Software Engineering
Modern Approaches
Eric Braude and Michael Bernstein
1
Chapter 24. Refactoring
2
© 2010 John Wiley & Sons Ltd.
The Software Development
Lifecycle
Learning Goals for This Chapter
Planning
Maintenance
Testing
Implementation
Requirements
analysis
Design
Phase most
relevant to this
chapter is shown in
bold
© 2010 John Wiley & Sons Ltd.
•
What is refactoring?
•
How does refactorings work at large
scales?
•
How do you refactor at the method level?
•
Can you reorganize classes using
refactoring? Reorganize data?
•
Can you refactor at the module/package
level?
•
In what way is refactoring essential for
agile projects?
•
How is refactoring used in non-agile
projects?
•
How does refactoring relate to design
patterns?
3
Using a Refactoring Wizard
Shift/Alt/R
© 2010 John Wiley & Sons Ltd.
4
Fowler’s Refactoring Taxonomy
1. Big Refactorings
2. Composing Methods
3. Moving Features Between Objects
4. Organizing Data
5. Dealing With Generalization
6. Simplifying Conditional Expressions
7. Making Method Calls Simpler
© 2010 John Wiley & Sons Ltd.
5
Big Refactorings 1: Tease Apart Inheritance *
Employee
SoftwareEmp
MaintceEmp
ClericalEmp
FulltimeSoftwareEmp
FulltimeMaintceEmp
FulltimeClericalEmp
ParttimeSoftwareEmp
ParttimeMaintceEmp
ParttimeClericalEmp
RetiredSoftwareEmp
RetiredMaintceEmp
RetiredClericalEmp
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Big Refactorings 1: Tease Apart Inheritance *
Employee
MaintceEmp
SoftwareEmp
ClericalEmp
FulltimeSoftwareEmp
FulltimeMaintceEmp
FulltimeClericalEmp
ParttimeSoftwareEmp
ParttimeMaintceEmp
ParttimeClericalEmp
RetiredSoftwareEmp
RetiredMaintceEmp
RetiredClericalEmp

SoftwareEmp
Employee
MaintceEmp
© 2010 John Wiley & Sons Ltd.
Status
ClericalEmp
Fulltime
Parttime Retired
*Fowlers’ taxonomy
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Big Refactorings 2*:
Convert Procedural Design to Objects
Control
startGame()
displayCharacter()
moveCharacter()
VideoGame
GameCharacter
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Big Refactorings 2*:
Convert Procedural Design to Objects
Control
startGame()
displayCharacter()
moveCharacter()

© 2010 John Wiley & Sons Ltd.
VideoGame
GameCharacter
VideoGame
start()
GameCharacter
display()
move()
*Fowlers’ taxonomy
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Big Refactorings 3*:
Separate Domain from
Presentation
Account
name
balance
…
displayStandard()
displayHTML()
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Big Refactorings 3*:
Separate Domain from
Presentation
Account
name
balance
…
displayStandard()
displayHTML()
StandardAccountGUI
display ()

Account
name
balance
…
HTMLAccountGUI
display ()
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Big Refactorings 4*: Extract
Hierarchy
Project
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Big Refactorings 4*: Extract
Hierarchy
Project
Project

SoftwareEngineeringProject
MobileApplication
CustomerEntertainmentProject
DesktopApplication
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Composing Methods*
• Extract method
• Inline method
• Inline temp (remove a temporary variable)
• Replace temp with query (i.e., a function)
• Introduce explaining variable (to replace
complicated expression)
• Split temporary variable (i.e., used more than
once)
• Remove assignment to parameters
• Replace method with method object
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
14
Moving Features Between Objects 1*
• Move Method
– Trades off method holding vs. usage
• Move Field
– Trades off holding vs. usage
• Extract Class
– Encapsulate a set of attributes and
methods of a class
• Inline Class
– Opposite of Extract Class
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Moving Features Between Objects 2*
• Hide Delegate
– Hide class dependencies from client
classes**
• Remove Middle Man
– Opposite of Hide Delegate
*Fowlers’ taxonomy
** See below
© 2010 John Wiley & Sons Ltd.
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Hide Delegates
Client
© 2010 John Wiley & Sons Ltd.
Class1
Class2
method2 ()
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Hide Delegates
Class1
Client

Class2
method2 ()
Class1
method1()
Client
Key:
= changed
© 2010 John Wiley & Sons Ltd.
delegate
Class2
method2()
delegate.method2()
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Organizing Data 1*
• Self Encapsulate Field
– Change direct access of an attribute to accessor
use
• Replace Data Value with Object
• Change Value to Reference
class Order { Customer customer;….
class Order { private Customer getCustomer(
String ….)
• Change Reference to Value
• Replace Array with Object
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Organizing Data 2*
• Change Unidirectional Association to
Bidirectional
– (Only if necessary.) Install backpointer.
• Change Bidirectional Association to
Unidirectional
– Find a way to drop; consider third party
• Replace “Magic Number” with Constant
• Encapsulate Field
– public attribute to private/accessor
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
20
Organizing Data 3*
• Replace Record with Data Class
– Simplest object with private data field,
accessor
• Replace Type Code with Class
Account
…
type
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Organizing Data 3*
• Replace Record with Data Class
– Simplest object with private data field,
accessor
• Replace Type Code with Class
Account
…
type
Account
…
© 2010 John Wiley & Sons Ltd.
AccountType
REGULAR: AccountType
BIG_DISCOUNT: AccountType
SMALL_DISCOUNT: AccountType
*Fowlers’ taxonomy
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• Replace Type Code with Subclass
Organizing
Data 4*
Account
…
type
• Replace Type Code with State/Strategy
Account
…
type
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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• Replace Type Code with Subclass
Account
…
type
Organizing
Data 4*
Account
…

