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Design Patterns in Java Tutorial

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DESIGN PATTERNS IN JAVA TUTORIAL Simply Easy Learning by tutorialspoint.com

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ABOUT THE TUTORIAL

Design Patterns in Java Tutorial Design patterns represent the best practices used by experienced object-oriented software developers. Design patterns are solutions to general problems that software developers faced during software development. These solutions were obtained by trial and error by numerous software developers over quite a substantial period of time.

This tutorial will take you through step by step approach and examples using Java while learning Design Pattern concepts.

Audience This reference has been prepared for the experienced developers to provide best solutions to certain problems faced during software development and for un-experienced developers to learn software design in an easy and faster way.

Prerequisites Before proceeding with this tutorial you should have a good understanding of Java programming language. A basic understanding of Eclipse IDE is also required because all the examples have been compiled using Eclipse IDE.

Copyright & Disclaimer Notice

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This tutorial may contain inaccuracies or errors and tutorialspoint provides no guarantee regarding the accuracy of the site or its contents including this tutorial. If you discover that the tutorialspoint.com site or this tutorial content contains some errors, please contact us at [email protected]

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Table of ContentsDesign Patterns in Java Tutorial ................................................ i Audience ................................................................................... i Prerequisites ............................................................................. i Copyright & Disclaimer Notice ................................................... i Design Pattern Overview ....................................................... 1 Common platform for developers ............................................................................................ 1 Best Practices ........................................................................................................................ 2 Factory Pattern ....................................................................... 3 Implementation ............................................................................................ 3 Class Diagram ............................................................................................. 3 Steps ........................................................................................................... 4 Abstract Factory Pattern ........................................................ 7 Implementation ............................................................................................ 7 Class Diagram ............................................................................................. 8 Steps ........................................................................................................... 9 Singleton Design Pattern ..................................................... 14 Implementation .......................................................................................... 14 Class Diagram ........................................................................................... 15 Steps ......................................................................................................... 16 Builder Design Pattern ......................................................... 18 Implementation .......................................................................................... 18 Class Diagram ........................................................................................... 19 Steps ......................................................................................................... 19 Prototype Design Pattern ..................................................... 24 Implementation .......................................................................................... 24 Class Diagram. .......................................................................................... 25 Steps ......................................................................................................... 26 Adapter Design Pattern ........................................................ 29 Implementation .......................................................................................... 29 Class Diagram ........................................................................................... 30 Steps ......................................................................................................... 31 Bridge Design Pattern .......................................................... 34

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Implementation .......................................................................................... 34 Class Diagram ........................................................................................... 35 Steps ......................................................................................................... 36 Filter Design Pattern ............................................................ 38 Implementation .......................................................................................... 38 Class Diagram ........................................................................................... 39 Steps ......................................................................................................... 40 Composite Design Pattern .................................................... 45 Implementation .......................................................................................... 45 Class Diagram ........................................................................................... 46 Steps ......................................................................................................... 47 Decorator Design Pattern ..................................................... 49 Implementation .......................................................................................... 49 Class Diagram ........................................................................................... 50 Steps ......................................................................................................... 51 Faade Design Pattern ......................................................... 54 Implementation .......................................................................................... 54 Class Diagram ........................................................................................... 55 Steps ......................................................................................................... 56 Flyweight Design Pattern .................................................... 58 Implementation .......................................................................................... 58 Class Diagram ........................................................................................... 59 Steps ......................................................................................................... 60 Proxy Design Pattern ........................................................... 63 Implementation .......................................................................................... 63 Class Diagram ........................................................................................... 64 Steps ......................................................................................................... 65 Chain of Responsibility Design Pattern ............................... 67 Implementation .......................................................................................... 67 Class Diagram ........................................................................................... 68 Steps ......................................................................................................... 69 Command Design Pattern .................................................... 72 Implementation .......................................................................................... 72 Class Diagram ........................................................................................... 73 Steps ......................................................................................................... 74 Interpreter Design Pattern .................................................. 77 Implementation .......................................................................................... 77 Class Diagram ........................................................................................... 78

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Steps ......................................................................................................... 79 Iterator Design Pattern ........................................................ 82 Implementation .......................................................................................... 82 Class Diagram ........................................................................................... 83 Steps ......................................................................................................... 84 Mediator Design Pattern ...................................................... 86 Implementation .......................................................................................... 86 Class Diagram ........................................................................................... 87 Steps ......................................................................................................... 88 Memento Design Pattern ...................................................... 90 Implementation .......................................................................................... 90 Class Diagram ........................................................................................... 91 Steps ......................................................................................................... 92 Observer Design Pattern ...................................................... 94 Implementation .......................................................................................... 94 Class Diagram ........................................................................................... 95 Steps ......................................................................................................... 96 State Design Pattern ............................................................ 99 Implementation .......................................................................................... 99 Class Diagram ......................................................................................... 100 Steps ....................................................................................................... 101 Null Object Design Pattern ................................................ 103 Implementation ........................................................................................ 103 Class Diagram ......................................................................................... 104 Steps ....................................................................................................... 105 Strategy Design Pattern ..................................................... 108 Implementation ........................................................................................ 108 Class Diagram ......................................................................................... 109 Steps ....................................................................................................... 110 Template Design Pattern ................................................... 112 Implementation ........................................................................................ 112 Class Diagram ......................................................................................... 113 Steps ....................................................................................................... 114 Visitor Design Pattern ........................................................ 116 Implementation ........................................................................................ 116 Class Diagram ......................................................................................... 117 Steps ....................................................................................................... 118 MVC Design Pattern .......................................................... 121

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Implementation ........................................................................................ 121 Class Diagram ......................................................................................... 122 Steps ....................................................................................................... 123 Business Delegate Design Pattern ..................................... 126 Implementation ........................................................................................ 126 Class Diagram ......................................................................................... 127 Steps ....................................................................................................... 128 Composite Entity Design Pattern ...................................... 131 Implementation ........................................................................................ 131 Class Diagram ......................................................................................... 132 Steps ....................................................................................................... 133 Data Access Object Design Pattern .................................... 136 Implementation ........................................................................................ 136 Class Diagram ......................................................................................... 137 Steps ....................................................................................................... 138 Front Controller Design Pattern ........................................ 141 Implementation ........................................................................................ 141 Class Diagram ......................................................................................... 142 Steps ....................................................................................................... 143 Intercepting Filter Design Pattern .................................... 145 Implementation ........................................................................................ 145 Class Diagram ......................................................................................... 146 Steps ....................................................................................................... 147 Service Locator Design Pattern .......................................... 150 Implementation ........................................................................................ 150 Class Diagram ......................................................................................... 151 Steps ....................................................................................................... 152 Transfer Object Design Pattern ......................................... 156 Implementation ........................................................................................ 156 Class Diagram ......................................................................................... 157 Steps ....................................................................................................... 158 About tutorialspoint.com .................................................... 161

TUTORIALS POINT Simply Easy Learning Page 1

Design Pattern Overview This chapter gives a basic idea about Design Patterns starting with their origin, their evaluation over time and their classifications.

