Refactoring: Improve the design of existing code

REFACTORING
Improving the Design of Existing code
Software Engineering II Class (Lect. 6)
Valerio Maggio, Ph.D. Student
A.A. 2011/12
Prof. Marco Faella

• What does “Refactoring” means?
‣ When should you refactor your code/design?
‣ Why should you refactor your code/design?
‣ Refactoring Principles
• Typical Refactorings
• Bad Smells & Solutions
OUTLINE

(CODE) REFACTORING
• The art of safely improving the design of existing code [Fowler09]
• Implications: [Fowler09]
‣ Refactoring does not include any change to the system
‣ Refactoring is not “Rewriting from scratch”
‣ Refactoring is not just any restructuring intended to improve the code
Refactoring: (Deﬁnition) Refactoring is the process of changing a
software system in such a way that it does not alter the external behavior of
the code yet improves its internal structure. [Fowler02]

REFACTORING
• Basic Metaphor:
‣ Start with an existing code and make it better;
‣ Change the internal structure while preserving the overall
semantics.
• Goals:
‣ Reduce near-duplicate code
‣ Improve comprehension and maintainability while reducing
coupling

THE REFACTORING CYCLE
start with working, tested code
while the design can be simplified do:
choose the worst smell
select a refactoring that addresses that smell
apply the refactoring
check that the tests still pass
•KWs:
•Tested Code
‣Importance of Testing in Refactoring
‣Why testing?
•(Code) Smells
‣Fragments of code that contain some design mistake

• Refactor when you add a function
• Refactor when you ﬁx a bug
• Refactor for Greater Understanding
WHEN SHOULDYOU
REFACTOR?
The Rule of Three: The ﬁrst time you do something, you just do it.
The second time you do something similar, you wince at the duplication, but
you do the duplicate thing anyway. The third time you do something similar,
you refactor. [Fowler09]

• Refactoring improves the design of software
‣ Without refactoring the design of the program will decay
‣ Poorly designed code usually takes more code to do the
same things
• Refactoring makes software easier to understand
‣ In most software development environments, somebody
else will eventually have to read your code
• Refactoring helps you ﬁnd bugs
WHY SHOULDYOU
REFACTOR?

• Programs that are hard to read, are hard to modify.
• Programs that have duplicated logic, are hard to modify.
• Programs that require additional behavior requiring changing
code are hard to modify.
• Programs with complex conditional logic are hard to modify.
• Refactoring makes code more readable
WHY REFACTORING WORKS?

Good O-O Practices
REFACTORING

• Short methods
‣ Simple method logic
• Few instance variables
• Clear object responsibilities
‣ State the purpose of the class in one sentence
‣ No super-intelligent objects
GOOD SIGNS OF OO
THINKING

• The Dependency Inversion Principle
‣Depend on abstractions, not concrete implementations
•Write to an interface, not a class
• The Interface Segregation Principle
‣Many small interfaces are better than one “fat” one
• The Acyclic Dependencies Principle
‣Dependencies between package must not form cycles.
•Break cycles by forming new packages
SOME PRINCIPLES

• The Common Closure Principle
‣Classes that change together, belong together
•Classes within a released component should share common
closure.That is, if one needs to be changed, they all are likely
to need to be changed.
• The Common Reuse Principle
‣Classes that aren’t reused together don’t belong together
•The classes in a package are reused together. If you reuse
one of the classes in a package, you reuse them all.
PACKAGES, MODULES AND
OTHER

TYPICAL REFACTORINGS
Refactoring in Action

REFACTORINGSTABLE
Class Refactorings Method Refactorings
Attribute
Refactorings
add (sub)class to hierarchy add method to class add variable to class
rename class rename method rename variable
remove class remove method remove variable
extract class push method down push variable down
push method up pull variable up
add parameter to method create accessors
move method abstract variable
extract code in new method
replace parameter with method

• Behavior on a superclass is relevant only for some of its
subclasses.
• Move it to those subclasses.
PUSH METHOD DOWN
Engineer
+ getQuota()
SalesMan
Employee
+ getQuota()
Employee
Salesman Engineer