RegularAccount
WholesaleAccount
• Replace Type Code with State/Strategy
Account
…
type

Account
…
RegularAccountType
© 2010 John Wiley & Sons Ltd.
AccountType
…
WholesaleAccountType
*Fowlers’ taxonomy
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Dealing with Generalization 1*
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Dealing with Generalization 2*
• Extract Subclass
Order
quantity
discount
minimum
type
• Extract Superclass
Manager
name
salary
numSupervisees
© 2010 John Wiley & Sons Ltd.
Engineer
name
salary
skillSet

*Fowlers’ taxonomy
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Dealing with Generalization 2*
• Extract Subclass
Order
quantity
discount
minimum
type
• Extract Superclass
Manager
name
salary
numSupervisees
© 2010 John Wiley & Sons Ltd.
Engineer
name
salary
skillSet

Order
quantity
WholesaleOrder
discount
minimum
Employee
name
salary

Manager
numSupervisees
Engineer
skillSet
*Fowlers’ taxonomy
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Dealing with Generalization 3*
• Extract Interface
Manager
name
salary
numSupervisees
billRate
Engineer
name
salary
skillSet
billRate
• Collapse Hierarchy
– Inherited class not special enough
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
28
Dealing with Generalization 3*
• Extract Interface
Manager
name
salary
numSupervisees
billRate
Billable
getRate()
Engineer
name
salary
skillSet
billRate
• Collapse Hierarchy
Employee
getName()
getSalary()

Manager
numSupervisees
Engineer
skillSet
– Inherited class not special enough
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Dealing with Generalization 4*:
Form Template Method
BicycleAssemblyInstructions
writeBikeInstructions()
TricycleAssemblyInstructions
writeTrikeInstructions()
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Dealing with Generalization 4*:
Form Template Method
BicycleAssemblyInstructions
writeBikeInstructions()

TricycleAssemblyInstructions
writeTrikeInstructions()
© 2010 John Wiley & Sons Ltd.
AssemblyInstructions
writePrep()
writeSafety()
writeWrapUp()
writeManual()
BicycleAssemblyInstructions
writePrep()
writeSafety()
writeWrapUp()
TricycleAssemblyInstructions
writePrep()
writeSafety()
writeWrapUp()
*Fowlers’ taxonomy
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Fowler: Dealing with Generalization*
• Replace Inheritance with Delegation
Record
lastChanged()
Account
• Replace Delegation with Inheritance
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Fowler: Dealing with Generalization*
• Replace Inheritance with Delegation
Record
lastChanged()

Account
record.lastChanged()
Account
lastChanged()
record
Record
lastChanged()
• Replace Delegation with Inheritance
*Fowlers’ taxonomy
© 2010 John Wiley & Sons Ltd.
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Introducing Façade
V
U
W
© 2010 John Wiley & Sons Ltd.
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Introducing Façade
V
U
W
package
V
U
F
W
© 2010 John Wiley & Sons Ltd.
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U
V
© 2010 John Wiley & Sons Ltd.
Introduce Module
Refactoring
36
U
V
Introduce Module
Refactoring
1
VI
U
V
© 2010 John Wiley & Sons Ltd.
37
Trnsctn
Cust

Modularize Refactoring
Applied 2:
Transaction/Customer
ICust
ICust

Trnsctn
Cust
Trnsctn
© 2010 John Wiley & Sons Ltd.
CustFaç
Cust
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Reference Types
Trnsctn
Cust
• Trnsctn has attribute of type Cust
– Replacing Cust cust; with ICust cust;
OK if …
• Cust has only private attributes (i.e., use via methods only)
• Trnsctn does not instantiate Cust instances
• Trnsctn has method with parameter of type Cust
– Replacing myMethod(Cust ) with myMethod(ICust )
OK if …
• Cust has only private attributes (i.e., use via methods only)
• Trnsctn has method returning type Cust
– Replacing Cust myMethod(); with ICust myMethod();
OK
• Trnsctn has method with variable of type Cust
– Replacing Cust cust; with ICust cust;
matter
© 2010 John Wiley & Sons Ltd.
Unresolved as a general
39
Agile Methods
Obtain high-level
requirements
Obtain requirements for
next period’s* segment
Refactor to
clean up
* Typically 1-6 weeks
© 2010 John Wiley & Sons Ltd.
Refactor to
accommodate
new
requirements
Modify code and test code base
to handle additional requirements
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