Design pattern

Design Pattern
Thibaut de Broca

History
1977: Christopher Alexander introduces the idea of patterns:
successful solutions to problems. (Pattern Language)
1987: Ward Cunningham and Kent Beck leverage
Alexander’s idea in the context of an OO language.
1987: Eric Gamma’s dissertation on importance of patterns
and how to capture them.
1994: The book ====>
Definition
In software engineering, a software design pattern is a general reusable solution to a
commonly occurring problem within a given context in software design. It is not a
finished design that can be transformed directly into source or machine code.

‘Avoid Tight coupling’
Fundamental Principles

‘Code as modular black boxes’ && ‘Single responsibility principles’

‘Program to an interface and not to an implementation’
The class shouldn’t care about which class they use, but more which
behaviour they will use.
To send messages from class to class, send interfaces not classes.
Be careful ! Don’t over-use interface => 1 class != 1 interface

public class Main {
public static void main(String[] args) {
Animal[] animals = new Animal[3];
animals[0] = new Dog();
animals[1] = new Cat();
animals[2] = new Fox();
for (Animal animal : animals) {
animal.speak();
}
}
}
An interface is a reference type in Java.
It is similar to class. It is a collection of abstract methods. A class implements an interface,
thereby inheriting the abstract methods of the interface. Along with abstract methods, an
interface may also contain constants, default methods, static methods, and nested types.
Interlude: Interface - Recall
Animal
Dog Cat Fox
public interface Animal {
public void speak();
}
public class Dog implements Animal {
public void speak() {System.out.println("Wouaf Wouaf!");}
}
public class Cat implements Animal {
public void speak() {System.out.println("Miaouh!");}
}
public class Fox implements Animal {
public void speak() {
System.out.println("What does the Fox say ?");
}
}

‘Favor Object Composition over inheritance’
The class shouldn’t care about which class they use, but more which
behaviour they will use.
To send messages from class to class, send interfaces not classes.
Bad use of inheritance:
My personal opinion is that there is no "better" or "worse" principle to design. There is "appropriate" and "inadequate" design for the concrete task. In
other words - I use both inheritance or composition, depending on the situation. The goal is to produce smaller code, easier to read, reuse and
eventually extend further.
class Stack extends ArrayList {
public void push(Object value) { … }
public Object pop() { … }
}
1
4
54
2
class Stack extends {
private ArrayList myList;
public void push(Object value) { … }
public Object pop() { … }
}

‘A design must be open for extension but closed for modification’
(Open/Close Principle)
=> Once a class is written, you should not have to modify it to add new
functionality.

‘Avoid Duplication’

- SRP: Single Responsibility Principle
- OCP: Open Close Principle
- LSP: Liskov Substitution Principle. (Derived classes must be substitutable for their
base classes).
- ISP: Interface Segregation Principle. (Client should not be forced to depend upon
interfaces they do not use).
- DIP: Dependency Inversion Principle. High level modules should not depend upon
low level modules. Both should depend upon abstraction).

Keep in mind: “When Coding, 80% of your time is spent by reading code”. So take
care of formatting and naming !
Others Principles

- DRY: Don’t Repeat Yourself. Can Lead to maintenance nightmare
- YAGNI: You ain’t gonna need it. Do not add functionality until necessary.
- KISS: Keep It Simple, Stupid. Avoid accidental complexity.
- Boy Scout Rules: Always leave the campground cleaner than you found it.
Others Principles

Class Diagram
· Upper part contains the name of the class
· Middle part describes attributes of the class
· Bottom part describes the methods of the class
public class Example {
public static int CONSTANT= 0;
private String attribute1;
public String method() {
return attribute1;
}
private static void privateMethod(int c) {
CONSTANT+=c;
}
}
UML - Recalls

Relations between classes
The simplest of relation :
· Class A depends on Class B if, at some point, it uses Class B
· Symbolized by dotted line and and open arrow indicating the dependency direction
public class A {
public String method(B b) {
return b.toString();
}
}
public class B {
public toString() {
return "B";
}
}
UML - Recalls

Association
Stronger dependency from class A to class B, namely, A uses and contains an instance of class B:
· It is represented by a plain line and an open arrow
· This association can have a cardinality 0..1 or 1..n
· It is directed , in the example below A knows B, but B does not know anything about A
public class A {
private B b;
public void setB(B b) {
this.b = b;
}
}
public class B {
public toString() {
return "B";
}
}
UML - Recalls