Design patterns represent the best practices used by experienced object-oriented software developers. Design patterns are solutions to general problems that software developers faced during software development. These solutions were obtained by trial and error by numerous software developers over quite a substantial period of time.

Gang of Four (GOF) In 1994, four authors Erich Gamma, Richard Helm; Ralph Johnson und John Vlissides published a book titled Design Patterns - Elements of Reusable Object-Oriented Software which initiated the concept of Design Pattern in Software development.

These authors are collectively known as Gang of Four (GOF). According to these authors design patterns are primarily based on the following principles of object orientated design.

x Program to an interface not an implementation

x Favor object composition over inheritance

Usage of Design Pattern Design Patterns have two main usages in software development.

Common platform for developers

Design patterns provide a standard terminology and are specific to particular scenario. For example, a singleton design pattern signifies use of single object so all developers familiar with

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single design pattern will make use of single object and they can tell each other that program is following a singleton pattern.

Best Practices

Design patterns have been evolved over a long period of time and they provide best solutions to certain problems faced during software development. Learning these patterns helps un-experienced developers to learn software design in an easy and faster way.

Types of Design Pattern As per the design pattern reference book Design Patterns - Elements of Reusable Object-Oriented Software, there are 23 design patterns. These patterns can be classified in three categories: Creational, Structural and behavioral patterns. We'll also discuss another category of design patterns: J2EE design patterns.

S.N. Pattern & Description

1 Creational Patterns These design patterns provides way to create objects while hiding the creation logic, rather than instantiating objects directly using new operator. This gives program more flexibility in deciding which objects need to be created for a given use case.

2 Structural Patterns These design patterns concern class and object composition. Concept of inheritance is used to compose interfaces and define ways to compose objects to obtain new functionalities.

3 Behavioral Patterns These design patterns are specifically concerned with communication between objects.

4 J2EE Patterns These design patterns are specifically concerned with the presentation tier. These patterns are identified by Sun Java Center.


Factory Pattern This section describes factory pattern and its implementation.

Factory pattern is one of most used design pattern in Java. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.

In Factory pattern, we create object without exposing the creation logic to the client and refer to newly created object using a common interface.

Implementation We're going to create a Shape interface and concrete classes implementing the Shape interface. A factory class ShapeFactory is defined as a next step.

FactoryPatternDemo, our demo class will use ShapeFactory to get a Shape object. It will pass information (CIRCLE / RECTANGLE / SQUARE) to ShapeFactory to get the type of object it needs. Class Diagram

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Steps Use the following steps to implement the above mentioned design pattern.

Step 1 Create an interface.

Shape.java public interface Shape { void draw(); }

Step 2 Create concrete classes implementing the same interface.

Rectangle.java public class Rectangle implements Shape { @Override public void draw() { System.out.println("Inside Rectangle::draw() method."); } } Square.java public class Square implements Shape { @Override public void draw() { System.out.println("Inside Square::draw() method."); } } Circle.java public class Circle implements Shape { @Override public void draw() { System.out.println("Inside Circle::draw() method."); } }


Step 3 Create a Factory to generate object of concrete class based on given information.

ShapeFactory.java public class ShapeFactory { //use getShape method to get object of type shape public Shape getShape(String shapeType){ if(shapeType == null){ return null; } if(shapeType.equalsIgnoreCase("CIRCLE")){ return new Circle(); } else if(shapeType.equalsIgnoreCase("RECTANGLE")){ return new Rectangle(); } else if(shapeType.equalsIgnoreCase("SQUARE")){ return new Square(); } return null; } }

Step 4 Use the Factory to get object of concrete class by passing an information such as type.

FactoryPatternDemo.java public class FactoryPatternDemo { public static void main(String[] args) { ShapeFactory shapeFactory = new ShapeFactory(); //get an object of Circle and call its draw method. Shape shape1 = shapeFactory.getShape("CIRCLE"); //call draw method of Circle shape1.draw(); //get an object of Rectangle and call its draw method. Shape shape2 = shapeFactory.getShape("RECTANGLE"); //call draw method of Rectangle shape2.draw(); //get an object of Square and call its draw method. Shape shape3 = shapeFactory.getShape("SQUARE"); //call draw method of circle shape3.draw(); } }


Step 5 Verify the output.

Inside Circle::draw() method. Inside Rectangle::draw() method. Inside Square::draw() method.


Abstract Factory Pattern This section describes abstract factory pattern and its implementation.

Abstract Factory patterns works around a super-factory which creates other factories. This factory is also called as Factory of factories. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.

In Abstract Factory pattern an interface is responsible for creating a factory of related objects, without explicitly specifying their classes. Each generated factory can give the objects as per the Factory pattern.

Implementation We're going to create a Shape and Color interfaces and concrete classes implementing these interfaces. We creates an abstract factory class AbstractFactory as next step. Factory classesShapeFactory and ColorFactory are defined where each factory extends AbstractFactory. A factory creator/generator class FactoryProducer is created. AbstractFactoryPatternDemo, our demo class uses FactoryProducer to get a AbstractFactory object. It will pass information (CIRCLE / RECTANGLE / SQUARE for Shape) to AbstractFactory to get the type of object it needs. It also passes information (RED / GREEN / BLUE for Color) to AbstractFactory to get the type of object it needs.

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Class Diagram



Step 1 Create an interface for Shapes.



Rectangle.java public class Rectangle implements Shape { @Override public void draw() { System.out.println("Inside Rectangle::draw() method."); } } Square.java public class Square implements Shape { @Override public void draw() { System.out.println("Inside Square::draw() method."); } } Circle.java public class Circle implements Shape { @Override public void draw() { System.out.println("Inside Circle::draw() method."); } }

Step 3 Create an interface for Colors.


Color.java public interface Color { void fill(); }

Step4 Create concrete classes implementing the same interface.

Red.java public class Red implements Color { @Override public void fill() { System.out.println("Inside Red::fill() method."); } } Green.java public class Green implements Color { @Override public void fill() { System.out.println("Inside Green::fill() method."); } } Blue.java public class Blue implements Color { @Override public void fill() { System.out.println("Inside Blue::fill() method."); } }

Step 5 Create an Abstract class to get factories for Color and Shape Objects.

AbstractFactory.java public abstract class AbstractFactory { abstract Color getColor(String color); abstract Shape getShape(String shape) ; }


Step 6 Create Factory classes extending AbstractFactory to generate object of concrete class based on given information.

