Question

If you have a Java Singleton that looks like this:

public class MySingleton {

private static MySingleton instance;

private int member;

public static MySingleton getInstance(){
    if(instance==null){
        instance = new MySingleton();
    }
    return instance;
}

private MySingleton(){
    //empty private constructor
}

public int getMemberA(){
    return member;
}

public int getMemberB(){
    return instance.member;
}

}

…is there a difference between getMemberA and getMemberB? That is, is there a difference between accessing the member with instance.xxx and just xxx?

Note: I am aware of the pros and cons of using the Singleton pattern!

Answer 1

Yes, there’s a difference.

Your singleton implementation isn’t currently threadsafe, which means it’s possible to call getMemberB() on an instance other than the one referred to by instance, at which point you’ll get a different result.

If your implementation were thread-safe (so genuinely only one instance could ever be created) then they’d be equivalent, and the simpler form would be much preferred.