Code Refactoring

CODE REFACTORING
LINGI2252 – PROF. KIM MENS
* Slides mostly based on Martin Fowler’s book: 
Refactoring: Improving the Design of Existing Code. ©Addison Wesley, 2000
*
(These slides are part of the course LINGI2252 “Software Maintenance and Evolution”, given by Prof. Kim Mens at UCL, Belgium)

Refactoring: 
Improving the Design of Existing Code
One of the best references on software refactoring, 
with illustrative examples in Java:
Refactoring: Improving the Design of Existing Code. 
Martin Fowler. Addison Wesley, 2000. ISBN: 0201485672
See also www.refactoring.com
Overview of this presentation
A. Refactoring basics
B. Categories of refactoring
C. Words of warning
2

A. REFACTORING BASICS
* Based on Chapter 2 of Martin Fowler’s book: 
*

What is refactoring?
A refactoring is a software transformation that
preserves the external behaviour of the software;
improves the internal structure of the software.
It is a disciplined way to clean up code that minimises the
chances of introducing bugs.
4

Deﬁnition of Refactoring [Fowler2000]
[noun] “a change made to the internal structure of
software to make it easier to understand and cheaper to
modify without changing its observable behaviour”
[verb] “to restructure software by applying a series of
refactorings without changing its observable behaviour”
typically with the purpose of making the software
easier to understand and modify
5

Why should you refactor?
To improve the design of software
To counter code decay (software ageing)
refactoring helps code to remain in shape
To increase software comprehensibility
To ﬁnd bugs and write more robust code
To increase productivity (program faster)
on a long term basis, not on a short term basis
7

Why should you refactor?
To reduce costs of software maintenance
To reduce testing
automatic refactorings are guaranteed to be behaviour
preserving
To prepare for / facilitate future customisations
To turn an OO application into a framework
To introduce design patterns in a behaviourally preserving way
8

When should you refactor?
Whenever you see the need for it
Do it all the time in little bursts
Not on a pre-set periodical basis
Apply the rule of three
1
st
time : implement from scratch
2
nd
time : implement something similar by code duplication
3
rd
time : do not implement similar things again, but refactor
9

Refactor when adding new features or functions
Especially if feature is difficult to integrate with the existing
code
Refactor during bug fixing
If a bug is very hard to trace, refactor first to make the code
more understandable, so that you can understand better where
the bug is located
Refactor during code reviews
10

Refactoring also ﬁts naturally in the agile methods
philosophy
Is needed to address the principle "Maintain simplicity"
Wherever possible, actively work to eliminate
complexity from the system
By refactoring the code
11

What do you tell the manager?
When (s)he’s technically aware (s)he’ll understand
why refactoring is important.
When (s)he’s interested in quality, (s)he’ll understand
that refactoring will improve software quality.
When (s)he’s only interested in the schedule, don’t
tell that you’re doing refactoring, just do it anyway.
In the end refactoring will make you more
productive.
12

When shouldn’t you refactor?
When the existing code is such a mess that although you could refactor
it, it would be easier to rewrite everything from scratch instead.
When you are too close to a deadline.
The productivity gain would appear after the deadline and thus
be too late.
However, when you are not close to a deadline you should never
put off refactoring because you don’t have the time.
Not having enough time usually is a sign that refactoring is
needed.
13

B. CATEGORIES OF REFACTORINGS
* Based on Martin Fowler’s book: 
*

Categories of refactorings
Small refactorings
(de)composing methods
moving features between objects
organizing data
simplifying conditional expressions
dealing with generalisation
simplifying method calls
Big refactorings
Tease apart inheritance
Extract hierarchy
Convert procedural design to objects
Separate domain from presentation
15

Small refactorings
(de)composing methods [9 refactorings]
moving features between objects [8 refactorings]
organizing data [16 refactorings]
simplifying conditional expressions [8 refactorings]
dealing with generalisation [12 refactorings]
simplifying method calls [15 refactorings]
16

