Check if a collection contains an object, comparing by reference

Question

The Collection.contains() method check if a collection contains a given object, using the .equals() method to perform the comparison.

From Java7 Javadoc

Answer 1

There is some kind of workaround...

You can use an IdentityHashMap, with Void as values (or whatever else -- your choice). You'd then use contains() on its .keySet() to check the presence of an element (or .containsKey() on the map directly).

A second workaround would be to use Guava and Equivalence.identity(); however your Collection will have to have elements of type Equivalence.Wrapper and you'd have to wrap before checking...

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Curiously enough, the JDK does not provide an IdentityHashSet. This is rather strange considering that the internal implementation of HashSet uses a HashMap, so one has to wonder why they didn't do the same for IdentityHashMap...

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Side note: the documentation of Collection is misleading; not all Collection implementations rely on .equals(). See, for instance, SortedSet or SortedMap. And, of course, IdentityHashMap.