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Design Patterns in Java

Design pattern are basic building block to efficiently design application as per requirement.
Broadly we divide it into 3 categories.
  • Creational
  • Structural
  • Behavioral.


Creational
  • Singleton
  • Prototype
  • Factory
  • Factory method
  • abstract factory
  • Builder


Creational


Creational pattern are based upon strategy we apply to control creation of object.
If we want to create object which can't be created again we call it singleton. Its like resource which we don't want be duplicated like connection.we make constructor private to achieve our objective.


Below patterns comes under Creational design pattern.




Structural (A,B,C,D,F,F,P)
  • Adapter,
  • bridge,
  • Composite, 
  • Decorator, 
  • Facade,
  • Flyweight, 
  • Proxy


Behavioural (MM,II,SS,CC,TV,O)



  • Command, 
  • Chain of responsibility,
  • Mediator, 
  • Memento,
  • Iterator, 
  • Interpreter,
  • Strategy,
  • State, 
  • Template, 
  • Visitor, 
  • Observer



J2EE Design Patern (Apart from GORM)


  • MVC ,
  • Front controller , 
  • DAO , 
  • Service locator , 
  • Business delegate




Sample Code:


package com.java.singleton;

public class DemoSingleton {
    
    private static DemoSingleton instanceOfSingleton;
    /**
     * Create private constructor
     */
    private DemoSingleton(){
        
    }
    /**
     * Create a static method to get instance.
     */
    public static DemoSingleton getInstance(){
        if(instanceOfSingleton == null){
            instanceOfSingleton = new DemoSingleton();
        }
        return instanceOfSingleton;
    }
    
    public void getSomeResource(){
        // do something here
        System.out.println(" get me ...");
    }
    
    public static void main(String a[]){
        DemoSingleton ds = DemoSingleton.getInstance();
        ds.getSomeResource();
    }
}


Factory Pattern

Factory of what? Of classes. In simple words, if we have a super class and n sub-classes, and based on data provided, we have to return the object of one of the sub-classes, we use a factory pattern.























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What are design patterns? 

So basically these are just collection of best practices to solve recurring problem by different kind of class arrangement. These arrangement could be based on creation of object or structural basis or on basis of required behavior.




Creational Design Pattern
Prototype Designpattern we will like to use this designpattern whenever creation of initial object & setting required default values is costly. So we just create 1 object & later on whenever more objects required we can provide initial object by just cloning of given object.Java Example is java.lang.Object#clone()
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Singleton When we need one instance per JVM. Example : Java Run time environment Java.lang.System.  java.lang.StringBuilder, Spring Beans are singleton …