Existing Java collection that supports custom criteria for uniqueness?

Question

I'd like to utilize a unique java collection that can accept a strategy for determining if member objects are "equal" on collection initialization.

The reason I need to do this is because the equals method of the class that I need to add to this collection is already implemented to satisfy other (more appropriate) functionality. In a specific case, the criteria for uniqueness in this collection instance needs to check only one variable of the class as opposed to a number of variables that are checked in the equals method. I would prefer to avoid decorating the objects as I am gathering them from disparate libraries and it would be costly to loop through for decoration (and it may muddy my code).

I realize this would not be a Set as it would break the Java contract for Set, but I just feel as though this problem must have been encountered previously. I figured Guava or Apache Collections would have provided something, but no luck it seems. Does anybody know of any available library that does provide this type of functionality? Should I be entertaining a different solution altogether?

WHy don't you just use a wrapper class and a classical `Set`? Or is that not an option? — fge, Jan 10 '13 at 17:10
Guava [rejected the idea explicitly](https://code.google.com/p/guava-libraries/issues/detail?id=576). — Louis Wasserman, Jan 10 '13 at 17:12
@fge A wrapper class for the member objects? I explained in question. (I may ultimately decide that is the way to go, nevertheless) — smp7d, Jan 10 '13 at 17:18
@smp7d I see nothing in the original question which prompts me to think that it cannot be done. Comparing apples and oranges for equality only if they have the same color, for instance, looks to me as what you want is a class which can compare colors -- this does not prevent the underlying apple and orange to exist. — fge, Jan 10 '13 at 17:21
@Louis hmm - that's pretty much the answer I just gave. I think in this case - where the implementation would be limited to the scope of the problem - there isn't too much scope for widespread confusion. Also good documentation will help! — Tom McIntyre, Jan 10 '13 at 17:26
@fge It certainly CAN be done. I just want to avoid it for efficiency purposes. I updated my wording in the question. — smp7d, Jan 10 '13 at 17:34

score 3 · Answer 1 · answered Jan 10 '13 at 17:10

3

Can you use a Custom Comparator and a TreeSet or TreeMap? Or use a Map where the Key has your criteria? A HashSet is just a wrapper for a HashMap so using a map instead should be much more expensive.

answered Jan 10 '13 at 17:10

Peter Lawrey

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I can use a custom Comparator. I didn't really consider that as I always thought of a Comparator as a means of ordering, which I don't need. Looking at the docs, I see that TreeSet breaks the Set contract. I would like to avoid using a Map as I conceptually have no need for Key-Value pairs. I may use a map internally if I decide to implement my own collection. – smp7d Jan 10 '13 at 19:55
You have a need to have a custom idea of what is unique about an object. This is the key. You also want to store the original object unaltered. This is the Value. There is a common tenancy to use Set or List with complex rules rather than use a simple Map. – Peter Lawrey Jan 11 '13 at 08:38

score 1 · Answer 2 · answered Jan 10 '13 at 17:16

That is not really practical. Consider for instance two instances of a class C which you consider equivalent.

Now you do:

set.add(c1);
set.remove(c2);

Should the set be empty after that? What about .retainAll(), .removeAll()?

Your best bet here is to create your own class which wraps over class C, deletages whatever is needed to be delegated, and have this wrapper class implement .hashCode() and .equals() (and possibly Comparable of itself too). With such a class, you can just go on and use classical sets and maps.

score 1 · Answer 3 · answered Jan 10 '13 at 17:23

Guava has an Equivalence, which lets you define whether two objects are equivalent.

It also has Equivalence.Wrapper which wraps arbitrary objects and delegates equals() and hashCode() to the implementations in the equivalence, rather than their own.

So you could do something like this:

public class MySet<T> implements Set<T> {

    private final Equivalence<T> equivalence;

    private final Set<Wrapper<T>> delegate = new HashSet<Wrapper<T>>();

    public MySet(Equivalence<T> equivalence) {
        this.equivalence = equivalence;
    }

    public boolean add(T t) {
        return delegate.add(equivalence.wrap(t));
    }

    // other Set methods

}

Yes, I can, and thank you for the implementation. But, does no library already provide an implementation like this? — smp7d, Jan 10 '13 at 17:38

Existing Java collection that supports custom criteria for uniqueness?

3 Answers3