ShapeFactory.java public class ShapeFactory extends AbstractFactory { @Override public Shape getShape(String shapeType){ if(shapeType == null){ return null; } if(shapeType.equalsIgnoreCase("CIRCLE")){ return new Circle(); } else if(shapeType.equalsIgnoreCase("RECTANGLE")){ return new Rectangle(); } else if(shapeType.equalsIgnoreCase("SQUARE")){ return new Square(); } return null; } @Override Color getColor(String color) { return null; } } ColorFactory.java public class ColorFactory extends AbstractFactory { @Override public Shape getShape(String shapeType){ return null; } @Override Color getColor(String color) { if(color == null){ return null; } if(color.equalsIgnoreCase("RED")){ return new Red(); } else if(color.equalsIgnoreCase("GREEN")){ return new Green(); } else if(color.equalsIgnoreCase("BLUE")){ return new Blue(); } return null; } }


Step 7 Create a Factory generator/producer class to get factories by passing an information such as Shape or Color

FactoryProducer.java public class FactoryProducer { public static AbstractFactory getFactory(String choice){ if(choice.equalsIgnoreCase("SHAPE")){ return new ShapeFactory(); } else if(choice.equalsIgnoreCase("COLOR")){ return new ColorFactory(); } return null; } }

Step 8 Use the FactoryProducer to get AbstractFactory in order to get factories of concrete classes by passing information such as type.

AbstractFactoryPatternDemo.java public class AbstractFactoryPatternDemo { public static void main(String[] args) { //get shape factory AbstractFactory shapeFactory = FactoryProducer.getFactory("SHAPE"); //get an object of Shape Circle Shape shape1 = shapeFactory.getShape("CIRCLE"); //call draw method of Shape Circle shape1.draw(); //get an object of Shape Rectangle Shape shape2 = shapeFactory.getShape("RECTANGLE"); //call draw method of Shape Rectangle shape2.draw(); //get an object of Shape Square Shape shape3 = shapeFactory.getShape("SQUARE"); //call draw method of Shape Square shape3.draw(); //get color factory AbstractFactory colorFactory = FactoryProducer.getFactory("COLOR");


//get an object of Color Red Color color1 = colorFactory.getColor("RED"); //call fill method of Red color1.fill(); //get an object of Color Green Color color2 = colorFactory.getColor("Green"); //call fill method of Green color2.fill(); //get an object of Color Blue Color color3 = colorFactory.getColor("BLUE"); //call fill method of Color Blue color3.fill(); } }


Inside Circle::draw() method. Inside Rectangle::draw() method. Inside Square::draw() method. Inside Red::fill() method. Inside Green::fill() method. Inside Blue::fill() method.


Singleton Design Pattern This section describes singleton pattern and its implementation.

Singleton pattern is one of the simplest design patterns in Java. This type of design pattern comes under creational pattern as this pattern provides one of the best way to create an object.

This pattern involves a single class which is responsible to creates own object while making sure that only single object get created. This class provides a way to access its only object which can be accessed directly without need to instantiate the object of the class.

Implementation We're going to create a SingleObject class. SingleObject class have its constructor as private and have a static instance of itself. SingleObject class provides a static method to get its static instance to outside world.SingletonPatternDemo, our demo class will use SingleObject class to get a SingleObject object.

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Class Diagram



Step 1 Create a Singleton Class.

SingleObject.java public class SingleObject { //create an object of SingleObject private static SingleObject instance = new SingleObject(); //make the constructor private so that this class cannot be //instantiated private SingleObject(){} //Get the only object available public static SingleObject getInstance(){ return instance; } public void showMessage(){ System.out.println("Hello World!"); } }

Step 2 Get the only object from the singleton class.

SingletonPatternDemo.java public class SingletonPatternDemo { public static void main(String[] args) { //illegal construct //Compile Time Error: The constructor SingleObject() is not visible //SingleObject object = new SingleObject(); //Get the only object available SingleObject object = SingleObject.getInstance(); //show the message object.showMessage(); } }



Hello World!


Builder Design Pattern This section describes builder pattern and its implementation.

Builder pattern builds a complex object using simple objects and using a step by step approach. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.

A Builder class builds the final object step by step. This builder is independent of other objects.

Implementation We've considered a business case of fast-food restaurant where a typical meal could be a burger and a cold drink. Burger could be either a Veg Burger or Chicken Burger and will be packed by a wrapper. Cold drink could be either a coke or pepsi and will be packed in a bottle.

We're going to create an Item interface representing food items such as burgers and cold drinks and concrete classes implementing the Item interface and a Packing interface representing packaging of food items and concrete classes implementing the Packing interface as burger would be packed in wrapper and cold drink would be packed as bottle. We then create a Meal class having ArrayList of Item and a MealBuilder to build different types of Mealobject by combining Item. BuilderPatternDemo, our demo class will use MealBuilder to build a Meal.

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Class Diagram


Step 1 Create an interface Item representing food item and packing.

Item.java public interface Item { public String name(); public Packing packing(); public float price(); } Packing.java public interface Packing { public String pack(); }


Step 2 Create concreate classes implementing the Packing interface.

Wrapper.java public class Wrapper implements Packing { @Override public String pack() { return "Wrapper"; } } Bottle.java public class Bottle implements Packing { @Override public String pack() { return "Bottle"; } }

Step 3 Create abstract classes implementing the item interface providing default functionalities.

Burger.java public abstract class Burger implements Item { @Override public Packing packing() { return new Wrapper(); } @Override public abstract float price(); } ColdDrink.java public abstract class ColdDrink implements Item { @Override public Packing packing() { return new Bottle(); } @Override public abstract float price(); }


Step 4 Create concrete classes extending Burger and ColdDrink classes

VegBurger.java public class VegBurger extends Burger { @Override public float price() { return 25.0f; } @Override public String name() { return "Veg Burger"; } } ChickenBurger.java public class ChickenBurger extends Burger { @Override public float price() { return 50.5f; } @Override public String name() { return "Chicken Burger"; } } Coke.java public class Coke extends ColdDrink { @Override public float price() { return 30.0f; } @Override public String name() { return "Coke"; } } Pepsi.java public class Pepsi extends ColdDrink { @Override public float price() { return 35.0f; }


@Override public String name() { return "Pepsi"; } }

Step 5 Create a Meal class having Item objects defined above.

Meal.java import java.util.ArrayList; import java.util.List; public class Meal { private List items = new ArrayList(); public void addItem(Item item){ items.add(item); } public float getCost(){ float cost = 0.0f; for (Item item : items) { cost += item.price(); } return cost; } public void showItems(){ for (Item item : items) { System.out.print("Item : "+item.name()); System.out.print(", Packing : "+item.packing().pack()); System.out.println(", Price : "+item.price()); } } }

Step 6 Create a MealBuilder class, the actual builder class responsible to create Meal objects.

MealBuilder.java public class MealBuilder { public Meal prepareVegMeal (){ Meal meal = new Meal(); meal.addItem(new VegBurger()); meal.addItem(new Coke()); return meal; } public Meal prepareNonVegMeal (){ Meal meal = new Meal();


meal.addItem(new ChickenBurger()); meal.addItem(new Pepsi()); return meal; } }

Step 7 BuiderPatternDemo uses MealBuider to demonstrate builder pattern.

