enum and singletons - top level vs nested enum

Question

As is now well known the recommended way for creating a singleton in java is via an enum (see for instance here)

But (for example in this answer) it seems to be considered (by @MikeAdler who replied to me in the comments) the right thing to have the enum in the singleton class (see for instance here for a full example, or the code given below). I do not seem to really understand the need/use of this - can someone please elaborate (and preferably give the correct dialect for this idiom) ?

public class Enclosing {

    private  Enclosing() {}

    static enum Singleton {
        INSTANCE;

        private static final Enclosing  singleton = new Enclosing();

        public Enclosing getSingleton() {
            return singleton;
        }
    }
}

EDIT : one would get the singleton by Enclosing.Singleton.INSTANCE.getSingleton();

@DaveNewton : could you elaborate in an answer ? I remind you that enums are special classes - and that the singleton will be an instance of some class anyway. What does it mean a "singleton class" ? — Mr_and_Mrs_D, Oct 14 '12 at 14:32
If you need, or have, a specific class, and you want it to be a singleton, this is one of the easiest ways to implement that. I don't really need to be reminded what an enum is, but thanks. — Dave Newton, Oct 14 '12 at 14:36
@DaveNewton Oh don't get me wrong :) Say I was reminding myself :D - still I do not really get the difference - why not create the class like this and not as a top level enum ? — Mr_and_Mrs_D, Oct 14 '12 at 14:39
I told you--if you already have, or for some reason specifically need, a given class, and you don't want an enum. — Dave Newton, Oct 14 '12 at 14:42

score 2 · Accepted Answer · edited May 23 '17 at 12:04

2

You would nest a Singleton when you wanted to perform lazy-loading of it, say for testing reasons:

public class Singleton {
    public Enclosing getInstance() {
        return SingletonHolder.INSTANCE;
    }

    static enum SingletonHolder {
        INSTANCE;
    }
}

Read more about it here: http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom

edited May 23 '17 at 12:04

Community

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answered Oct 14 '12 at 14:33

Garrett Hall

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Makes sense - could you please elaborate on the mechanism ? As I see from your code the singleton would be created when the class `Singleton` is loaded for the first time while a top level enum would create the INSTANCE on application load ? – Mr_and_Mrs_D Oct 14 '12 at 14:36
Why is this better than using a plain old enum ? As I don't think it is. – NimChimpsky Oct 14 '12 at 14:39
@Nim : cause the enum singleton is only created when/if needed (I guess - not sure how the class loader works) – Mr_and_Mrs_D Oct 14 '12 at 14:41
Let's say SingletonHolder has static initialization that takes a long time. If I want to call other static methods on Singleton without going through initialization I can. This may be useful if I want to inject a different SingletonHolder INSTANCE for unit testing. – Garrett Hall Oct 14 '12 at 14:41
I don't see how static initializers are relevant. You can call static methods of enum without a new instance, and you can inject a different enum if you like. I don't get the advantage of your code at all. – NimChimpsky Oct 14 '12 at 14:45
An enum is lazy loaded anyway by the class loader. – NimChimpsky Oct 14 '12 at 15:44
@NimChimpsky : yes but it will be loaded as soon as any of its methods are called. Here you can call any of the static methods of `Singleton` and still not have the `SingletonHolder` initialized. Do read the linked wikipedia article (the How it works section) and please let me know if having objections. At Garrett Hall : I would appreciate some more concrete testing example - I do think I have to accept your answer though :) – Mr_and_Mrs_D Oct 14 '12 at 22:29
A very very nice explanation of the "Initialization-on-demand holder idiom" aka [Lazy Initialization Holder Class by Josh Bloch](http://youtu.be/pi_I7oD_uGI?t=39m34s). I'd still appreciate some testing code, showing the need for this pattern :) – Mr_and_Mrs_D Jun 01 '13 at 12:41

NimChimpsky · Answer 2 · 2012-10-14T14:31:13.807

1

public enum Foo {
   INSTANCE;
}

is the simplest and best way to get a singleton post-java 5. The code you posted is just unnecessarily complex, I don't see any advantage of it over just using an enum.

edited Oct 14 '12 at 14:31

answered Oct 14 '12 at 14:25

NimChimpsky

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Yes I know - iam wondering on the particular idiom of having this enum declared _nested_ – Mr_and_Mrs_D Oct 14 '12 at 14:30
@Mr_and_Mrs_D it just looks messy to me. – NimChimpsky Oct 14 '12 at 14:31
It may but it is used and probably has some uses - that's why I am asking :) – Mr_and_Mrs_D Oct 14 '12 at 14:33

enum and singletons - top level vs nested enum

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