Small Refactorings : 
(de)composing methods
1. Extract Method
2. Inline Method
3. Inline Temp
4. Replace Temp With Query
5. Introduce Explaining Variable
6. Split Temporary Variable
7. Remove Assignments to Parameter
8. Replace Method With Method Object
9. Substitute Algorithm
17
= we will zoom in on these
= home reading
Legend:

(De)composing methods: 
1. Extract Method
18
What? When you have a fragment of code that can be grouped together,
turn it into a method with a name that explains the purpose of the
method
Why? improves clarity, removes redundancy
Example:
public void accept(Packet p) { 
if ((p.getAddressee() == this) && 
(this.isASCII(p.getContents()))) 
this.print(p); 
else 
super.accept(p); }
public void accept(Packet p) {
if this.isDestFor(p) this.print(p);
else super.accept(p); }
public boolean isDestFor(Packet p) {
return
((p.getAddressee() == this) &&
(this.isASCII(p.getContents()))); }
Beware of local variables !

(Opposite of Extract Method)
 
What? When a method’s body is just as clear as its name, put the method’s
body into the body of its caller and remove the method
Why? To remove too much indirection and delegation
Example:
int getRating(){
return moreThanFiveLateDeliveries();
} 
boolean moreThanFiveLateDeliveries(){
return _numberOfLateDeliveries > 5;
}
(De)composing methods : 
2. Inline Method
19
int getRating(){
return (_numberOfLateDeliveries > 5);
}

3. Inline Temp
20
What? When you have a temp that is assigned once with a simple
expression, and the temp is getting in the way of refactorings, 
replace all references to that temp with the expression. 
Why? (Part of Replace Temp with Query refactoring)
 
Example:

return (anOrder. basePrice() > 100)
double basePrice = anOrder.basePrice();
return (basePrice > 100)

4. Replace Temp with Query
21
What? When you use a temporary variable to hold the result of an
expression, extract the expression into a method and replace all 
references to the temp with a method call
Why? Cleaner code
Example:

double basePrice = _quantity * _itemPrice;
if (basePrice > 1000)
return basePrice * 0.95;
else
return basePrice * 0.98;
if (basePrice() > 1000)
return basePrice() * 0.95;
else
return basePrice() * 0.98;
…
double basePrice(){
return _quantity * _itemPrice;
}

5. Introduce Explaining Variable
22
What? When you have a complex expression, put the result of the 
(parts of the) expression in a temporary variable with a name
that explains the purpose
Why? Breaking down complex expressions for clarity
Example:
if ( (platform.toUpperCase().indexOf(“MAC”) > -1) &&
(browser.toUpperCase().indexOf(“IE”) > -1) &&
wasInitialized() && resize > 0 )
{
//ACTION
}
final boolean isMacOs = platform.toUpperCase().indexOf(“MAC”) > -1;
final boolean isIEBrowser = browser.toUpperCase().indexOf(“IE”) > -1;
final boolean wasResized = resize > 0;
if (isMacOs && isIEBrowser && wasInitialized() && wasResized){
//ACTION
}

6. Split Temporary Variable
23
What? When you assign a temporary variable more than once,
but it is not a loop variable nor a collecting temporary
variable, make a separate temporary variable for each
assignment
Why? Using temps more than once is confusing
Example:

double temp = 2 * (_height + _width);
System.out.println (temp);
temp = _height * _width;
System.out.println (temp);
final double perimeter =
2 * (_height + _width);
System.out.println (perimeter);
final double area = _height * _width;
System.out.println (area);

7. Remove Assignments To Parameter
24
What? When the code assigns to a parameter,
use a temporary variable instead
Why? Lack of clarity and confusion between “pass by value”
and “pass by reference”
 
Example:

int discount (int inputVal, int quantity,
int yearToDate){
if (inputVal > 50) inputVal -= 2;
... MORE CODE HERE ... int discount (int inputVal, int quantity,
int yearToDate){
int result = inputVal;
if (inputVal > 50) result -= 2;
... MORE CODE HERE ...