BuilderPatternDemo.java public class BuilderPatternDemo { public static void main(String[] args) { MealBuilder mealBuilder = new MealBuilder(); Meal vegMeal = mealBuilder.prepareVegMeal(); System.out.println("Veg Meal"); vegMeal.showItems(); System.out.println("Total Cost: " +vegMeal.getCost()); Meal nonVegMeal = mealBuilder.prepareNonVegMeal(); System.out.println("\n\nNon-Veg Meal"); nonVegMeal.showItems(); System.out.println("Total Cost: " +nonVegMeal.getCost()); } }


Veg Meal Item : Veg Burger, Packing : Wrapper, Price : 25.0 Item : Coke, Packing : Bottle, Price : 30.0 Total Cost: 55.0 Non-Veg Meal Item : Chicken Burger, Packing : Wrapper, Price : 50.5 Item : Pepsi, Packing : Bottle, Price : 35.0 Total Cost: 85.5


Prototype Design Pattern This section describes prototype pattern and its implementation.

Prototype pattern refers to creating duplicate object while keeping performance in mind. This type of design pattern comes under creational pattern as this pattern provides one of the best way to create an object.

This pattern involves implementing a prototype interface which tells to create a clone of the current object. This pattern is used when creation of object directly is costly. For example, a object is to be created after a costly database operation. We can cache the object, returns its clone on next request and update the database as as and when needed thus reducing database calls.

Implementation We're going to create an abstract class Shape and concrete classes extending the Shape class. A class ShapeCache is defined as a next step which stores shape objects in a Hashtable and returns their clone when requested. PrototypPatternDemo, our demo class will use ShapeCache class to get a Shape object.

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Class Diagram.



Step 1 Create an abstract class implementing Clonable interface. Shape.java public abstract class Shape implements Cloneable { private String id; protected String type; abstract void draw(); public String getType(){ return type; } public String getId() { return id; } public void setId(String id) { this.id = id; } public Object clone() { Object clone = null; try { clone = super.clone(); } catch (CloneNotSupportedException e) { e.printStackTrace(); } return clone; } }

Step 2 Create concrete classes extending the above class.

Rectangle.java public class Rectangle extends Shape { public Rectangle(){ type = "Rectangle"; } @Override public void draw() { System.out.println("Inside Rectangle::draw() method.");


} } Square.java public class Square extends Shape { public Square(){ type = "Square"; } @Override public void draw() { System.out.println("Inside Square::draw() method."); } } Circle.java public class Circle extends Shape { public Circle(){ type = "Circle"; } @Override public void draw() { System.out.println("Inside Circle::draw() method."); } }

Step 3 Create a class to get concrete classes from database and store them in a Hashtable. ShapeCache.java import java.util.Hashtable; public class ShapeCache { private static Hashtable shapeMap = new Hashtable(); public static Shape getShape(String shapeId) { Shape cachedShape = shapeMap.get(shapeId); return (Shape) cachedShape.clone(); } // for each shape run database query and create shape // shapeMap.put(shapeKey, shape); // for example, we are adding three shapes public static void loadCache() { Circle circle = new Circle(); circle.setId("1"); shapeMap.put(circle.getId(),circle); Square square = new Square();


square.setId("2"); shapeMap.put(square.getId(),square); Rectangle rectangle = new Rectangle(); rectangle.setId("3"); shapeMap.put(rectangle.getId(),rectangle); } }

Step 4 PrototypePatternDemo uses ShapeCache class to get clones of shapes stored in a Hashtable. PrototypePatternDemo.java public class PrototypePatternDemo { public static void main(String[] args) { ShapeCache.loadCache(); Shape clonedShape = (Shape) ShapeCache.getShape("1"); System.out.println("Shape : " + clonedShape.getType()); Shape clonedShape2 = (Shape) ShapeCache.getShape("2"); System.out.println("Shape : " + clonedShape2.getType()); Shape clonedShape3 = (Shape) ShapeCache.getShape("3"); System.out.println("Shape : " + clonedShape3.getType()); } }


Shape : Circle Shape : Square Shape : Rectangle


Adapter Design Pattern This section describes adapter pattern and its implementation.

Adapter pattern works as a bridge between two incompatible interfaces. This type of design pattern comes under structural pattern as this pattern combines the capability of two independent interfaces.

This pattern involves a single class which is responsible to join functionalities of independent or incompatible interfaces. A real life example could be a case of card reader which acts as an adapter between memory card and a laptop. You plugins the memory card into card reader and card reader into the laptop so that memory card can be read via laptop.

We are demonstrating use of Adapter pattern via following example in which an audio player device can play mp3 files only and wants to use an advanced audio player capable of playing vlc and mp4 files.

Implementation We've an interface MediaPlayer interface and a concrete class AudioPlayer implementing theMediaPlayer interface. AudioPlayer can play mp3 format audio files by default. We're having another interface AdvancedMediaPlayer and concrete classes implementing theAdvancedMediaPlayer interface.These classes can play vlc and mp4 format files. We want to make AudioPlayer to play other formats as well. To attain this, we've created an adapter class MediaAdapter which implements the MediaPlayer interface and uses AdvancedMediaPlayerobjects to play the required format. AudioPlayer uses the adapter class MediaAdapter passing it the desired audio type without knowing the actual class which can play the desired format. AdapterPatternDemo, our demo class will useAudioPlayer class to play various formats.

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Class Diagram



Step 1 Create interfaces for Media Player and Advanced Media Player.

MediaPlayer.java public interface MediaPlayer { public void play(String audioType, String fileName); } AdvancedMediaPlayer.java public interface AdvancedMediaPlayer { public void playVlc(String fileName); public void playMp4(String fileName); }

Step 2 Create concrete classes implementing the AdvancedMediaPlayer interface. VlcPlayer.java public class VlcPlayer implements AdvancedMediaPlayer{ @Override public void playVlc(String fileName) { System.out.println("Playing vlc file. Name: "+ fileName); } @Override public void playMp4(String fileName) { //do nothing } } Mp4Player.java 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. Name: "+ fileName); } }


Step 3 Create adapter class implementing the MediaPlayer interface. MediaAdapter.java 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); } } }

Step 4 Create concrete class implementing the MediaPlayer interface. AudioPlayer.java 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); } //mediaAdapter is providing support to play other file formats else if(audioType.equalsIgnoreCase("vlc") || audioType.equalsIgnoreCase("mp4")){ mediaAdapter = new MediaAdapter(audioType); mediaAdapter.play(audioType, fileName); } else{ System.out.println("Invalid media. "+ audioType + " format not supported"); } }


}

Step 5 Use the AudioPlayer to play different types of audio formats.

AdapterPatternDemo.java public class AdapterPatternDemo { public static void main(String[] args) { 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"); } }


Playing mp3 file. Name: beyond the horizon.mp3 Playing mp4 file. Name: alone.mp4 Playing vlc file. Name: far far away.vlc Invalid media. avi format not supported


Bridge Design Pattern This section describes bridge pattern and its implementation.

Bridge is used where we need to decouple an abstraction from its implementation so that the two can vary independently. This type of design pattern comes under structural pattern as this pattern decouples implementation class and abstract class by providing a bridge structure between them.

This pattern involves an interface which acts as a bridge which makes the functionality of concrete classes independent from interface implementer classes. Both types of classes can be altered structurally without affecting each other.

We are demonstrating use of Bridge pattern via following example in which a circle can be drawn in different colors using same abstract class method but different bridge implementer classes.

