Introduction to Refactoring

Refactoring
Improving the design of existing code …
…. without changing the behavior!

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Why do good developers write bad software?
• Project and budget pressure lead to taking shortcuts
• You realize there are better ways to do something
• Requirements change over time
!

• When it’s hard to change update your code this lead to less optimal design

e fix our
w
How do
?
oftware
s


How do w

e know ou
r
software
is “bad”
… when it
works fin
e!

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Software development is not a Jenga game
• Single Responsibility Principle
• Just because you can, doesn’t mean you should.

• Open Closed Principle
• Open chest surgery is not needed when putting on a coat.

• Liskov Substitution Principle
• If it looks like a duck, quacks like a duck, 
but needs batteries – you probably have the wrong abstraction

• Interface Segregation Principle
• You want me to plug this in, where?

• Dependency Inversion Principle
• Would you solder a lamp directly to the electrical wiring in a wall?


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Single Responsibility Principle
• A responsibility can be deﬁned as a reason to change
!
!
!
!
!
!
!

• Just because you can, doesn’t mean you should.


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Open Closed Principle
• open for extension, but closed for modiﬁcation
!
!
!
!
!
!
!

• Open chest surgery is not needed when putting on a coat.


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Liskov Substitution Principle
• use a subtype without changing the correctness of the program
!
!
!
!
!
!
!

• If it looks like a duck, quacks like a duck, 
but needs batteries – you probably have the wrong abstraction

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Interface Segregation Principle
• states that no client should be forced to depend on methods it does not use
!
!
!
!
!
!
!

• You want me to plug this in, where?


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Dependency Inversion Principle
• no dependencies on implementations only on abstractions
!
!
!
!
!
!
!

• Would you solder a lamp directly to the electrical wiring in a wall?


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Dependency Inversion Example
class EventLogWriter
{
public void Write(string message)
{
//Write to event log here
}
}

!

class AppPoolWatcher
{
// Handle to EventLog writer to write to the logs
EventLogWriter writer = null;

!

// This function will be called when the app pool has problem
public void Notify(string message)
{
if (writer == null)
{
writer = new EventLogWriter();
}
writer.Write(message);
}
}

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EventLogWriter
AppPoolWatcher
// This can be
watcher.Action

!

writer = new EventLogWriter();
watcher = new AppPoolWatcher();
done in some class
= writer;

// This can be done in some other class
watcher.Notify("Sample message to log");


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class AppPoolWatcher
{
// Handle to EventLog writer to write to the logs
INofificationAction action = null;

!

!

public INofificationAction Action
{
get
{
return action;
}
set
{
action = value;
}
}
// This function will be called when the app pool has problem
public void Notify(string message)
{
action.ActOnNotification(message);
}

}

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Refactoring
Improving the design of existing code
Without changing the behavior


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Refactoring
Released in 2000, already a
golden oldie, a book I recommend
everyone to read.
!
The book deﬁnes the technique
of refactoring and is a reference
for refactoring types.


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Code Smells
Refactoring Cheat Codes


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Code Smells
• The term code smells was introduced by Kent Beck, based on an important
lesson he learned from his Grandmother about children and diapers. 
 
„If it stinks, change it”—Grandma Beck


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Code Smells
• Duplicated Code

• Primitive Obsession

• Inappropriate Intimacy

• Long Method

• Switch Statements

• Large Class

• Parallel Inheritance
Hierarchies

• Alternative Classes with
Different Interfaces

• Long Parameter List
• Divergent Change
• Shotgun Surgery
• Feature Envy
• Data Clumps


• Incomplete Library Class

• Lazy Class

• Data Class

• Speculative Generality

• Refused Bequest

• Temporary Field

• Comments

• Middle Man

• Dead Code

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Comments and Naming
• Comments

• Rename Method

• Add comments only to explain purpose

• Extract Method

• If comments need to explain how code work, it
smells.

!

• Instead use meaningful names, e.g.: 
schedule.add(Course course)

!
!

!

!

!

!

• Type Embedded in Name

• Rename Method

• Uncommunicative Name
• Inconsistent Names


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Excess and Duplication
• Excess
• Long Method
• Long Parameter List
• Large Class
• Long methods, large classes and endless
parameters make code unreadable.
!

• Duplicated Code

• Extract Method
• Replace Temp with Query
• Introduce Parameter Object
• Extract Class
• Extract Subclass

!
!
!
• Replace Parameter with Method
• Pull Up Field
• Form Template Method
• Substitute Algorithm


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The Past and the Future
• Dead Code

• Delete Code

• or worse, commented code

!

• Delete it, we have version control for that!

!

!
!

• Speculative Generality

• Collapse Hierarchy

• Don't attempt to predict future needs with
unnecessary generalization.

• Inline Class
• Remove Parameter
• Rename Method

YAGNI!