8. Replace Method with Method Object
25
What? When you have local variables but cannot use
extract method, turn the method into its own object,
with the local variables as its ﬁelds
Why? Extracting pieces out of large methods makes things more
comprehensible
Example:

Order
price()
PriceCalculator
primaryBasePrice
secondaryBasePrice
compute()
return new PriceCalculator(this). compute()
1
Order
price()
double primaryBasePrice;
double secondaryBasePrice;
// long computation

9. Substitute Algorithm
26
What? When you want to replace an algorithm with a clearer
alternative, replace the body of the method with the
new algorithm
Why? To replace complicated algorithms with clearer ones
Example:

String foundPerson(String[] people){
for (int i = 0; i < people.length; i++){
if (people[i]. equals (“John”) ) {
return “John”;
}
if (people[i]. equals (“Jack”) ) {
return “Jack”;
}
}
}
String foundPerson(String[] people){
List candidates = Array.asList(new String[] {“John”, “Jack”})
for (int i = 0; i < people.length; i++)
if (candidates[i]. contains (people[i]))
return people[i];
}

Small refactorings
27

moving features between objects
1. Move Method
2. Move Field
3. Extract Class
4. Inline Class
5. Hide Delegate
6. Remove Middle Man
7. Introduce Foreign Method
8. Introduce Local Extension
= we will zoom in on these
= home reading
Legend:
28

Moving features between objects : 
1,2. Move Method / Field
29
What? When a method (resp. field) is used by or uses more features of
another class than its own, create a similar method (resp. field) 
in the other class; remove or delegate original method (resp. field) 
and redirect all references to it. 
Why? Essence of refactoring 
Example:

Class 1
aMethod()
Class 2
Class 1
Class 2
aMethod()

3. Extract Class
30
What? When you have a class doing work that should be done by two,
create a new class and move the relevant ﬁelds and methods to 
the new class
Why? Large classes are hard to understand
Example:

PhoneNumber
areaCode
number
getPhoneNumber
1
Person
name
officeAreaCode
officeNumber
homeAreaCode
homeNumber
getOfficePhone
getHomPhone
Person
name
getOfficePhone
getHomePhone
phone

4. Inline Class
31
What? When you have a class that does not do very much, move
all its features into another class and delete it
Why? To remove useless classes (as a result of other refactorings)
Example:
PhoneNumber
areaCode
number
getPhoneNumber()
1
Person
name
getPhoneNumber()
office-
phone
1
Person
name
officeAreaCode
officeNumber
getPhoneNumber()

5. Hide Delegate
32
What? When you have a client calling a delegate class of an object,
create methods on the server to hide the delegate
Why? Increase encapsulation
Example:
Person
getDepartment()
Department
getManager()
Person
getManager()
Client Class Client Class
Department
getManager()

6. Remove Middle Man
33
What? When a class is doing too much simple delegation, get the
client to call the delegate directly
Why? To remove too much indirection (as a result of other
refactorings)
Example:
Person
getManager()
Department
Person
getDepartment()
Department
getManager()
Client ClassClient Class

What? When a server class needs an additional method, but you
cannot modify the class, create a method in the client
class with an instance of the server class as its ﬁrst argument
Why? To introduce one additional service
Example:
Date newStart = new Date (previousEnd.getYear(),
previousEnd.getMonth(), previousEnd.getDate() + 1);
Date newStart = nextDay(previousEnd);
private static Date nextDay(Date arg) {
return new Date (arg.getYear(),
arg.getMonth(), arg.getDate() + 1);
}
7. Introduce Foreign Method
34

What? When a server class needs several additional methods but you cannot 
modify the class, create a new class containing the extra methods;
make the extension class a subclass or wrapper
Why? To introduce several additional services
Example:
Client Class
nextDayDate(Date): Date
MfDate
nextDay(): Date
Date
8. Introduce Local Extension
35