Implementation We've an interface DrawAPI interface which is acting as a bridge implementer and concrete classes RedCircle, GreenCircle implementing the DrawAPI interface. Shape is an abstract class and will use object of DrawAPI. BridgePatternDemo, our demo class will use Shape class to draw different colored circle.

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Class Diagram



Step 1 Create bridge implementer interface.

DrawAPI.java public interface DrawAPI { public void drawCircle(int radius, int x, int y); }

Step 2 Create concrete bridge implementer classes implementing the DrawAPI interface. RedCircle.java public class RedCircle implements DrawAPI { @Override public void drawCircle(int radius, int x, int y) { System.out.println("Drawing Circle[ color: red, radius: " + radius +", x: " +x+", "+ y +"]"); } } GreenCircle.java public class GreenCircle implements DrawAPI { @Override public void drawCircle(int radius, int x, int y) { System.out.println("Drawing Circle[ color: green, radius: " + radius +", x: " +x+", "+ y +"]"); } }

Step 3 Create an abstract class Shape using the DrawAPI interface. Shape.java public abstract class Shape { protected DrawAPI drawAPI; protected Shape(DrawAPI drawAPI){ this.drawAPI = drawAPI; } public abstract void draw(); }


Step 4 Create concrete class implementing the Shape interface. Circle.java public class Circle extends Shape { private int x, y, radius; public Circle(int x, int y, int radius, DrawAPI drawAPI) { super(drawAPI); this.x = x; this.y = y; this.radius = radius; } public void draw() { drawAPI.drawCircle(radius,x,y); } }

Step 5 Use the Shape and DrawAPI classes to draw different colored circles. BridgePatternDemo.java public class BridgePatternDemo { public static void main(String[] args) { Shape redCircle = new Circle(100,100, 10, new RedCircle()); Shape greenCircle = new Circle(100,100, 10, new GreenCircle()); redCircle.draw(); greenCircle.draw(); } }


Drawing Circle[ color: red, radius: 10, x: 100, 100] Drawing Circle[ color: green, radius: 10, x: 100, 100]


Filter Design Pattern This section describes filter pattern and its implementation.

Filter pattern or Criteria pattern is a design pattern that enables developers to filter a set of objects, using different criteria, chaining them in a decoupled way through logical operations. This type of design pattern comes under structural pattern as this pattern is combining multiple criteria to obtain single criteria.

Implementation We're going to create a Person object, Criteria interface and concrete classes implementing this interface to filter list of Person objects. CriteriaPatternDemo, our demo class uses Criteria objects to filter List of Person objects based on various criteria and their combinations.

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Step 1 Create a class on which criteria is to be applied.

Person.java public class Person { private String name; private String gender; private String maritalStatus; public Person(String name,String gender,String maritalStatus){ this.name = name; this.gender = gender; this.maritalStatus = maritalStatus; } public String getName() { return name; } public String getGender() { return gender; } public String getMaritalStatus() { return maritalStatus; } }

Step 2 Create an interface for Criteria.

Criteria.java import java.util.List; public interface Criteria { public List meetCriteria(List persons); }


Step 3 Create concrete classes implementing the Criteria interface. CriteriaMale.java import java.util.ArrayList; import java.util.List; public class CriteriaMale implements Criteria { @Override public List meetCriteria(List persons) { List malePersons = new ArrayList(); for (Person person : persons) { if(person.getGender().equalsIgnoreCase("MALE")){ malePersons.add(person); } } return malePersons; } } CriteriaFemale.java import java.util.ArrayList; import java.util.List; public class CriteriaFemale implements Criteria { @Override public List meetCriteria(List persons) { List femalePersons = new ArrayList(); for (Person person : persons) { if(person.getGender().equalsIgnoreCase("FEMALE")){ femalePersons.add(person); } } return femalePersons; } } CriteriaSingle.java import java.util.ArrayList; import java.util.List; public class CriteriaSingle implements Criteria { @Override public List meetCriteria(List persons) { List singlePersons = new ArrayList(); for (Person person : persons) { if(person.getMaritalStatus().equalsIgnoreCase("SINGLE")){ singlePersons.add(person); } } return singlePersons; } }


AndCriteria.java import java.util.List; public class AndCriteria implements Criteria { private Criteria criteria; private Criteria otherCriteria; public AndCriteria(Criteria criteria, Criteria otherCriteria) { this.criteria = criteria; this.otherCriteria = otherCriteria; } @Override public List meetCriteria(List persons) { List firstCriteriaPersons = criteria.meetCriteria(persons); return otherCriteria.meetCriteria(firstCriteriaPersons); } } OrCriteria.java import java.util.List; public class AndCriteria implements Criteria { private Criteria criteria; private Criteria otherCriteria; public AndCriteria(Criteria criteria, Criteria otherCriteria) { this.criteria = criteria; this.otherCriteria = otherCriteria; } @Override public List meetCriteria(List persons) { List firstCriteriaItems = criteria.meetCriteria(persons); List otherCriteriaItems = otherCriteria.meetCriteria(persons); for (Person person : otherCriteriaItems) { if(!firstCriteriaItems.contains(person)){ firstCriteriaItems.add(person); } } return firstCriteriaItems; } }


Step 4 Use different Criteria and their combination to filter out persons.

CriteriaPatternDemo.java public class CriteriaPatternDemo { public static void main(String[] args) { List persons = new ArrayList(); persons.add(new Person("Robert","Male", "Single")); persons.add(new Person("John","Male", "Married")); persons.add(new Person("Laura","Female", "Married")); persons.add(new Person("Diana","Female", "Single")); persons.add(new Person("Mike","Male", "Single")); persons.add(new Person("Bobby","Male", "Single")); Criteria male = new CriteriaMale(); Criteria female = new CriteriaFemale(); Criteria single = new CriteriaSingle(); Criteria singleMale = new AndCriteria(single, male); Criteria singleOrFemale = new OrCriteria(single, female); System.out.println("Males: "); printPersons(male.meetCriteria(persons)); System.out.println("\nFemales: "); printPersons(female.meetCriteria(persons)); System.out.println("\nSingle Males: "); printPersons(singleMale.meetCriteria(persons)); System.out.println("\nSingle Or Females: "); printPersons(singleOrFemale.meetCriteria(persons)); } public static void printPersons(List persons){ for (Person person : persons) { System.out.println("Person : [ Name : " + person.getName() +", Gender : " + person.getGender() +", Marital Status : " + person.getMaritalStatus() +" ]"); } } }


Males: Person : [ Name : Robert, Gender : Male, Marital Status : Single ] Person : [ Name : John, Gender : Male, Marital Status : Married ]


Person : [ Name : Mike, Gender : Male, Marital Status : Single ] Person : [ Name : Bobby, Gender : Male, Marital Status : Single ] Females: Person : [ Name : Laura, Gender : Female, Marital Status : Married ] Person : [ Name : Diana, Gender : Female, Marital Status : Single ] Single Males: Person : [ Name : Robert, Gender : Male, Marital Status : Single ] Person : [ Name : Mike, Gender : Male, Marital Status : Single ] Person : [ Name : Bobby, Gender : Male, Marital Status : Single ] Single Or Females: Person : [ Name : Robert, Gender : Male, Marital Status : Single ] Person : [ Name : Diana, Gender : Female, Marital Status : Single ] Person : [ Name : Mike, Gender : Male, Marital Status : Single ] Person : [ Name : Bobby, Gender : Male, Marital Status : Single ] Person : [ Name : Laura, Gender : Female, Marital Status : Married ]


Composite Design Pattern This section describes composite pattern and its implementation.

