Singleton pattern

Definition: At any point of time we ensure that there is only object created for the class

Example:
participant classes:

• Singleton.java
• SingletonClient.java(client)

Diagram:

Singleton.java

• This class includes a static member variable singleton with private scope
• the constructor is private so that no one can instantiate it
• we have a getCurrentInstance() method to instantiate the class. If the instance is already available then the instance is returned to the caller otherwise it is newly created

public class Singleton {

 private static Singleton singleton = null;

 private Singleton(){

 }

 public static synchronized Singleton getCurrentInstance(){
 if(singleton == null){
 return new Singleton();
 }
 else{
 return singleton;
 }
 }

 public void sayHello(){
 System.out.println("Hello");
 }
}

SingletonClient.java

use getCurrentInstance() method to get the instance of Singleton class and use it.

 public class SingletonClient {

 public static void main(String[] args) {

 Singleton singleton =  Singleton.getCurrentInstance();
 singleton.sayHello();
 }
}
 

contract:

cannot instatiate more than once.
getCurrentInstance() return only one instance at any point of time