Java Enum Singleton

Question

Building on what has been written in SO question Best Singleton Implementation In Java and specifically talking about using enum to create a singleton, what are the differences/pros/cons between (constructor left out)

public enum Elvis {
    INSTANCE;
    private int age;


    public int getAge() {
        return age;
    }
}

and then called Elvis.INSTANCE.getAge()

and

public enum Elvis {
    INSTANCE;
    private int age;


    public static int getAge() {
        return INSTANCE.age;
    }
}

and called Elvis.getAge()

Answer 1

Suppose you're binding to something which will use the properties of any object it's given - you can pass Elvis.INSTANCE very easily, but you can't pass Elvis.class and expect it to find the property (unless it's deliberately coded to find static properties of classes).

Basically you only use the singleton pattern when you want an instance. If static methods work okay for you, then just use those and don't bother with the enum.

Answer 2

(Stateful) Singletons are generally used to pretend not to be using static variables. If you don't actually use the publicly static variable then you will fool less people.

Answer 3

A great advantage is when your singleton must implements an interface. Following your example:

public enum Elvis implements HasAge {
    INSTANCE;
    private int age;

    @Override
    public int getAge() {
        return age;
    }
}

With:

public interface HasAge {
    public int getAge();
}

It can't be done with statics...

Answer 4

I would choose the option which is the simplest and clearest. This is somewhat subjective, but if you don't know what is clearest, just go for the shortest option.