Composite pattern is used where we need to treat a group of objects in similar way as a single object. Composite pattern composes objects in term of a tree structure to represent part as well as whole hierarchies. This type of design pattern comes under structural pattern as this pattern creates a tree structure of group of objects.

This pattern creates a class contains group of its own objects. This class provides ways to modify its group of same objects.

We are demonstrating use of Composite pattern via following example in which show employees hierarchy of an organization.

Implementation We've a class Employee which acts as composite pattern actor class. CompositePatternDemo, our demo class will use Employee class to add department level hierarchy and print all employees.

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Step 1 Create Employee class having list of Employee objects. Employee.java import java.util.ArrayList; import java.util.List; public class Employee { private String name; private String dept; private int salary; private List subordinates; // constructor public Employee(String name,String dept, int sal) { this.name = name; this.dept = dept; this.salary = salary; subordinates = new ArrayList(); } public void add(Employee e) { subordinates.add(e); } public void remove(Employee e) { subordinates.remove(e); } public List getSubordinates(){ return subordinates; } public String toString(){ return ("Employee :[ Name : "+ name +", dept : "+ dept + ", salary :" + salary+" ]"); } }

Step 2 Use the Employee class to create and print employee hierarchy. CompositePatternDemo.java public class CompositePatternDemo { public static void main(String[] args) { Employee CEO = new Employee("John","CEO", 30000);


Employee headSales = new Employee("Robert","Head Sales", 20000); Employee headMarketing = new Employee("Michel","Head Marketing", 20000); Employee clerk1 = new Employee("Laura","Marketing", 10000); Employee clerk2 = new Employee("Bob","Marketing", 10000); Employee salesExecutive1 = new Employee("Richard","Sales", 10000); Employee salesExecutive2 = new Employee("Rob","Sales", 10000); CEO.add(headSales); CEO.add(headMarketing); headSales.add(salesExecutive1); headSales.add(salesExecutive2); headMarketing.add(clerk1); headMarketing.add(clerk2); //print all employees of the organization System.out.println(CEO); for (Employee headEmployee : CEO.getSubordinates()) { System.out.println(headEmployee); for (Employee employee : headEmployee.getSubordinates()) { System.out.println(employee); } } } }


Employee :[ Name : John, dept : CEO, salary :30000 ] Employee :[ Name : Robert, dept : Head Sales, salary :20000 ] Employee :[ Name : Richard, dept : Sales, salary :10000 ] Employee :[ Name : Rob, dept : Sales, salary :10000 ] Employee :[ Name : Michel, dept : Head Marketing, salary :20000 ] Employee :[ Name : Laura, dept : Marketing, salary :10000 ] Employee :[ Name : Bob, dept : Marketing, salary :10000 ]


Decorator Design Pattern This section describes decorator pattern and its implementation.

Decorator pattern allows adding new functionality an existing object without altering its structure. This type of design pattern comes under structural pattern as this pattern acts as a wrapper to existing class.

This pattern creates a decorator class which wraps the original class and provides additional functionality keeping class methods signature intact.

We are demonstrating use of Decorator pattern via following example in which we'll decorate a shape with some color without alter shape class.

Implementation We're going to create a Shape interface and concrete classes implementing the Shape interface. We then create a abstract decorator class ShapeDecorator implementing the Shape interface and havingShape object as its instance variable. RedShapeDecorator is concrete class implementing ShapeDecorator. DecoratorPatternDemo, our demo class will use RedShapeDecorator to decorate Shape objects.

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Rectangle.java public class Rectangle implements Shape { @Override public void draw() { System.out.println("Shape: Rectangle"); } } Circle.java public class Circle implements Shape { @Override public void draw() { System.out.println("Shape: Circle"); } }

Step 3 Create abstract decorator class implementing the Shape interface. ShapeDecorator.java public abstract class ShapeDecorator implements Shape { protected Shape decoratedShape; public ShapeDecorator(Shape decoratedShape){ this.decoratedShape = decoratedShape; } public void draw(){ decoratedShape.draw(); }


}

Step 4 Create concrete decorator class extending the ShapeDecorator class. RedShapeDecorator.java 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"); } }

Step 5 Use the RedShapeDecorator to decorate Shape objects. DecoratorPatternDemo.java public class DecoratorPatternDemo { public static void main(String[] args) { 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(); } }



Circle with normal border Shape: Circle Circle of red border Shape: Circle Border Color: Red Rectangle of red border Shape: Rectangle Border Color: Red


Faade Design Pattern This section describes facade pattern and its implementation.

Facade pattern hides the complexities of the system and provides an interface to the client using which the client can access the system. This type of design pattern comes under structural pattern as this pattern adds an interface to exiting system to hide its complexities.

This pattern involves a single class which provides simplified methods which are required by client and delegates calls to existing system classes methods.

Implementation We're going to create a Shape interface and concrete classes implementing the Shape interface. A facade class ShapeMaker is defined as a next step. ShapeMaker class uses the concrete classes to delegates user calls to these classes.FacadePatternDemo, our demo class will use ShapeMaker class to show the results.

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Rectangle.java public class Rectangle implements Shape { @Override public void draw() { System.out.println("Rectangle::draw()"); } } Square.java public class Square implements Shape { @Override public void draw() { System.out.println("Square::draw()"); } } Circle.java public class Circle implements Shape { @Override public void draw() { System.out.println("Circle::draw()"); } }


Step 3 Create a facade class.

ShapeMaker.java 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(); } }

Step 4 Use the facade to draw various types of shapes.

FacadePatternDemo.java public class FacadePatternDemo { public static void main(String[] args) { ShapeMaker shapeMaker = new ShapeMaker(); shapeMaker.drawCircle(); shapeMaker.drawRectangle(); shapeMaker.drawSquare(); } }


Circle::draw() Rectangle::draw() Square::draw()


Flyweight Design Pattern This section describes flyweight pattern and its implementation.

Flyweight pattern is primarily used to reduce the number of objects created, to decrease memory footprint and increase performance. This type of design pattern comes under structural pattern as this pattern provides ways to decrease objects count thus improving application required objects structure.

Flyweight pattern try to reuse already existing similar kind objects by storing them and creates new object when no matching object is found. We'll demonstrate this pattern by drawing 20 circle of different locations but we'll creating only 5 objects. Only 5 colors are available so color property is used to check already existing Circle objects.

