How to create a Singleton Class in java

//  Example of Singleton Pattern in java //

package com.singletondemo;

class SingletonDB {

public static volatile SingletonDB getInstance;

private SingletonDB() {

   if (getInstance != null) {

               throw (new RuntimeException("instance access only 

                            createInstance()"));

    }

}

public static SingletonDB createInstance() {

if (getInstance == null) {

synchronized (SingletonDB.class) {

if (getInstance == null) {

getInstance = new SingletonDB();

}

}

}

return getInstance;

}

}

public class Main {

public static void main(String[] args) {

SingletonDB db = SingletonDB.createInstance();

System.out.println(db);

}

}