Author: saqibkhan

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 plugin 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 have a MediaPlayer interface and a concrete class AudioPlayer implementing the MediaPlayer interface. AudioPlayer can play mp3 format audio files by default.

We are having another interface AdvancedMediaPlayer and concrete classes implementing the AdvancedMediaPlayer 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 have created an adapter class MediaAdapter which implements the MediaPlayer interface and uses AdvancedMediaPlayer objects 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 use AudioPlayer class to play various formats.

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);
}

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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");
   }
}

Step 6

Verify the output.

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

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 ways 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, an 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 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.

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;
   }
}

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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<String, Shape> shapeMap  = new Hashtable<String, Shape>();

   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());		
   }
}

Step 5

Verify the output.

Shape : Circle
Shape : Square
Shape : Rectangle

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 have 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 are 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 Meal objects by combining Item. BuilderPatternDemo, our demo class will use MealBuilder to build a Meal.

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();
}

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Step 2

Create concrete 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<Item> items = new ArrayList<Item>();	

   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());
   }
}

Step 8

Verify the output.

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

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 ways to create an object.

This pattern involves a single class which is responsible to create an object while making sure that only single object gets 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.

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!");
   }
}

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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();
   }
}

Step 3

Verify the output.

Hello World!

Abstract Factory patterns work 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 are going to create a Shape interface and a concrete class implementing it. We create an abstract factory class AbstractFactory as next step. Factory class ShapeFactory is defined, which 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.

Step 1

Create an interface for Shapes.

Shape.java

public interface Shape {
   void draw();
}

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Step 2

Create concrete classes implementing the same interface.

RoundedRectangle.java

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

RoundedSquare.java

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

Rectangle.java

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

Step 3

Create an Abstract class to get factories for Normal and Rounded Shape Objects.

AbstractFactory.java

public abstract class AbstractFactory {
   abstract Shape getShape(String shapeType) ;
}

Step 4

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.equalsIgnoreCase("RECTANGLE")){
     return new Rectangle();         
  }else if(shapeType.equalsIgnoreCase("SQUARE")){
     return new Square();
  }	 
  return null;
   }
}

RoundedShapeFactory.java

public class RoundedShapeFactory extends AbstractFactory {
   @Override
   public Shape getShape(String shapeType){    
  if(shapeType.equalsIgnoreCase("RECTANGLE")){
     return new RoundedRectangle();         
  }else if(shapeType.equalsIgnoreCase("SQUARE")){
     return new RoundedSquare();
  }	 
  return null;
   }
}

Step 5

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

FactoryProducer.java

public class FactoryProducer {
   public static AbstractFactory getFactory(boolean rounded){   
  if(rounded){
     return new RoundedShapeFactory();         
  }else{
     return new ShapeFactory();
  }
   }
}

Step 6

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

AbstractFactoryPatternDemo.java

public class AbstractFactoryPatternDemo {
   public static void main(String[] args) {
  //get shape factory
  AbstractFactory shapeFactory = FactoryProducer.getFactory(false);
  //get an object of Shape Rectangle
  Shape shape1 = shapeFactory.getShape("RECTANGLE");
  //call draw method of Shape Rectangle
  shape1.draw();
  //get an object of Shape Square 
  Shape shape2 = shapeFactory.getShape("SQUARE");
  //call draw method of Shape Square
  shape2.draw();
  //get shape factory
  AbstractFactory shapeFactory1 = FactoryProducer.getFactory(true);
  //get an object of Shape Rectangle
  Shape shape3 = shapeFactory1.getShape("RECTANGLE");
  //call draw method of Shape Rectangle
  shape3.draw();
  //get an object of Shape Square 
  Shape shape4 = shapeFactory1.getShape("SQUARE");
  //call draw method of Shape Square
  shape4.draw();
  
   }
}

Step 7

Verify the output.

Inside Rectangle::draw() method.
Inside Square::draw() method.
Inside RoundedRectangle::draw() method.
Inside RoundedSquare::draw() method.

Factory pattern is one of the most used design patterns 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.

Step 1

Create an interface.

Shape.java

public interface Shape {
   void draw();
}

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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 square
  shape3.draw();
   }
}

Step 5

Verify the output.

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

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.

What is Gang of Four (GOF)?

In 1994, four authors Erich Gamma, Richard Helm, Ralph Johnson and 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.

• Program to an interface not an implementation
• 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 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 unexperienced developers to learn software design in an easy and faster way.

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Types of Design Patterns

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