Home Java Beginners Singleton Design Pattern

Singleton Design Pattern
Posted on: July 6, 2007 at 12:00 AM

This example explores how to implement a SingletonPattern on a class in java.

Singleton Design Pattern

This example explores how to implement a SingletonPattern on a class in java.

The Singleton design pattern ensures that only one instance of a class is created, it provides a global point of access to the object and allow multiple instances in the future without affecting a singleton class's clients. To ensure that only one instance of a class is created we make SingletonPattern as static. getInstance() method returns a single instance of the class. We create a private default constructor to avoid outside modification.

This example gets instances of SingletonPattern two times but the method getInstance() will return the same object without creating a new one.

The code of the program is given below:


public class SingletonPattern{

  private static SingletonPattern instance;

  private SingletonPattern(){} 

public static synchronized SingletonPattern getInstance(){

  if (instance == null)

  {

  instance = new SingletonPattern();

  }

  return instance;

  }

public static void main(String arg[]){

  System.out.println("The output of two instance:");

  SingletonPattern sp=new SingletonPattern();

  System.out.println("First Instance: "+sp.getInstance());

  sp=new SingletonPattern();

  System.out.println("Second Instance:"+sp.getInstance());

  }

}

The output of the program is given below:

C:\rajesh\kodejava>javac SingletonPattern.java

C:\rajesh\kodejava>java SingletonPattern
The output of two instance:
First Instance: SingletonPattern@3e25a5
Second Instance:SingletonPattern@3e25a5

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