Correct Singleton Class implemnetaion

This is a very basic design pattern question asked by the interviewer. There are multiple ways to implement Singleton class using Java.

Here is a correct way of implementation:

===
class Singleton {
 private volatile static Singleton _instance;//do not initialize here

private Singleton() { //mark it private to prevent Singleton object to be instantiated from outside

}

public static Singleton getInstance() {

 if (_instance == null) { 

  synchronized (Singleton.class) { 

   if (_instance == null) { 

    _instance = new Singleton();

    }

   }

  }

 return _instance;

}
===