Question

I have implemented the Singleton class as below:

public class Singleton {

    private static  Singleton instance = null;


    private Singleton() { 
    }

private synchronized static void createInstance() {
    instance = new Singletone();
}


    public static Singleton getInstance() {
        if(instance == null){
            createInstance();
        }
        return instance;
    }

}

But I want to know if it is a correct implementation of a singleton.
Are there any problem in multithreaded environment.

Answer 1

Your implementation is almost correct. The problem is that it is not thread-safe. 2 separate threads may enter getInstance() simultaneously, check that instance is null and then create 2 instances of your class. Here is the fix:

public static synchronized Singletone getInstance() {
    if(instance == null){
        createInstance();
    }
    return instance;
} 

Please pay attention on word synchronized.