Small refactorings
36

organizing data
1. Encapsulate ﬁeld
2. Replace data value with
object
3. Change value to reference
4. Change reference to value
5. Replace array with object
6. Duplicate observed data
7. Change unidirectional
association to bidirectional
8. Change bidirectional
association to unidirectional
9. Replace magic number with
symbolic constant
10. Encapsulate collection
11. Replace record with data class
12. Replace subclass with ﬁelds
13-16. Replace type code with
class / subclass / state /
strategy 37

38
Organizing Data : 
1. Encapsulate Field
private String name; 
public String getName() { 
return this.name; } 
public void setName(String s) { 
this.name = s; }
public String name;
What? There is a public ﬁeld. Make it private and provide accessors.
Why? Encapsulating state increases modularity, and facilitates code reuse 
and maintenance.
When the state of an object is represented as a collection of private 
variables, the internal representation can be changed without
modifying the external interface
Example:

39
private Document doc; 
public String getContents() { 
return this.doc.getContents(); } 
public void setContents(String s) { 
this.doc.setContents(s); }
public class Document { 
private String contents;
public String getContents() {
return this.contents; }
public void setContents(String s) { 
this.contents = s; } 
}
private String contents;
public String getContents() {
return this.contents; }
public void setContents(String s) {
this.contents = s; }
2. Replace Data Value with Object

40
13. Replace Type Code with Subclass
What? An immutable type code affects the behaviour of a class
Example:
Employee
const Engineer=0
const Salesman=1
const Manager=2
type:Int
Employee
Engineer Salesman Manager

41
When? If subclassing cannot be used, e.g. because of dynamic
type changes during object lifetime (e.g. promotion of employees) 
Example:
Employee
const Engineer=0
const Salesman=1
const Manager=2
type:Int
Employee EmployeeType
Engineer Salesman Manager
15,16. Replace Type Code with State/Strategy
Makes use of state pattern or strategy design pattern

42
12. Replace Subclass with Fields
What? Subclasses vary only in methods that return constant data
Solution: Change methods to superclass ﬁelds and eliminate subclasses
Example:

Person
sex: [M, F]
Person
Male Female
Similar to replace inheritance with aggregation

Small refactorings
43

simplifying conditional expressions
1. Decompose conditional
2. Consolidate conditional expression
3. Consolidate duplicate conditional fragments
4. Remove control ﬂag
5. Replace nested conditional with guard clauses
6. Replace conditional with polymorphism
7. Introduce null objects
8. Introduce assertion
44

Small refactorings
45

dealing with generalisation
1. Push down method / ﬁeld
2. Pull up method / ﬁeld / constructor body
3. Extract subclass / superclass / interface
4. Collapse hierarchy
5. Form template method
6. Replace inheritance with delegation (and vice versa)
46

Dealing with Generalisation: 
1. Push Down Method
When behaviour on a superclass is relevant only for some of its subclasses,
move it to those subclasses
Engineer
Employee
getQuota
Salesman
47
Salesman
getQuota
Employee
Engineer

ASCIIPrinter PSPrinter
Printserver
accept
ASCIIPrinter
accept
PSPrinter
accept
Printserver
2. Pull Up Method
Simple variant: look for methods with same name in subclasses that do not appear in
superclass
More complex variant: do not look at the name but at the behaviour of the method
If the method that is being pulled up already exists in the superclass as an abstract method,
make it concrete with the common behaviour
48

When you have 2 classes with similar features
PrintServer FileServer FileserverPrintserver
Outputserver
3. Extract Superclass
49

Small refactorings
50

simplifying method calls
1. Rename method
2. Add parameter
3. Remove parameter
4. Separate query from modiﬁer
5. Parameterize method
6. Replace parameter with method
7. Replace parameter with explicit
methods
8. Preserve whole object
9. Introduce parameter object
10. Remove setting method
11. Hide method
12. Replace constructor with factory
method
13. Encapsulate downcast
14. Replace error code with exception
15. Replace exception with test
51

