Using enums to implement utility classes and singletons

Question

Peter Lawrey writes about Two Uses of Enums that most people forget on his blog.

First of all, I hadn't forgotten - I hadn't even realised :)

These approaches are nice and concise - are there any benefits other than conciseness compared with the more traditional ways of achieving the same thing, such as using final classes with private constructors for utility classes?

Also, are there any issues (apart from confusing programmers who aren't expecting it)?

Answer 1

It would seem more intuitive to me to use enums for real enumerations.

Answer 2

I don't really agree with the first use of an enum from that post. If you want an uninstantiable utility class, just give it a private constructor. It's that simple, and the enum provides no added benefit in that situation that I see.

The use of enums for singletons in utility classes is great, but I would generally try to keep the fact that an enum is being used an internal implementation detail. See, for example, Guava's Predicates class which uses an enum to enforce a single instance of certain Predicates like alwaysTrue(). It does not expose the enum to users though.

As far as other benefits: yes, there are other benefits such as built-in serializability and absolutely enforcing a single instance of an enum constant per classloader, even when deserializing.