Aggregation
Our example models Rockband and members. A rockband can have several members. And Members can be in several Rockand.
The addition of the diamond adds information about the life of the objects
· The empty diamond signifies that ‘Member’ objects can be shared between several ‘RockBand’ objects
· When an ‘RockBand’ object is destroyed, then the instances of ‘Member’ it was associated with do not disappear.
UML - Recalls

Composition
· The plain/full diamond indicates that the contained objects are not shared
· When the container object is destroyed, the contained objects disappear with it
UML - Recalls
class Tattoo {
String description;
public Tattoo(String description) {
this.description = description;
}
}
class Member {
private String name;
private List<Tattoo> tattoos = new
ArrayList<>();
[...]
public ink(String description) {
tattoos.add(new Tattoo(description));
}
}
class RockBand {
private List<Member> members = new ArrayList<>();
public void addMember(Member m) {
members.add(m);
}
public Member getMember(int num) {
return members.get(num);
}
public void toString() {
[..]
}
}

Inheritance / Generalization
· Inheritance enables to create classes hierarchies sharing same attributes
· Children classes also have the attributes and methods of the parent classes,
· These attributes and methods are defined for the objects the children classes.
· This mechanism enables Polymorphism
Properties
· Transitivity: If B inherits from A and C inherits from B then C inherits from A
· Not reflexive: No class can inherit from itself
· Not symmetric: If B inherits from A, A does not inherit from B
· Not cyclic : If B inherits from A and C inherits from B then A can't inherit from C
UML - Recalls

Inheritance / Generalization - Code
public class Animal {
private String name;
public Animal(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
}
public class Cat extends Animal {
public Cat(String name) {
super(name); // Call to the parent class constructor
}
public void meow() {
System.out.println("Miaou " + super.getName() + " Miaou");
}
}
public class Dog extends Animal {
public Dog(String name) {
super(name); // Call to the parent class constructor
}
public void bark() {
System.out.println("Wouf " + super.getName() + " Wouf");
}
}
UML - Recalls

Pattern Catalogue
5 mains classes

Creational Pattern
Creational design patterns are design patterns that deal with object creation
mechanisms, trying to create objects in a manner suitable for the situation.
- Factory Method Pattern
- Abstract Factory
- Singleton
- Builder
- Prototype

Intent:
It allows to create objects without specifying their class (method whose main goal
is to create a class instance).
Application:
- A class can not anticipate the type of objects it must create (i.e.: framework).
- A class wants its subclasses to specify the type of objects it creates.
- A class needs controls over the creation of its objects.
Factory Method Pattern
// Problem: A depends on B
public class A {
public void doIt() {
B b = new B()
}
}

Structure:
public abstract class Animal() {}
public class Dog extend Animal() {}
public class Cat extend Animal() {}
public class Dolphin extend Animal() {}
public interface AnimalFactory {
abstract public Animal create(String type);
}
public AnimalFactory () {
public Animal create(String type) {
switch(type) {
case "Dog": return new Dog(); break;
case "Cat": return new Cat(); break;
case "Dolphin": return new Dolphin(); break;
default : throw Exception("Animal " + type + " is unknown.");
}
}
}
public Main() {
Animalfactory factory = new Animalfactory();
Animal cat = factory.create("cat");
}
}
Factory Method Pattern

Intent:
We want to have only one instance of a class.
Application:
- Instantiate an object can take time and
memory.
- We only need one instance.
// Example:
// Problem: For factory, we only need one
instance
Singleton Pattern

Singleton Pattern
Structure:
The trick is to use a static variable to
remember the instance.
Then, why not to create a static method ?
The big difference between a singleton and a bunch of static
methods is that singletons can implement interfaces, so you can
pass around the singleton as if it were "just another"
implementation.
MyClass
- static MyClass myClass;
static getInstance(): MyClass
C
public class AnimalFactory {
private AnimalFactory();
private static AnimalFactory animalFactory;
public static AnimalFactory getInstance() {
if (animalFactory == null) {
animalFactory = new AnimalFactory();
}
return animalFactory;
}
//…...
}
public Main() {
Animal cat = Animalfactory.getInstance().create("cat");
}
}