Simplifying method calls : 
9. Introduce Parameter Object
Group parameters that belong together in a
separate object
Customer
amountInvoicedIn(from:Date,to:Date)
amountReceivedIn(from:Date,to:Date)
amountOverdueIn(from:Date,to:Date)
Customer
amountInvoicedIn(r:DateRange)
amountReceivedIn(r:DateRange)
amountOverdueIn(r:DateRange)
DataRange
from : Date
to : Date
52

Simplifying method calls: 
14. Replace Error Code with Exception
What? When a method returns a special code to indicate an
error, throw an exception instead
Why? Clearly separate normal processing from error processing
Example:

int withdraw(int amount) {
if (amount > balance)
return -1
else
{balance -= amount;
return 0}
}
void withdraw(int amount) throws BalanceException {
if (amount > balance) throw new BalanceException();
balance -= amount;
}
53

Categories of refactorings
(according to [Fowler2000])
Small refactorings
(de)composing methods [9]
moving features between objects [8]
organizing data [16]
simplifying conditional expressions [8]
dealing with generalisation [12]
simplifying method calls [15]
Big refactorings
Tease apart inheritance
Extract hierarchy
Convert procedural design to objects
Separate domain from presentation
54

Big refactorings
Require a large amount of time (> 1 month)
Require a degree of agreement among the
development team
No instant satisfaction, no visible progress
55

Big Refactorings
1. Tease apart inheritance
2. Extract hierarchy
3. Convert procedural design to objects
4. Separate domain from presentation

Big refactorings: 
Problem
A tangled inheritance hierarchy that is doing 2 jobs at
once
Solution
Create 2 separate hierarchies and use delegation to invoke
one from the other
57

Approach
Identify the different jobs done by the hierarchy.
Extract least important job into a separate hierarchy.
Use extract class to create common parent of new hierarchy.
Create appropriate subclasses.
Use move method to move part of the behaviour from the
old hierarchy to the new one.
58

59
Window
FullXWin FullMSWin IconXWin
Full Iconised
IconMSWin
WindowImpl
FullXWin
Window
IconisedMSWin
1

Related design patterns
Bridge
decouples an abstraction from its
implementation so that the two can vary
independently
Strategy / Visitor / Iterator / State
60

Problem
An overly-complex class that is doing too much work,
at least in part through many conditional statements.
Solution
Turn class into a hierarchy where each subclass
represents a special case.
61

Approach
Create a subclass for each special case.
Use one of the following refactorings to return the appropriate subclass
for each variation:
replace constructor with factory method
replace type code with subclasses
replace type code with state/strategy
Take methods with conditional logic and apply:
replace conditional with polymorphism
62

Calculating electricity bills.
Lots of conditional logic needed to cover many different cases:
different charges for summer/winter
different tax rates
different billing plans for personal / business / government / …
reduced rates for persons with 
disabilities or social security
Customer
Billing Scheme
2. Extract hierarchy (example)
63

3. Convert procedural design 
into objects
Problem
You have code written in a procedural style.
Solution
Turn the data records into objects, break up the behaviour, and
move the behaviour to the objects.
Smaller refactorings used
extract method, move method, …
64

4. Separate domain 
from presentation
Goal
Change a two-tier design (user interface/database) into a a
three-tier one (UI/business logic/database).
Solution
Separate domain logic into separate domain classes.
Smaller refactorings used
extract method, move method/ﬁeld, duplicate observed data, …
65

C. REFACTORING TOOLS
*

CODE REFACTORING – REFACTORING TOOLS
AUTOMATED CODE REFACTORING TOOLS
Available for all major programming languages 
(and OO programming languages in particular)
Java : IntelliJ IDEA, Eclipse, NetBeans, JDeveloper, …
JavaScript : WebStorm, Eclipse, …
C++ : VisualStudio, Eclipse, …
ObjectiveC and SWIFT : XCode
.NET : VisualStudio
Smalltalk, PHP, Ruby, Python, C#, Delphi, …
67

