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

From:
"Mike Schilling" <mscottschilling@hotmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 27 Aug 2008 17:19:20 -0700
Message-ID:
<%7mtk.25637$Ri.18514@flpi146.ffdc.sbc.com>
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.

e.g.:
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
exist.)

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,
Mates>();

    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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