Design pattern tutorial

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.

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Table of Contents
Design 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
Façade 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

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



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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.");
}
}

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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();
}
}

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Step 5
Verify the output.
Inside Circle::draw() method.
Inside Rectangle::draw() method.
Inside Square::draw() method.



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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




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

Step 1
Create an interface for Shapes.
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 an interface for Colors.

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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) ;
}

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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;
}
}

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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");


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//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();
}
}

Step 9
Verify the output.
Inside Circle::draw() method.
Inside Rectangle::draw() method.

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


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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





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

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();
}
}

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Step 3
Verify the output.
Hello World!

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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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