Re: final fields of an enum that refer to

Joshua Cranmer <Pidgeot18@verizon.invalid>
Mon, 25 Aug 2008 08:47:51 -0400
Andreas Leitgeb wrote:

 enum Thing {
   T1, T2;
   final Thing mate;

What could I do, to make T2 "final"ly the mate of T1
and vice versa ?

Strictly speaking, having each store a final Object reference to each
other is impossible, since a final variable is only set at construction,
and an object has to be created before it can be referred to. [1]

That does not preclude some workable alternatives. Possibilities that
would still work:

1. Make the mate a String. Because of the way enums work, you get the
same effect if you do |Thing.valueOf(mate)|.

2. If you drop finality, you can still initialize in the constructor.

enum Thing {
   T1, T2(T1);

   private Thing() { this(null); }
   private Thing(Thing mate) {
     this.mate = mate;
     assert mate.mate = null;
     mate.mate = this;

   private Thing mate;

An enum initialization allows you to reference any variable declared to
the left of the current one but not anything to the right, so |T1(T2),
T2;| is impossible.

3. A less agreeable one, IMO. You can use some form of static-time
initialization, either manually setting the variables (ew?) or using a
map of some sort.

4. As you mentioned, you can also do abstract methods, but that seems
rather burdensome for such a feature.

[1] Well, if you let the |this| pointer escape initialization in a
controlled manner, and you have proper factory setup, it could be done:

public class Widget {
   private Map<String, Widget> knownWidgets;

   private final Widget(String qualifier, String mate) {
     knownWidgets.put(qualifier, this);
     this.mate = makeWidget(mate);

   private final Widget mate;

   public static Mate makeWidget(String qualifier) {
     if (knownWidgets.get(qualifier))
       return knownWidgets.get(qualifier);
     String mate = /* determine this somehow */;
     return new Widget(qualifier, mate);

That doesn't look too pleasant, is incomplete, and also violates the
dictum of not letting the |this| pointer escape the constructor. On the
other hand, it does show that it is possible in theory, but I think such
a setup would be rather fragile in actuality.

Such a model, furthermore, is difficult or probably even impossible to
emulate for enums, since they are the first things initialized in the
static initializer, and I think that either the compiler or the runtime
environment would complain if you started trying to play around with
them in the necessary clever ways.
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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