Question

Java defines a Set interface where contains() is defined as following:

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

The Collection interface defines contains() as following:

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)).

I need a Java ‘instance set’ where contains() is based on == and not equals(). In other words, a set of hard instances where two different objects A and B where A.equals(B) could coexist in this same set, since A!=B.

Is such an ‘instance set’ delivered in Java or in some public library? I can’t find anything, but may be someone knows better on SO. If not, I’ll implement it. Thanks.

Answer 1

There is no direct “instance set” in the JRE.

But there is an IdentityHashMap, which implements a “instance map” according to your terminology.

And there is a method called Collections.newSetFromMap() which can create a Set from an arbitrary Map implementation.

So you can easily build your own instance set like this:

Set<MyType> instanceSet = Collections.newSetFromMap(new IdentityHashMap<MyType,Boolean>());