Singleton Design Pattern by example

Singleton design pattern is most commonly used among all available java design patterns. It is a Creational patterns. Basic purpose of this design pattern is to maintain only one instance of a class throughout the application. This class may represent a common shared resource, functionality etc.

When to Use ? You should consider singleton design pattern in following scenario

• A resource is shared across all application component and only one instance of resource is possible.
• Global variables are being passed across all part of application
• Multiple instances representing same kind of information.

How to Use ? It is very simple to implement this design pattern. Following are he key requirements and possible solution for implementing singleton design pattern.

• Only one instance should be available at the time – restrict the class instantiation using private scoped constructor.
• Everybody should get same instance – maintain a private static instance variable so that only one instance will be available
• Easy way to access this instance – provide a public static method which returns instance of class. If instance is already there it return the same otherwise create new one.

Putting all together in sequence

1. Declare a class with a private constructor.
2. Maintain a self reference and declare as static.
3. Provide a static method to access this self reference. This method check whether self reference is instantiated or not , if it already instantiated then return the same otherwise  instantiate it.

UML : 

Code :

Singleton Class :

  1: package demo.singleton;
  2: /**
  3:  * Singleton Resource Class
  4:  * @author Yogesh
  5:  */
  6: public class SingletonResource {
  7:     private static SingletonResource resorce =null;
  8: 
  9:     public static SingletonResource getInstance(){
 10:         if(resorce==null){
 11:             resorce = new SingletonResource();
 12:         }
 13:         return resorce;
 14:     }
 15: 
 16:     private SingletonResource(){
 17:         // your initializing code here
 18:     }
 19: }
 20: 

Test Class : 

  1: public class Test {
  2: 
  3:     public static void main(String[] args){
  4:         SingletonResource resource1 = SingletonResource.getInstance();
  5:         SingletonResource resource2 = SingletonResource.getInstance();
  6:         if(resource1==resource2){
  7:             System.out.println("Both are equals");
  8:         } else {
  9:             System.out.println("Both are not equals");
 10:         }
 11:     }
 12: }

Output :  Both are equals