LIMITATIONS OF MOST REFACTORING TOOLS
Only support for primitive refactorings
class refactorings
add (sub)class to hierarchy, rename class, remove class
method refactorings
add to class, rename, remove, push down, pull up, add parameter, move to
component, extract code
variable refactorings
add to class, rename, remove, push down, pull up, create accessors, abstract
variable
Often no support for higher-level refactorings
68

REFACTORING IN ECLIPSE
The refactoring tool in Eclipse supports a number of
transformations described in Martin Fowler's book
Refactoring can be accessed via the Refactor menu.
Refactoring commands are also available from the context
menus in many views or appear as quick assists.

SUPPORTED REFACTORING ACTIONS IN ECLIPSE (2016)
Rename, Move, Change Method Signature
Extract Method, Extract Local Variable, Extract Constant
Inline, Move Type to New File, Use Supertype Where Possible
Convert Anonymous Class to Nested, Convert Local Variable to Field
Extract Superclass, Extract Interface, Extract Class
Push Down, Pull Up, Encapsulate Field
Introduce Parameter Object, Introduce Indirection
Introduce Factory, Introduce Parameter
Generalize Declared Type , Infer Generic Type Arguments
(and more)

CODE REFACTORING – REFACTORING TOOLS 71

D. WORDS OF WARNING
*

CODE REFACTORING – WORDS OF WARNING
A WORD OF WARNING (1)
Know what you are doing
If not applied well, refactoring may decrease quality rather
than improve it
75

“Bad smells” are symptoms that something is wrong
Refactoring are supposed to remove “bad smells”
PhoneNumber
areaCode
number
getPhoneNumber
1
Person
name
officeAreaCode
officeNumber
homeAreaCode
homeNumber
getOfficePhone
getHomPhone
Person
name
getOfficePhone
getHomePhone
phone
1
EXTRACT 
CLASS
SMELLS LIKE A
TOO “LARGE CLASS” SMELLS BETTER
NOW…
76

Refactoring should not introduce new smells
HumanBeing
Person
name
getOfficePhone
getHomePhone
EXTRACT 
SUPERCLASS
Person
name
getOfficePhone
getHomePhone
SMELLS LIKE A
TOO ABSTRACT CLASS
77

NEXT SESSION: INTRODUCTION TO “BAD SMELLS”
Bad code smells
indicate that your code is ripe for refactoring
Refactoring is about
how to change code
Bad smells are about
when to modify it
78

Independently applied refactorings can introduce subtle
merge conﬂicts
79
Bank
Account
Loan
handles
Company
Agency
Account
Loan
handles
Bank Company
represents
Bank
Account
Loan
handles
Company
Safe
EXTRACT 
CLASS
CREATE
SUBCLASS
REFACTORING CONFLICT :
In the new version, Safe should not
be handled by Bank, but by Agency

CODE REFACTORING
POSSIBLE QUESTIONS
▸ Give a deﬁnition of refactoring in your own words 
and illustrate it with a concrete example of a refactoring.
▸ Explain why you should refactor.
▸ Explain when (= at what moment) refactoring should (or should not) be performed.
▸ Like refactoring, performance optimisation does not usually change the behaviour of code
(other than its speed); it only alters the internal structure. So how does it differ from refactoring?
▸ Explain and illustrate one of the following refactorings in detail:
▸ Extract Method, Move Method, Extract Class, Replace Type Code with Subclass, Replace
Subclass with Fields, Pull Up Method, Introduce Parameter Object
▸ Give a concrete example of how a refactoring could accidentally reduce quality.
▸ Give a concrete example of how to independently applied refactorings could accidentally
introduce a subtle merge conﬂict.

Code Refactoring

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