Java: Retrieving an element from a HashSet

Question

Hoping someone can explain why I cannot retrieve an element from a HashSet.

Consider my HashSet containing a list of MyHashObjects with their hashCode() and equals() methods overridden correctly.

What I was hoping to do was to construct a MyHashObject myself, and set the relevant hash code properties to certain values. I can query the HashSet to see if there "equivalent" objects in the set using the contains() method. So even though contains() returns true for the 2 objects, they may not be == true.

How come then there is no get() method similar to how the contains() works?

Interested to know the thinking behind this API decision

Answer 1

If you know what element you want to retrieve, then you already have the element. The only question for a Set to answer, given an element, is whether it contains() it or not.

If you want to iterator over the elements, just use a Set.iterator().

It sounds like what you're trying to do is designate a canonical element for an equivalence class of elements. You can use a Map<MyObject,MyObject> to do this. See this SO question or this one for a discussion.

If you are really determined to find an element that .equals() your original element with the constraint that you MUST use the HashSet, I think you're stuck with iterating over it and checking equals() yourself. The API doesn't let you grab something by its hash code. So you could do:

MyObject findIfPresent(MyObject source, HashSet<MyObject> set) {    if (set.contains(source)) {       for (MyObject obj : set) {         if (obj.equals(source))            return obj;       }     }    return null; } 

Brute force and O(n) ugly, but if that's what you need to do...