Implementation We're going to create a Shape interface and concrete class Circle implementing the Shape interface. A factory class ShapeFactory is defined as a next step. ShapeFactory have a HashMap of Circle having key as color of the Circle object. Whenever a request comes to create a circle of particular color to ShapeFactory. ShapeFactory checks the circle object in itsHashMap, if object of Circle found, that object is returned otherwise a new object is created, stored in hashmap for future use and returned to client. FlyWeightPatternDemo, our demo class will use ShapeFactory to get a Shape object. It will pass information (red / green / blue/ black / white) to ShapeFactory to get the circle of desired color it needs.

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Step 2 Create concrete class implementing the same interface.

Circle.java public class Circle implements Shape { private String color; private int x; private int y; private int radius; public Circle(String color){ this.color = color; } public void setX(int x) { this.x = x; } public void setY(int y) { this.y = y; } public void setRadius(int radius) { this.radius = radius; } @Override public void draw() { System.out.println("Circle: Draw() [Color : " + color +", x : " + x +", y :" + y +", radius :" + radius); } }


Step 3 Create a Factory to generate object of concrete class based on given information.

ShapeFactory.java import java.util.HashMap; public class ShapeFactory { private static final HashMap circleMap = new HashMap(); public static Shape getCircle(String color) { Circle circle = (Circle)circleMap.get(color); if(circle == null) { circle = new Circle(color); circleMap.put(color, circle); System.out.println("Creating circle of color : " + color); } return circle; } }

Step 4 Use the Factory to get object of concrete class by passing an information such as color.

FlyweightPatternDemo.java public class FlyweightPatternDemo { private static final String colors[] = { "Red", "Green", "Blue", "White", "Black" }; public static void main(String[] args) { for(int i=0; i < 20; ++i) { Circle circle = (Circle)ShapeFactory.getCircle(getRandomColor()); circle.setX(getRandomX()); circle.setY(getRandomY()); circle.setRadius(100); circle.draw(); } } private static String getRandomColor() { return colors[(int)(Math.random()*colors.length)]; } private static int getRandomX() { return (int)(Math.random()*100 ); } private static int getRandomY() { return (int)(Math.random()*100); } }



Creating circle of color : Black Circle: Draw() [Color : Black, x : 36, y :71, radius :100 Creating circle of color : Green Circle: Draw() [Color : Green, x : 27, y :27, radius :100 Creating circle of color : White Circle: Draw() [Color : White, x : 64, y :10, radius :100 Creating circle of color : Red Circle: Draw() [Color : Red, x : 15, y :44, radius :100 Circle: Draw() [Color : Green, x : 19, y :10, radius :100 Circle: Draw() [Color : Green, x : 94, y :32, radius :100 Circle: Draw() [Color : White, x : 69, y :98, radius :100 Creating circle of color : Blue Circle: Draw() [Color : Blue, x : 13, y :4, radius :100 Circle: Draw() [Color : Green, x : 21, y :21, radius :100 Circle: Draw() [Color : Blue, x : 55, y :86, radius :100 Circle: Draw() [Color : White, x : 90, y :70, radius :100 Circle: Draw() [Color : Green, x : 78, y :3, radius :100 Circle: Draw() [Color : Green, x : 64, y :89, radius :100 Circle: Draw() [Color : Blue, x : 3, y :91, radius :100 Circle: Draw() [Color : Blue, x : 62, y :82, radius :100 Circle: Draw() [Color : Green, x : 97, y :61, radius :100 Circle: Draw() [Color : Green, x : 86, y :12, radius :100 Circle: Draw() [Color : Green, x : 38, y :93, radius :100 Circle: Draw() [Color : Red, x : 76, y :82, radius :100 Circle: Draw() [Color : Blue, x : 95, y :82, radius :100


Proxy Design Pattern This section describes proxy pattern and its implementation.

In Proxy pattern, a class represents functionality of another class. This type of design pattern comes under structural pattern.

In Proxy pattern, we create object having original object to interface its functionality to outer world.

Implementation We're going to create a Image interface and concrete classes implementing the Image interface.ProxyImage is a a proxy class to reduce memory footprint of RealImage object loading. ProxyPatternDemo, our demo class will use ProxyImage to get a Image object to load and display as it needs.

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Image.java public interface Image { void display(); }


RealImage.java public class RealImage implements Image { private String fileName; public RealImage(String fileName){ this.fileName = fileName; loadFromDisk(fileName); } @Override public void display() { System.out.println("Displaying " + fileName); } private void loadFromDisk(String fileName){ System.out.println("Loading " + fileName); } } ProxyImage.java public class ProxyImage implements Image{ private RealImage realImage; private String fileName; public ProxyImage(String fileName){ this.fileName = fileName; } @Override public void display() { if(realImage == null){ realImage = new RealImage(fileName); }


realImage.display(); } }

Step 3 Use the ProxyImage to get object of RealImage class when required. ProxyPatternDemo.java public class ProxyPatternDemo { public static void main(String[] args) { Image image = new ProxyImage("test_10mb.jpg"); //image will be loaded from disk image.display(); System.out.println(""); //image will not be loaded from disk image.display(); } }


Loading test_10mb.jpg Displaying test_10mb.jpg Displaying test_10mb.jpg


Chain of Responsibility Design Pattern This section describes chain of responsibility pattern and its implementation.

As the name suggest, the chain of responsibility pattern creates a chain of receiver objects for a request. This pattern decouples sender and receiver of a request based on type of request. This pattern comes under behavioral patterns.

In this pattern, normally each receiver contains reference to another receiver. If one object cannot handle the request then it passes the same to the next receiver and so on.

Implementation We've created an abstract class AbstractLogger with a level of logging. Then we've created three types of loggers extending the AbstractLogger. Each logger checks the level of message to its level and print accordingly otherwise does not print and pass the message to its next logger.

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Step 1 Create an abstract logger class.

AbstractLogger.java public abstract class AbstractLogger { public static int INFO = 1; public static int DEBUG = 2; public static int ERROR = 3; protected int level; //next element in chain or responsibility protected AbstractLogger nextLogger; public void setNextLogger(AbstractLogger nextLogger){ this.nextLogger = nextLogger; } public void logMessage(int level, String message){ if(this.level


ErrorLogger.java public class ErrorLogger extends AbstractLogger { public ErrorLogger(int level){ this.level = level; } @Override protected void write(String message) { System.out.println("Error Console::Logger: " + message); } } FileLogger.java public class FileLogger extends AbstractLogger { public FileLogger(int level){ this.level = level; } @Override protected void write(String message) { System.out.println("File::Logger: " + message); } }

Step 3 Create different types of loggers. Assign them error levels and set next logger in each logger. Next logger in each logger represents the part of the chain.

ChainPatternDemo.java public class ChainPatternDemo { private static AbstractLogger getChainOfLoggers(){ AbstractLogger errorLogger = new ErrorLogger(AbstractLogger.ERROR); AbstractLogger fileLogger = new FileLogger(AbstractLogger.DEBUG); AbstractLogger consoleLogger = new ConsoleLogger(AbstractLogger.INFO); errorLogger.setNextLogger(fileLogger); fileLogger.setNextLogger(consoleLogger); return errorLogger; } public static void main(String[] args) { AbstractLogger loggerChain = getChainOfLoggers(); loggerChain.logMessage(AbstractLogger.INFO, "This is an information."); loggerChain.logMessage(AbstractLogger.DEBUG,


"This is an debug level information."); loggerChain.logMessage(AbstractLogger.ERROR, "This is an error information."); } }


Standard Console::Logger: This is an information. File::Logger: This is an debug level information. Standard Console::Logger: This is an debug level information. Error Console::Logger: This is an error information. File::Logger: This is an error information. Standard Console::Logger: This is an error information.


