Private constructor and instances - Multiple choice

Question

I am trying to figure out the answer to the following MC question. I have tried looking for an answer on google but people seem to have different answers for this problem. Can someone please explain their answer.

public class Gingleton {
    private static Gingleton INSTANCE = null;

    public static Gingleton getInstance()
    {
        if ( INSTANCE == null )
        {
            INSTANCE = new Gingleton();
        }
        return INSTANCE;
    }

    private Gingleton() {
    }
}

• More than one instance of Gingleton can be created (My choice)

• A Gingleton will never be created

• The constructor is private and can't be called

• value can be garbage collected, and the call to getInstance may return garbage data

Answer 1

New instance creation in getInstance() is not synchronized in any way, so it IS possible that more than one instance will be created in mulithreaded environment. To ensure only one instance you should do:

public class Gingleton {

    // volatile
    private static volatile Gingleton INSTANCE = null;

    public static Gingleton getInstance()
    {
        if ( INSTANCE == null )
        {
            synchronized (Gingleton.class) {  // Synchronized
                if ( INSTANCE == null )
                {
                    INSTANCE = new Gingleton();
                }
            }
        }
        return INSTANCE;
    }

    private Gingleton() {
    }
}