Re: enum singleton

Mark Space <>
Thu, 30 Apr 2009 08:17:07 -0700
Thufir wrote:

would this be better done as an enum singleton?

package guestbook;

import javax.jdo.JDOHelper;
import javax.jdo.PersistenceManagerFactory;

public final class PMF {
    private static final PersistenceManagerFactory pmfInstance =

    private PMF() {}

    public static PersistenceManagerFactory get() {
        return pmfInstance;

I'd say "no." I tried using enums as classes in a little personal
project, and I didn't like the result. You can't add abstract classes
to different types of enums, you can't subclass an enum type (often
desirable) and in general enums are just fiddly to work with. Putting
an interface on several enum types is not a substitute for having a
first-class class to work with.

Just like Roedy's question a few weeks back about reading/writing enums
to/from a file as strings (I think that was it), it's almost always
easier, and better, to roll your own enum-like type and not have to put
up with the inherent restrictions the language places on enums.

If it's 100% clear that a class must be an enum, use an enum. If you're
not sure, look hard at rolling your own.

In short, the implementation above is fine. I'd leave it exactly like
it is.

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