Singleton pattern

import java.util.*;

class Singleton{

    private static Singleton obj;

    private Singleton(){

    }
    public static Singleton getSingleton(){
        if(obj == null){

            synchronized(Singleton.class){
                if(obj == null){
                    // in case not already created by another thread
                    obj = new Singleton();
                }

            }

        }
        return obj;
    }
}


/*
 * 1. private constructor so no new object creation
 * 2. obj creation using static method
 * 3. ob ject is static
 *
 * 4. Thread safety -> synchronized block instead of synchronized method
 * to sync only required code
 */

public class Main{
    public static void main(String args[]) throws Exception{
        // singleton design pattern -> only one object is used
        Singleton obj1 = Singleton.getSingleton();
        System.out.println(obj1.hashCode());

        Singleton obj2 = Singleton.getSingleton();
        System.out.println(obj2.hashCode());

        // same hashCode means same object
    }
}