Comments on: How to write a Singleton? (2/5)

By: Sumit

Sumit — Tue, 04 Aug 2009 22:59:22 +0000

@ PPOW: I thought Josh Bloch’s suggestion was not

enum Singleton { INSTANCE }

but

private static Singleton createInstance() {
return EnumSet.of ( new Singleton() );
}

By: ppow

ppow — Tue, 04 Aug 2009 18:42:57 +0000

It is simpler, but you lose the advantage of having control over the way the singleton is accessed. With a get method it will be easier to incorporate future changes – what if you change your mind and want to have a lazy initialization? Maybe want to add logging on every get / count the numbers of gets? Maybe at some point you will want to drop the singleton quality and just give new objects every time someone request one? Without a access method you do not have those possibilities…

By: Kees

Kees — Tue, 04 Aug 2009 16:09:29 +0000

Simpler is to have the INSTANCE public:

public class Singleton {
public static final Singleton INSTANCE = new Singleton();

// Remember that Singleton constructor should be private
private Singleton() {}
}

By: ppow

ppow — Tue, 28 Jul 2009 17:21:10 +0000

Yes, I know Bloch’s opinion about that but I do not agree with him because of several things:
- with enum your singleton cannot extend any other class (you can only implement interfaces)
- your singleton object is exactly of the enum class that contains it (you cannot separate the class that gives you access to singleton with the singleton class)
- your singleton is of type Enum and because of that it has many useless and confusing methods (oridinal(), name()…)
- you do not control access to the singleton by a get/access method (see the comments about benefits from method like that in Effective Java #1)
The only gain of doing this with enum is the serialization issue… but if you do need that I would strongly suggest dealing with readResolve() as it is more readable what kind of issue you are solving this way.

By: Dave Macpherson

Dave Macpherson — Tue, 28 Jul 2009 14:53:36 +0000

The enum implementation certainly is the simplest, and if your singleton is also serializable, then unless you provide a readResolve() implementation, it’s also the only way to guarantee that it forever remains a singleton (see Item #3 of Bloch’s Effective Java).

By: Dimitris Andreou

Dimitris Andreou — Thu, 23 Jul 2009 02:38:43 +0000

>I show today how to write it in a simplest and safest way possible

Actually, this is the simplest and safest way possible:

enum Singleton { INSTANCE }