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:

boolean contains(Object o)

Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e)).

Is there a smart way to check if a collection contains an object o, but comparing by reference instead (i.e. o==e)?

Of course I can iterate through the collection and make the check, I'm looking for an existing function which can do that.

Clarifications:

• I want to perform this operation regardless the equals() implementation of the object in the collection.
• I don't want to change the objects in the collection nor the collection itself.

Edit:

Even though my question is about a general solution for Collection implementations, specific cases for Collection sub-interfaces would also be appreciated.

Answer 1

For those of us using Java 8, Collection#stream() is a clean option:

collection.stream().anyMatch(x -> x == key) 

Answer 2

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<X> 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.