Write a singleton class in Java

Singleton class means you can create only one object for the given class. You can create a singleton class by making its constructor as private, so that you can restrict the creation of the object. Provide a static method to get instance of the object, wherein you can handle the object creation inside the class only. In this example we are creating object by using static block.

package com.java2novice.algos;
 
public class MySingleton {
 
    private static MySingleton myObj;
     
    static{
        myObj = new MySingleton();
    }
     
    private MySingleton(){
     
    }
     
    public static MySingleton getInstance(){
        return myObj;
    }
     
    public void testMe(){
        System.out.println("Hey.... it is working!!!");
    }
     
    public static void main(String a[]){
        MySingleton ms = getInstance();
        ms.testMe();
    }
}

Implementing Singleton class with getInstance() method

// Java program implementing Singleton class
// with getInstance() method
class Singleton
{
    // static variable single_instance of type Singleton
    private static Singleton single_instance = null;
 
    // variable of type String
    public String s;
 
    // private constructor restricted to this class itself
    private Singleton()
    {
        s = "Hello I am a string part of Singleton class";
    }
 
    // static method to create instance of Singleton class
    public static Singleton getInstance()
    {
        if (single_instance == null)
            single_instance = new Singleton();
 
        return single_instance;
    }
}
 
// Driver Class
class Main
{
    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();
 
        // changing variable of instance x
        x.s = (x.s).toUpperCase();
 
        System.out.println("String from x is " + x.s);
        System.out.println("String from y is " + y.s);
        System.out.println("String from z is " + z.s);
        System.out.println("\n");
 
        // changing variable of instance z
        z.s = (z.s).toLowerCase();
 
        System.out.println("String from x is " + x.s);
        System.out.println("String from y is " + y.s);
        System.out.println("String from z is " + z.s);
    }
}

Output:

String from x is HELLO I AM A STRING PART OF SINGLETON CLASS
String from y is HELLO I AM A STRING PART OF SINGLETON CLASS
String from z is HELLO I AM A STRING PART OF SINGLETON CLASS


String from x is hello i am a string part of singleton class
String from y is hello i am a string part of singleton class
String from z is hello i am a string part of singleton class