designed
object-oriented code
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Therefore, a switch statement is a simple and easily detected
“bad smell”
Of course, not all uses of switch are bad
A switch statement should not be used to distinguish between
various kinds of object
There are several well-defined refactorings for this case
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The simplest is the creation of subclasses
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Example 1, continued
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class Animal {
final int MAMMAL = 0, BIRD = 1, REPTILE = 2;
int myKind; // set in constructor
...
String getSkin() {
switch (myKind) {
case MAMMAL: return "hair";
case BIRD: return "feathers";
case REPTILE: return "scales";
default: return "integument";
}
}
}
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Example 1, improved
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class Animal {
String getSkin() { return "integument"; }
}
class Mammal extends Animal {
String getSkin() { return "hair"; }
}
class Bird extends Animal {
String getSkin() { return "feathers"; }
}
class Reptile extends Animal {
String getSkin() { return "scales"; }
}
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How is this an improvement?
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Adding a new animal type, such as Amphibian, does
not require revising and recompiling existing code
Mammals, birds, and reptiles are likely to differ in other
ways, and we’ve already separated them out (so we
won’t need more switch statements)
We’ve gotten rid of the flags we needed to tell one kind
of animal from another
Basically, we’re now using Objects the way they were
meant to be used
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JUnit tests
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As we refactor, we need to run JUnit tests to ensure
that we haven’t introduced errors
public void testGetSkin() {
assertEquals("hair", myMammal.getSkin());
assertEquals("feathers", myBird.getSkin());
assertEquals("scales", myReptile.getSkin());
assertEquals("integument", myAnimal.getSkin());
}
This should work equally well with either
implementation
The setUp() method of the test fixture may need to be
modified
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Bad Smell Examples
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You should refactor any time you detect a “bad smell” in the
code
Examples of bad smells include:
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Duplicate Code
Long Methods
Large Classes
Long Parameter Lists
Multi location code changes
Feature Envy
Data Clumps
Primitive Obsession
We will discuss most or all of these later
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Eclipse refactorings
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Eclipse can perform several
refactorings for you:
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Rename (just about anything)
Change method signature
Move class to another package
Pull up (into a superclass)
Push down (into subclasses)
Extract interface
Generalize types
Use supertype where possible
Infer generic type arguments
Inline method call
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Extract method
Extract local variable
Extract constant
Introduce parameter
Introduce factory
Convert local variable to field
Encapsulate field
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The End
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