• You have methods with identical results on subclasses.
• Move them to the superclass
PUSH METHOD UP
+ getName()
Salesman
Employee
+ getName()
Engineer
+ getName()
Employee
Salesman Engineer

• A method needs more information from its caller.
• Add a parameter for an object that can pass on this
information.
ADD PARAMETERTO
METHOD
+getContact()
Customer
+getContact(Date)
Customer

EXTRACT METHOD
void printOwing() {

printBanner();

//print details

System.out.println ("name:
" + _name);
System.out.println ("amount
" + getOutstanding());
}
void printOwing() {

printBanner();
printDetails(getOutstanding());
}
void printDetails (double outstanding) {
System.out.println ("name: " + _name);
System.out.println ("amount " + outstanding);
}

BAD SMELLS IN CODE
If it stinks, change it.

BAD SMELLS IN CODE
• By now you have idea of what refactoring is!
• But, you have no concrete indication on when refactoring
should be applied!
Bad Smells: (General Deﬁnition) Pieces of code that are
wrong (in some sense) and that are ugly to see. [Fowler09]

BAD SMELLS IN CODE (1)
Symptoms Bad Smell Name
Missing Inheritance or delegation Duplicated code
Inadequate decomposition Long Method
Too many responsibility Large/God Class
Object is missing Long Parameter List
Missing polymorphism TypeTests

Same class changes differently
depending on addition
Divergent Change
Small changes affects too many
objects
Shotgun Surgery
Method needing too much
information from another object
Feature Envy
Data that are always used together Data Clumps
Changes in one hierarchy require
change in another hierarchy
Parallel Inheritance Hierarchy

Class that does too little Lazy Class
Class with too many delegating
methods
Middle Man
Attributes only used partially under
certain circumstances
Temporary Field
Coupled classes, internal
representation dependencies
Message Chains
Class containing only accessors Data Classes

DUPLICATED CODE
Problem: Finding the same code structure
in more than one place
Solution: Perform Extract Method and
invoke the code from both places
Problem: Having the same expression in
two sibling subclasses
Solution: Perform Extract Method in both
classes and the Pull Up Field
(a)
(b)

LONG METHOD
Problem: Finding a very long method
Solution: Perform Extract Method
and improve responsibilities distribution
Problem: Finding a very long method whose
statements operates on different parameters/variables
Solution: Extract Class
(a)
(b)

LARGE CLASS
Problem: Finding a class that does too much (too
many responsibilities)
Solution: Perform Extract (Sub)Class and improve
responsibilities distribution
Rule of Thumb:
• When a class is trying to do too much, it often
shows up as too many instance variables.
• When a class has too many instance variables,
duplicated code cannot be far behind

LONG PARAMETER LIST
Problem: Finding a method that has a very long
parameter list
Solution: Replace Parameters with Methods
or Introduce Parameter Object
‣ [Old School] Pass everything as a parameter
instead of using global data.
public void marry(String name, int age,
boolean gender, String name2,
int age2, boolean gender2) {...}

DIVERGENT CHANGE
Problem: Divergent change occurs when one
class is commonly changed in different ways for
different reasons
Solution: identify everything that changes for a
particular cause and use Extract Class to put them
all together
‣ Violation of Separation of Concerns principle

SHOTGUN SURGERY
Problem: Every time you make a kind of change, you
have to make a lot of little changes to a lot of different
classes.
Solution: Use Move Method and Move Field to
put all the changes in a single class.
• When the changes are all over the place they are
hard to ﬁnd, and its easy to miss an important change.
• Results of Copy&Paste programming

FEATURE ENVY
A classic [code] smell is a method that seems more interested
in a class other than the one it is in.The most common focus
of the envy is the data. [Fowler02]
Problem: Finding a (part of a) method that makes heavy use
of data and methods from another class
Solution: Use Move/Extract Method to put it
in the more desired class.