Creational Pattern
- Factory Method Pattern
- Abstract Factory
- Singleton
- Builder
- Prototype (très utilisé dans Javascript)

Behavioral Pattern
Behavioral patterns are concerned with the assignment of responsibilities between
objects, or, encapsulating behavior in an object and delegating requests to it.
- Observer
- Strategy
- State
- Visitor
- Memento
- Mediator
- Iterator

Intent:
Supports a relation One To Many so that
several objects get notified when an
object changes its state and can react.
Application:
- Widely use with IHM when several
elements of the view change when
the model changes.
Observer Pattern

Roles: subject and several observers
- Subject:
- Can modify his state.
- He notifies observers when he has
changed.
- Can give his new state
- Observers
- Can subscribe/unsubscribe to a subject
Observer Pattern
subscribe

public abstract class AbstractSubject {
private List<Observer> observers = new ArrayList<Observer>();
public void attach(Observer observer){observers.add(observer);}
public void notifyAllObservers(){
for (Observer observer : observers) {
observer.update();
}
}
}
public class Subject extends AbstractSubject{
private int state;
public int getState() {return state;}
public void setState(int state) {
this.state = state;
notifyAllObservers();
}
}
public class BinaryObserver extends Observer{
public BinaryObserver(Subject subject){
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println( "Binary String: " + Integer.toBinaryString(
subject.getState() ) );
}
}
public class ObserverPatternDemo {
AbstractSubject subject = new Subject();
new HexaObserver(subject);
new OctalObserver(subject);
new BinaryObserver(subject);
System.out.println("First state change: 15");
subject.setState(15);
System.out.println("Second state change: 10");
subject.setState(10);
}
}
Observer Pattern

Observer Pattern
Abstract Subject => Classe java.util.Observable
- addObserver(Observer obs)
- notifyObservers();
Concrete Subject => extends Observable
- When state change, you should call notifyObservers();
Abstract Observer: Interface java.util.Observer
- update(Observable obs, Object args);
Concrete Observer: implements java.util.Observer

Behavioral Pattern
- Observer
- Strategy
- State
- Visitor
- Memento
- Mediator
- Iterator
- Interpreter

Intent:
It allows to switch between differents algorithms to accomplish a task.
Application:
- Different variants of an algorithm
- Many related classes differ only in their behaviour
Strategy Pattern

Subject => Interface to outside world
Strategy (Algorithm) => common interface for the differents
algorithms.
Strategy Pattern

Program
- Input: read file
- Output: filtered file
Examples of 4 filters:
- No filtering
- Only words that start with t
- Only words longer than 5 characters
- Only words that are palindroms
Strategy Pattern - Example

public class StrategyPattern {
Context context = new Context();
String filename = args[0];
System.out.println("n* Default(Alll): ");
context.filter(filename);
System.out.println("n* Start with T: ");
context.changeStrategy(new StartWithT());
context.filter(filename);
}
}
public interface CheckStrategy {
public boolean check(String s);
}
public class All implements CheckStrategy {
@Override
public boolean check(String s) {
return true;
}
}
public class StartWithT implements CheckStrategy {
@Override
public boolean check(String s) {
return s != null && s.startWith("t");
}
}
public class Context {
private CheckStrategy strategy;
public Context() {this.strategy = new All();}
public void changeStrategy(CheckStrategy strategy) {
this.strategy = strategy;
}
public void filter(String filename) throws IOException {
BufferedReader infile = new BufferedReader(new
FileReader(filename));
String buffer = null;
while ((buffer = infile.readLine() != null) {
StringTokenizer word = new StringTokenizer(buffer);
while (words.hasMoreTokens()) {
String word = words.nextToken();
if (strategy.check(word)) {
Ssytem.out.println(word);
}
}
}
}
}
Strategy Pattern - Example

State Pattern
Intent:
Implements a state machine in an object-
oriented way.
It lets an object show other methods after a
change of internal state.
Application:
- an application is characterized by large and
numerous case statements. These cases
vector flow of control based on the state of
the application.

