How to Create a Singlton Class in Java by Example?

Singleton: Exactly one instance of a class. Define a class that has only one instance and provide a global point of access of it.

Create a class singleton is very important in real time project. The Singleton's purpose is to control object creation, limiting the number of objects to one only. If we are creating a object of a class again and again , that will definitely increase the memory and finally our application will get slow.

For Example: If we have restriction that we can create only one instance of a database connection. In this case we will create only one object of database and used everywhere.

Implementation of Singleton class:

Singleton.java

package com.java.deepak.singleton;

public class Singleton
{
    private static Singleton instance;

    private Singleton()
    {
    }

    public synchronized static Singleton getInstance()
    {
        if ( instance == null )
        {
            instance = new Singleton();
            System.out.println( "New Object Created...!!!" );
        }
        else
        {
            System.out.println( "Object Already in Memory..." );
        }
        return instance;
    }
}

• We will make constructor as private. So than we can not create an object outside of a class.
  
  
• Create a private method to create a object of a class. By using this method, we will create an object outside of class.

SingletonClient.java

package com.java.deepak.singleton;

public class SingletonClient
{
    public static void main( String[] args )
    {
        Singleton s1 = Singleton.getInstance();
        Singleton s2 = Singleton.getInstance();
    }
}

Here we are calling singleton class two times.

Output:



In output, we can see that when we call method first time then new object will create but when we call second time then existing object will return.