Java Singleton Design Pattern : Questions

Question

I had an interview recently and he asked me about Singleton Design Patterns about how are they implemented and I told him that using static variables and static methods we can i

Answer 1

There are a few ways to implement a Singleton pattern in Java:

// private constructor, public static instance
// usage: Blah.INSTANCE.someMethod();
public class Blah {
    public static final Blah INSTANCE = new Blah();
    private Blah() {
    }
    // public methods
}

// private constructor, public instance method
// usage: Woo.getInstance().someMethod();
public class Woo {
    private static final Woo INSTANCE = new Woo();
    private Woo() {
    }
    public static Woo getInstance() {
        return INSTANCE;
    }
    // public methods
}

// Java5+ single element enumeration (preferred approach)
// usage: Zing.INSTANCE.someMethod();
public enum Zing {
    INSTANCE;
    // public methods
}

Given the examples above, you will have a single instance per classloader.

Regarding using a singleton in a cluster...I'm not sure what the definition of "using" is...is the interviewer implying that a single instance is created across the cluster? I'm not sure if that makes a whole lot of sense...?

Lastly, defining a non-singleton object in spring is done simply via the attribute singleton="false".