interface Statelike {
void writeName(StateContext context, String name);
}
class StateLowerCase implements Statelike {
@Override
public void writeName(final StateContext context, final String name) {
System.out.println(name.toLowerCase());
context.setState(new StateMultipleUpperCase());
}
}
class StateMultipleUpperCase implements Statelike {
/** Counter local to this state */
private int count = 0;
@Override
public void writeName(final StateContext context, final String name) {
System.out.println(name.toUpperCase());
/* Change state after StateMultipleUpperCase's writeName() gets invoked twice */
if (++count > 1) {context.setState(new StateLowerCase());}
}
}
class StateContext {
private Statelike myState;
StateContext() {
setState(new StateLowerCase());
}
void setState(final Statelike newState) {myState = newState;}
public void writeName(final String name) {
myState.writeName(this, name);
}
}
State Pattern
public class DemoOfClientState {
final StateContext sc = new StateContext();
sc.writeName("Monday");
sc.writeName("Tuesday");
sc.writeName("Wednesday");
sc.writeName("Thursday");
sc.writeName("Friday");
sc.writeName("Saturday");
sc.writeName("Sunday");
}
}

Structural Patterns
Design patterns that ease the design by identifying a simple way to realize
relationships between entities.
- Proxy
- Decorator
- Facade
- Adapter
- Aggregate
- Bridge

Proxy Pattern
Translation in French:
Proxy <=> ?????????
Intent:
- A proxy, in its most general form, is a
class functioning as an interface to
something else.
Application:
- The proxy could interface to
anything: a network connection, a
large object in memory, a file.
- For sensitive objects: can verify that
the caller has the authorization.

interface Image {
public void displayImage();
}
// On System A
class RealImage implements Image {
private String filename = null;
public RealImage(final String filename) {
this.filename = filename;
loadImageFromDisk();
}
private void loadImageFromDisk() {System.out.println("Loading " + filename);}
public void displayImage() {System.out.println("Displaying " + filename);}
}
// On System B
class ProxyImage implements Image {
private RealImage image = null;
private String filename = null;
public ProxyImage(final String filename) {
this.filename = filename;
}
public void displayImage() {
if (image == null) {
image = new RealImage(filename);
}
image.displayImage();
}
}
class ProxyExample {
public static void main(final String[] arguments) {
final Image image1 = new ProxyImage("HiRes_10MB_Photo1");
final Image image2 = new ProxyImage("HiRes_10MB_Photo2");
image1.displayImage(); // loading necessary
image1.displayImage(); // loading unnecessary
image2.displayImage(); // loading necessary
}
}
Proxy Pattern - Example

- Proxy
- Decorator
- Facade
- Adapter
- Aggregate
- Bridge
Structural Patterns

Decorator Pattern
Intent:
- A wrapper that adds functionality to a
class. It is stackable.
Application:
- The decorator pattern is often useful
for adhering to the “Single
Responsibility Principle”.
- It allows functionality to be divided
between classes with unique areas
of concern.

Decorator Pattern
public interface Shape {
void draw();
}
public class Rectangle implements Shape {
@Override
public void draw() {System.out.println("Shape: Rectangle");}
}
public class Circle implements Shape {
@Override
public void draw() {System.out.println("Shape: Circle");}
}
public abstract class ShapeDecorator implements Shape {
protected Shape decoratedShape;
public ShapeDecorator(Shape decoratedShape){
this.decoratedShape = decoratedShape;
}
public void draw(){decoratedShape.draw();}
}
public class RedShapeDecorator extends ShapeDecorator {
public RedShapeDecorator(Shape decoratedShape) {
super(decoratedShape);
}
@Override
public void draw() {
decoratedShape.draw();
setRedBorder(decoratedShape);
}
private void setRedBorder(Shape decoratedShape){
System.out.println("Border Color: Red");
}
public class DecoratorPatternDemo {
Shape circle = new Circle();
Shape redCircle = new RedShapeDecorator(new Circle());
Shape redRectangle = new RedShapeDecorator(new Rectangle());
System.out.println("Circle with normal border");
circle.draw();
System.out.println("nCircle of red border");
redCircle.draw();
System.out.println("nRectangle of red border");
redRectangle.draw();
}
}

Intent:
- Provide a unified interface to a set of
interfaces in a subsystem.
- Defines a higher-level interface that
makes the subsystem easier to use.
Application:
- A simple interface is required to
access a complex system.
- An entry point is needed to each
level of layered software.
Façade Pattern

