What is the difference between these two approaches of creating singleton Class ? //Double Checked Locking Code public static Singleton createInstance() { if(singleton == null){ synchronized(Singleton.class) { if(singleton == null) { singleton = new Singleton(); } } } return singleton; } //Single checked locking code public static Singleton createInstance() { synchronized(Singleton.class) { if(singleton == null) { singleton = new Singleton(); } } return singleton; }

Interview Questions and Answers

Q1. What is the difference between these two approaches of creating singleton Class ?

//Double Checked Locking Code
public static Singleton createInstance() {
   if(singleton == null){
      synchronized(Singleton.class) {
         if(singleton == null) {
            singleton = new Singleton();
         }
      }
   }
   return singleton;
}

//Single checked locking code
public static Singleton createInstance() {
   synchronized(Singleton.class) {
      if(singleton == null) {
         singleton = new Singleton();
      }
   }
   return singleton;
}

Design

Ans. In First Case , Lock for the synchronized block will be received only if singleton == null whereas in second case every thread will acquire the lock before executing the code.
The problem of synchronization with singleton will only happen when the object has not be instantiated. Once instantiated , the check singleton == null will always generate true and the same object will be returned and hence no problem. First condition will make sure that synchronized access ( acquiring locks ) will only take place if the object has not been created so far.

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