KISS

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Rename Method
// this method will return the maximum

void getMaximumCreditLimit() {

// credit limit.

return maximumCreditLimit;

void getIt() {

}

return _mCL;

void getInvoiceCreditLimit() {

}

int maximumCredit;

void getivcdlmt() {

float usedCredit;

int m;
float u;

=>

. .
}

. .
}


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Extract Method
void printOwing(double amount) {

void printOwing(double amount) {

printBanner();

printBanner();
printDetails(amount);

//print details

}

System.out.println ("name:" + _name);
System.out.println ("amount" + amount);
}

void printDetails (double amount) {

=>

System.out.println ("name:" + _name);
System.out.println ("amount" + amount);
}


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Replace Temp With Query
double basePrice = _quantity * _itemPrice;

if (basePrice() > 1000)

if (basePrice > 1000)

return basePrice() * 0.95;

return basePrice * 0.95;

else

else

return basePrice() * 0.98;


…
double basePrice() {

=>

return _quantity * _itemPrice;
}


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Introduce Parameter Object
double invoicedAmount(Date start, Date end) {

double invoicedAmount(Period range) {

double result;

double result;

for (Invoice invoice: invoices) {

for (Invoice invoice: invoices) {
if (range.contains(invoice.getDate()) {

if (invoice.getDate().after(start) ||

result += invoice.getAmount();

invoice.getDate().before(end)) {
}

result += invoice.getAmount();
}

}

return result;

}
return result;
}

=> !

}

class Period (Date start, Date end) {
. . .
boolean contains(Date date) {
return date.after(start) || date.before(end);
}
}


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Extract Class
class Person...
public String getName() {

class Person...
public String getName() {

return _name;
}
public String getTelephoneNumber() {
return ("(" + _officeAreaCode + ")"
+ _officeNumber);
}
String getOfficeAreaCode() {
return _officeAreaCode;
}
String getOfficeNumber() {
return _officeNumber;
}
private String _name;
private String _officeAreaCode;
private String _officeNumber;


return _name;

=> !

}
public String getTelephoneNumber(){
return _officeTelephone.getTelephoneNumber();
}
TelephoneNumber getOfficeTelephone() {
return _officeTelephone;
}
private String _name;
private TelephoneNumber _officeTelephone =
new TelephoneNumber();
class TelephoneNumber...
public String getTelephoneNumber() {
return ("(" + _areaCode + ") " + _number);
}
String getAreaCode() {
return _areaCode;
String getNumber() {
return _number;
}
private String _number;
private String _areaCode;

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Extract Subclass
class JobItem ...
public JobItem (int unitPrice,

class JobItem ...
public JobItem (int unitPrice) {

boolean isLabor,
Employee employee) {
_unitPrice = unitPrice;
_isLabor = isLabor;
_employee = employee;

!

}
public int getUnitPrice(){
return (_isLabor) ?
_employee.getRate():
_unitPrice;
}
...
class Employee...
public Employee (int rate) {
_rate = rate;
}
public int getRate() {
return _rate;
}
private int _rate;

_unitPrice = unitPrice;

!

=>

}
return _unitPrice;
}
...

class LaborItem ...
public LaborItem (Employee employee) {
super(0);
_employee = employee;
}
return_employee.getRate():
}
...

!

class Employee...
public Employee (int rate) {
_rate = rate;
}
public int getRate() {
return _rate;
}
private int _rate;


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Replace Parameter with Method
public double getPrice() {
int basePrice = _quantity * _itemPrice;
int discountLevel;
if (_quantity > 100) discountLevel = 2;
else discountLevel = 1;
double finalPrice =
discountedPrice (basePrice,discountLevel);
return finalPrice;
}

!

private double discountedPrice
(int basePrice, int discountLevel) {
if (discountLevel == 2)
else
}


public double getPrice() {
if (getDiscountLevel() == 2)
return getBasePrice() * 0.1;
else
return getBasePrice() * 0.05;
}

!

=>

private double getBasePrice() {
return _quantity * _itemPrice;
}

!

private int getDiscountLevel() {
if (_quantity > 100) return 2;
else return 1;
}

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Substitute Algorithm
String foundPerson(String[] people){
for (int i = 0; i < people.length; i++) {
if (people[i].equals ("Don")){
return "Don";
}
if (people[i].equals ("John")){
return "John";
}
if (people[i].equals ("Kent")){
return "Kent";
}
}
return "";
}

=>
String foundPerson(String[] people){
List candidates = Arrays.asList(new String[] {"Don", "John", "Kent"});
for (int i=0; i < people.length; i++)
if (candidates.contains(people[i]))
return people[i];
return "";
}


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References
• SOLID: 

http://en.wikipedia.org/wiki/SOLID_(object-oriented_design)
!
• SmellsToRefactorings: 
http://users.csc.calpoly.edu/~jdalbey/305/Lectures/SmellsToRefactorings
!
• Martin Fowler refactoring site: http://refactoring.com/
!
• Dependency Injection: http://martinfowler.com/articles/injection.html


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