Façade Pattern
public interface Shape {
void draw();
}
public class Rectangle implements Shape {
@Override
public void draw() {System.out.println("Rectangle::draw()");}
}
public class Square implements Shape {
@Override
public void draw() {System.out.println("Square::draw()");}
}
public class Circle implements Shape {
@Override
public void draw() {System.out.println("Circle::draw()");}
}
public class ShapeMaker {
private Shape circle;
private Shape rectangle;
private Shape square;
public ShapeMaker() {
circle = new Circle();
rectangle = new Rectangle();
square = new Square();
}
public void drawCircle(){circle.draw();}
public void drawRectangle(){rectangle.draw();}
public void drawSquare(){square.draw();}
}
public class FacadePatternDemo {
ShapeMaker shapeMaker = new ShapeMaker();
shapeMaker.drawCircle();
shapeMaker.drawRectangle();
shapeMaker.drawSquare();
}
Generic:
Example:

Intent:
- Allows the interface of an existing
class to be used as another interface
- Converts one interface to another so
that it matches what the client is
expecting
Application:
- Example: an adapter that converts the
interface of a Document Object Model
of an XML document into a tree
structure that can be displayed.
Adapter Pattern (Wrapper)

public interface MediaPlayer {
public void play(String audioType, String fileName);
}
public interface AdvancedMediaPlayer {
public void playVlc(String fileName);
public void playMp4(String fileName);
}
public class VlcPlayer implements AdvancedMediaPlayer{
@Override
public void playVlc(String fileName) {System.out.println("Playing vlc file: "+
fileName);}
@Override
public void playMp4(String fileName) {// do nothing}
}
public class Mp4Player implements AdvancedMediaPlayer{
@Override
public void playVlc(String fileName) {// do nothing}
@Override
public void playMp4(String fileName) {System.out.println("Playing mp4 file: "+
fileName);}
}
Example:
Adapter Pattern

public class MediaAdapter implements MediaPlayer {
AdvancedMediaPlayer advancedMusicPlayer;
public MediaAdapter(String audioType) {
If (audioType.equalsIgnoreCase("vlc") ) {
advancedMusicPlayer = new VlcPlayer();
} else if (audioType.equalsIgnoreCase("mp4")) {
advancedMusicPlayer = new Mp4Player();
}
}
@Override
public void play(String audioType, String fileName) {
if(audioType.equalsIgnoreCase("vlc")){
advancedMusicPlayer.playVlc(fileName);
} else if(audioType.equalsIgnoreCase("mp4")) {
advancedMusicPlayer.playMp4(fileName);
}
}
}
public class AudioPlayer implements MediaPlayer {
MediaAdapter mediaAdapter;
@Override
public void play(String audioType, String fileName) {
// inbuilt support to play mp3 music files
if(audioType.equalsIgnoreCase("mp3")) {
System.out.println("Playing mp3 file. Name: " + fileName);
} else if (audioType.equalsIgnoreCase("vlc") || audioType.equalsIgnoreCase("mp4")) {
// mediaAdapter is providing support to play other file formats
mediaAdapter = new MediaAdapter(audioType);
mediaAdapter.play(audioType, fileName);
} else {
System.out.println("Invalid media. " + audioType + " format not supported");
}
Example:
Adapter Pattern
public class AdapterPatternDemo {
AudioPlayer audioPlayer = new AudioPlayer();
audioPlayer.play("mp3", "beyond the horizon.mp3");
audioPlayer.play("mp4", "alone.mp4");
audioPlayer.play("vlc", "far far away.vlc");
audioPlayer.play("avi", "mind me.avi");
}
}

- Encapsulate what varies, OCP (Open Close Principle).
- 1 class, 1 responsability.
- Program against an interface, not an implementation, DIP.
- Prefer composition, over inheritance.
- Loose coupling, modular black boxes.
Golden OO-principles

Approach:
- Understand your design context
- Examine the patterns catalogue
- Identify and study related pattern
- Apply suitable pattern
Pitfalls:
- Selectins wrong patterns
- Inappropriate use of patterns
How to choose a Pattern ?

- Wikipedia
- tutorialspoint.com
- http://www.slideshare.net/mkruthika/software-design-patterns-ppt
- http://www.slideshare.net/HermanPeeren/design-patterns-illustrated/
- ...
Credits

Design pattern

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