What is the Java equivalent of .NET's IEquatable Interface?

Question

score 21 · Accepted Answer · answered Jun 10 '10 at 06:27

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Unfortunately I don't believe there is one - which is a pain in terms of providing hash maps etc with custom equality comparisons :(

Obviously there's Comparable<T> as an equivalent to IComparable<T> and Comparator<T> for IComparer<T>, but I don't believe there's any equivalent of IEqualityComparer<T> and IEquatable<T>.

There may be third party libraries providing the same sort of interface and maps which use them of course...

answered Jun 10 '10 at 06:27

Jon Skeet

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It wouldn't be that much of a stretch to create a delegating decorator which implements `Map` and your own `Equatable` (*is that even a word?*) interface though. – Esko Jul 12 '10 at 11:12
@Esko: Do you mean making the underlying map contain proxied keys, effectively? It's doable, but it would be ugly... and I suspect it would create issues when you tried to do weak maps etc. It would have been much nicer if it had been in there to start with. – Jon Skeet Jul 12 '10 at 13:40
Why not that way too. Proxying through `WeakReference` shouldn't be that much of an extra work, either. I do see your point though, C# has lots of these things readily implemented while Java likes to be the "barebones" language. I guess Google Collections may have something for this through its Predicate/Function system but I'm not sure... – Esko Jul 12 '10 at 14:44
@Esko: I don't believe I've seen any Google collections with the `IEqualityComparer` interface equivalent, which is really what's required for the most general purpose solution. (It's easy to build a comparer based on an equatable type.) – Jon Skeet Jul 12 '10 at 14:49
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The lack of an Equatable interface in the BCL is one of Java's many frustrating flaws - it essentially requires that you implement (or at least imply that you implement) a total order for a class even when it may not make any sense just to allow a type to be used as a key in a hashmap. – LBushkin Nov 30 '13 at 18:29

score 9 · Answer 2 · edited Jan 02 '12 at 10:37

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If you are just looking for that interface to implement the equals() method, you can override the object.equals() method directly.

edited Jan 02 '12 at 10:37

sra

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answered Jul 12 '10 at 11:06

Ahmed Mostafa Abdel-Baky

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a good override implementation of `equals()` can be found here: http://stackoverflow.com/questions/185937/overriding-the-java-equals-method-quirk – Julien Feb 21 '13 at 09:15

score 2 · Answer 3 · answered Dec 23 '11 at 18:52

The IEquatable<T> interface is primarily useful with generic classes that store many instances of what may be unboxed structure types; it's marginally useful with sealed types, and worse than useless with unsealed types. There's thus really wouldn't be any point to implementing IEquatable<T> in Java.

score 1 · Answer 4 · answered Oct 31 '14 at 19:38

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There's an equivalent type in Guava called Equivalence<T>.

answered Oct 31 '14 at 19:38

Craig Gidney

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What is the Java equivalent of .NET's IEquatable Interface?

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