Re: final field of an enum to refer to later-named instance.

"Mike Schilling" <>
Wed, 27 Aug 2008 17:19:20 -0700
Andreas Leitgeb wrote:

[This is a supercede, just to correct a broken Subject]

I've got an enum that contains Things that come in pairs.

I'd like each Thing to have a *final* reference to
it's mate, that in half of the cases appears later
in the list of Thing-instances.

enum Thing {
  T1, T2;
  final Thing mate;

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

I know some alternatives, myself, like just making the
reference non-final (and initialize it in the enum's
static {...} block) or defining an abstract method in
the enum like "abstract Thing getMate();" and implement
it for each instance to return its mate...

Please only followup, if you know a trick for final fields,
or if you know that it is strictly impossible that way.

Not having played with enums, I don't know what games you can play with
their constructors, but this kind of thing will work for a "normal" class.
(The trick is to construct the mate if and only if it doesn't already

import java.util.Map;
import java.util.HashMap;

public class Mates
    public final Mates myMate;
    private final int value;

    private static Map<Integer, Mates> instances = new HashMap<Integer,

    public Mates(int i)
        value = i;
        instances.put(i, this);
        int mateValue = (i % 2 == 0) ? i + 1 : i - 1;
        if (instances.get(mateValue) == null)
            new Mates(mateValue);
        myMate = instances.get(mateValue);

    public String toString()
        return Integer.valueOf(value).toString();

    public static void main(String[] args)
        Mates[] arr = new Mates[10];
        for (int i = 0; i < 10; i++)
            arr[i] = new Mates(i);

        for (int i = 0; i < 10; i++)
            System.out.println(arr[i] + " is the mate of " + arr[i].myMate);

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