What is a Singleton Class in Java ?

Singleton Class

    The Class which allows only to create a single object is known as singleton class in java. First time it will create an object and second time onward it will return the same object which is created earlier. This class object can be created only through a factory method, because it's constructor is private.

Example

package com.els.core;

import java.io.Serializable;

public class SingleObject implements Cloneable, Serializable

{

//create an object of SingleObject  

private static SingleObject instance =null;

//make the constructor private so that this class cannot be instantiated

private SingleObject()

{

//arguments

System.out.println("Singleton object created...");

}

//Get the only object available  

public static SingleObject getInstance() throws Exception

{

if(instance==null)

{

instance= new SingleObject();

}

return instance;

}

//non static method

public void showMessage()

{

System.out.println("Singleton Class!");

}

}

******

package com.els.core;

//creating object of singleton class

public class SingleTest

{

public static void main(String[] args)

{

try{

                        //first time creating object

SingleObject obj1 = SingleObject.getInstance();

                        //second time

SingleObject obj2 = SingleObject.getInstance();

System.out.println(obj1);

System.out.println(obj2);

                        obj1.showMessage();

}catch(Exception e){

e.printStackTrace();

}

}  

}

Compile both class and execute.

javac SingleObject 

javac SingleTest

java com.els.core.SingleTest

Output

Singleton object created...

com.els.core.SingleObject@f9f9d8

com.els.core.SingleObject@f9f9d8

Singleton Class!

Note : But the above class is not fully singleton class. We will discuss on next topic. Please share your feedback as comment.