Question

I understand that only one instance of any object according to .equals() is allowed in a Set and that you shouldn’t “need to” get an object from the Set if you already have an equivalent object, but I would still like to have a .get() method that returns the actual instance of the object in the Set (or null) given an equivalent object as a parameter.

Any ideas/theories as to why it was designed like this?

I usually have to hack around this by using a Map and making the key and the value same, or something like that.

EDIT: I don’t think people understand my question so far. I want the exact object instance that is already in the set, not a possibly different object instance where .equals() returns true.

As to why I would want this behavior, typically .equals() does not take into account all the properties of the object. I want to provide some dummy lookup object and get back the actual object instance in the Set.

Answer 1

While the purity argument does make the method get(Object) suspect, the underlying intent is not moot.

There are various class and interface families that slightly redefine equals(Object). One need look no further than the collections interfaces. For example, an ArrayList and a LinkedList can be equal; their respective contents merely need to be the same and in the same order.

Consequently, there are very good reasons for finding the matching element in a set. Perhaps a clearer way of indicating intent is to have a method like

public interface Collection<E> extends ... {
  ...
  public E findMatch(Object o) throws UnsupportedOperationException;
  ...
}

Note that this API has value broader that within Set.

As to the question itself, I don’t have any theory as to why such an operation was omitted. I will say that the minimal spanning set argument does not hold, because many operations defined in the collections APIs are motivated by convenience and efficiency.