What will happen if there are multiple threads accessing the Singleton class ?

We will make our code synchronized (using synchronized method)

public static synchronized Singleton getInstance() {

   if (singleton == null) {

     singleton = new Singleton();  

   }

   return singleton;

}

We can optimize above code using synchronized block

public static Singleton getInstance() {

   if (singleton == null)   {

     synchronized (Singleton.class) {

          singleton = new Singleton();

     }

   }

  return singleton;

}