What is Singleton class?

The Singleton pattern is a creational design pattern that ensures that only one instance of a class can be created and that the same instance is used every time the object is requested.

class Singleton {

    // Static variable reference of single_instance

    // of type Singleton

    private static Singleton single_instance = null;

    // Declaring a variable of type String

    public String s;

    // Constructor

    // Here we will be creating private constructor

    // restricted to this class itself

    private Singleton()

    {

        s = "Hello I am a string part of Singleton class";

    }

 // Static method

    // Static method to create instance of Singleton class

    public static synchronized Singleton getInstance()

    {

        if (single_instance == null)

            single_instance = new Singleton();

             return single_instance;

    }

}

 

// Class 2

// Main class

class TestingSinglTon {

    // Main driver method

    public static void main(String args[])

    {

        // Instantiating Singleton class with variable x

        Singleton x = Singleton.getInstance();

        // Instantiating Singleton class with variable y

        Singleton y = Singleton.getInstance();

        // Instantiating Singleton class with variable z

        Singleton z = Singleton.getInstance();

        // Printing the hash code for above variable as

        // declared

        System.out.println("Hashcode of x is " + x.hashCode());

        System.out.println("Hashcode of y is "+ y.hashCode());

        System.out.println("Hashcode of z is "+ z.hashCode());

        // Condition check

        if (x == y && y == z) {

            // Print statement

            System.out.println(

                "Three objects point to the same memory location on the heap i.e, to the same object");

        } else {

            // Print statement

            System.out.println(

                "Three objects DO NOT point to the same memory location on the heap");

        }

    }

}

Output:

====================================

Hashcode of x is 558638686

Hashcode of y is 558638686

Hashcode of z is 558638686

Three objects point to the same memory location on the heap i.e, to the same object