DATA CLUMPS
Problem: Finding same three or four data items together
in lots of places (Fields in a couple of classes,
Parameters in many method signatures)
Solution (1): look for where the clumps appear as ﬁelds
and use Extract Class to turn the clumps into an object
Solution (2): For method parameters use Introduce
Parameter Object to slim them down

PRIMITIVE OBSESSION
People new to objects are sometimes reluctant to use small objects
for small tasks, such as money classes that combine numbers and
currency ranges with an upper and lower.
Problem: Your primitive needs any additional data or
behavior.
Solution: Use Extract Class to turn primitives
into a Class.

TYPE TESTS
Problem: Finding a switch statement checking the
type of an instance object (no Polymorphism).
Solution: Use Extract Method to extract the switch
statement and then Move Method to get it into the
class where the polymorphism is needed
The essence of polymorphism is that instead of asking an
object what type it is and then invoking some behavior
based on the answer, you just invoke the behavior.

PARALLEL INHERITANCE
HIERARCHIES
Problem: every time you make a subclass of one class, you
have to make a subclass of another (special case of Shotgun
Surgery).
Solution: Use Move Method and Move Field to
collapse pairwise features and remove duplications.

LAZY CLASS
Problem: Finding a class that is not doing “enough”.
Solution: If you have subclasses that are not doing
enough try to use Collapse Hierarchy and nearly
useless components should be subjected to Inline Class

SPECULATIVE GENERALIZATION
Problem: You get this smell when someone says "I think we
need the ability to do this someday" and you need all sorts of
hooks and special cases to handle things that are not required.
Solution:
(1) If you have abstract classes that are not doing enough then use
Collapse Hierarchy
(2) Unnecessary delegation can be removed with Inline Class
(3) Methods with unused parameters should be subject to
Remove Parameter
(4) Methods named with odd abstract names should be repaired
with Rename Method

TEMPORARY FIELD
Problem: Finding an object in which an instance
variable is set only in certain circumstances.
(We usually expect an object to use all of its variables)
Solution: Use Extract Class to create a home for
these orphan variables by putting all the code that uses the
variable into the component.

MESSAGE CHAINS
Problem: Message chains occur when you see a client that
asks one object for another object, which the client then asks
for yet another object, which the client then asks for yet
another object, etc.
intermediate.getProvider().doSomething()
ch = vehicle->getChassis();
body = ch->getBody();
shell = body->getShell();
material = shell->getMaterial();
props = material->getProperties();
color = props->getColor();

ELIMINATE NAVIGATION CODE
…
engine.carburetor.fuelValveOpen = true
Engine
+carburetor
Car
-engine
+increaseSpeed()
Carburetor
+fuelValveOpen
Engine
-carburetor
+speedUp()
Car
-engine
+increaseSpeed()
…
engine.speedUp()
carburetor.fuelValveOpen = true
Carburetor
+fuelValveOpen
Car
-engine
+increaseSpeed()
Carburetor
-fuelValveOpen
+openFuelValve()
Engine
-carburetor
+speedUp()
carburetor.openFuelValve()fuelValveOpen = true

• One the major features of Objects is encapsulation
• Encapsulation often comes with delegation
• Sometimes delegation can go to far
• For example if you ﬁnd half the methods are delegated to
another class it might be time to use Remove Middle Man and
talk to the object that really knows what is going on
• If only a few methods are not doing much, use Inline Method to
inline them into the caller
• If there is additional behavior, you can use Replace Delegation
with Inheritance to turn the middle man into a subclass of the
real object
MIDDLE MAN

REMOVE MIDDLE MAN
Client
Department
+getManager()
Person
Client
+getDepartment()
Person
+getManager()
Department

WHAT’S NEXT?
• In the next class, we will:
a. Analyze together a (quite) real case study.
b.Focus on the design of each (Java) class
c. Apply some Refactoring directly into an IDE
• to understand what “Refactoring” means in practice
• to see Refactoring features embedded into Eclipse

REFERENCES
• [Fowler02] M. Fowler, K. Beck; Refactoring:
Improving the Design of Existing Code, Addison
Wesley, 2002
• [Fowler09] J. Fields, S. Harvie, M.Fowler, K. Beck;
Refactoring in Ruby,Addison Wesley, 2009
• http://martinfowler.com/refactoring/catalog/
index.html

Refactoring: Improve the design of existing code

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