Command Design Pattern This section describes command pattern and its implementation.

Command pattern is a data driven design pattern and falls under behavioral pattern category. A request is wrapped under a object as command and passed to invoker object. Invoker object looks for the appropriate object which can handle this command and pass the command to the corresponding object and that object executes the command.

Implementation We've created an interface Order which is acting as a command. We've created a Stock class which acts as a request. We've concrete command classes BuyStock and SellStock implementing Orderinterface which will do actual command processing. A class Broker is created which acts as a invoker object. It can take order and place orders. Broker object uses command pattern to identify which object will execute which command based on type of command. CommandPatternDemo, our demo class will use Broker class to demonstrate command pattern.

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Step 1 Create a command interface.

Order.java public interface Order { void execute(); }

Step 2 Create a request class.

Stock.java public class Stock { private String name = "ABC"; private int quantity = 10; public void buy(){ System.out.println("Stock [ Name: "+name+", Quantity: " + quantity +" ] bought"); } public void sell(){ System.out.println("Stock [ Name: "+name+", Quantity: " + quantity +" ] sold"); } }

Step 3 Create concrete classes implementing the Order interface. BuyStock.java public class BuyStock implements Order { private Stock abcStock; public BuyStock(Stock abcStock){ this.abcStock = abcStock; } public void execute() { abcStock.buy(); } }


SellStock.java public class SellStock implements Order { private Stock abcStock; public SellStock(Stock abcStock){ this.abcStock = abcStock; } public void execute() { abcStock.sell(); } }

Step 4 Create command invoker class.

Broker.java import java.util.ArrayList; import java.util.List; public class Broker { private List orderList = new ArrayList(); public void takeOrder(Order order){ orderList.add(order); } public void placeOrders(){ for (Order order : orderList) { order.execute(); } orderList.clear(); } }

Step 5 Use the Broker class to take and execute commands.

CommandPatternDemo.java public class CommandPatternDemo { public static void main(String[] args) { Stock abcStock = new Stock(); BuyStock buyStockOrder = new BuyStock(abcStock); SellStock sellStockOrder = new SellStock(abcStock); Broker broker = new Broker(); broker.takeOrder(buyStockOrder); broker.takeOrder(sellStockOrder); broker.placeOrders(); }


}


Stock [ Name: ABC, Quantity: 10 ] bought Stock [ Name: ABC, Quantity: 10 ] sold


Interpreter Design Pattern This section describes interpreter pattern and its implementation.

Interpreter pattern provides way to evaluate language grammar or expression. This type of pattern comes under behavioral patterns. This pattern involves implementing an expression interface which tells to interpret a particular context. This pattern is used in SQL parsing, symbol processing engine etc.

Implementation We're going to create an interface Expression and concrete classes implementing the Expressioninterface. A class TerminalExpression is defined which acts as a main interpreter of context in question. Other classes OrExpression, AndExpression are used to create combinational expressions. InterpreterPatternDemo, our demo class will use Expression class to create rules and demonstrate parsing of expressions.

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Step 1 Create an expression interface.

Expression.java public interface Expression { public boolean interpret(String context); }

Step 2 Create concrete classes implementing the above interface.

TerminalExpression.java public class TerminalExpression implements Expression { private String data; public TerminalExpression(String data){ this.data = data; } @Override public boolean interpret(String context) { if(context.contains(data)){ return true; } return false; } } OrExpression.java public class OrExpression implements Expression { private Expression expr1 = null; private Expression expr2 = null; public OrExpression(Expression expr1, Expression expr2) { this.expr1 = expr1; this.expr2 = expr2; } @Override public boolean interpret(String context) { return expr1.interpret(context) || expr2.interpret(context); } }


AndExpression.java public class AndExpression implements Expression { private Expression expr1 = null; private Expression expr2 = null; public AndExpression(Expression expr1, Expression expr2) { this.expr1 = expr1; this.expr2 = expr2; } @Override public boolean interpret(String context) { return expr1.interpret(context) && expr2.interpret(context); } }

Step 3 InterpreterPatternDemo uses Expression class to create rules and then parse them. InterpreterPatternDemo.java public class InterpreterPatternDemo { //Rule: Robert and John are male public static Expression getMaleExpression(){ Expression robert = new TerminalExpression("Robert"); Expression john = new TerminalExpression("John"); return new OrExpression(robert, john); } //Rule: Julie is a married women public static Expression getMarriedWomanExpression(){ Expression julie = new TerminalExpression("Julie"); Expression married = new TerminalExpression("Married"); return new AndExpression(julie, married); } public static void main(String[] args) { Expression isMale = getMaleExpression(); Expression isMarriedWoman = getMarriedWomanExpression(); System.out.println("John is male? " + isMale.interpret("John")); System.out.println("Julie is a married women? " + isMarriedWoman.interpret("Married Julie")); } }



John is male? true Julie is a married women? true


Iterator Design Pattern This section describes iterator pattern and its implementation.

Iterator pattern is very commonly used design pattern in Java and .Net programming environment. This pattern is used to get a way to access the elements of a collection object in sequential manner without any need to know its underlying representation.

Iterator pattern falls under behavioral pattern category.

Implementation We're going to create a Iterator interface which narrates navigation method and a Container interface which returns the iterator. Concrete classes implementing the Container interface will be responsible to implement Iterator interface and use it IteratorPatternDemo, our demo class will use NamesRepository, a concrete class implementation to print a Names stored as a collection in NamesRepository.

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Step 1 Create interfaces.

Iterator.java public interface Iterator { public boolean hasNext(); public Object next(); } Container.java public interface Container { public Iterator getIterator(); }

Step 2 Create concrete class implementing the Container interface. This class has inner class NameIteratorimplementing the Iterator interface. NameRepository.java public class NameRepository implements Container { public String names[] = {"Robert" , "John" ,"Julie" , "Lora"}; @Override public Iterator getIterator() { return new NameIterator(); } private class NameIterator implements Iterator { int index; @Override public boolean hasNext() { if(index < names.length){ return true; } return false; } @Override public Object next() { if(this.hasNext()){ return names[index++]; } return null; } }


}

Step 3 Use the NameRepository to get iterator and print names. IteratorPatternDemo.java public class IteratorPatternDemo { public static void main(String[] args) { NameRepository namesRepository = new NameRepository(); for(Iterator iter = namesRepository.getIterator(); iter.hasNext();){ String name = (String)iter.next(); System.out.println("Name : " + name); } } }


Name : Robert Name : John Name : Julie Name : Lora


Mediator Design Pattern This section describes mediator pattern and its implementation.

Mediator pattern is used

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