Defining muliple hash functions for the same object

Question

I have several Sets that store the objects of the same class, but I want to specify a different identity function for each of them (i.e. say, in one set A==B if A.x==B.x, while in another A==B if A.y==B.y).

Currently I use TreeSets with different Comparators defined for each. I am wondering how the same thing can be done if I want to switch to HashSets. Java does not allow passing a separate hash function in the same way it allows Comparators for Sorted/Tree-based Collections. The only way that I can think of doing it would involve creating a different wrapper class and implementing the hashCode() method in each for the elements of each HashSet. Is there a better way of doing this?

Answer 1

How about creating separate subclasses for each set. The only difference for each subclass would be the overridden hash function that meets your criteria.

Answer 2

I'm not aware of a better way to do it. Your proposed solution (wrapper classes with different comparison logic) sounds very reasonable.

You mentioned implementing the hashCode method - don't forget to implement equals as well.

Answer 3

There is another option: You can copy the source for HashMap and replace the method hash(Object key) with something else (for example, a call to a Hasher that works like the Comparator of the TreeMap).

Answer 4

You could try using THashSet in GNU Trove, This support multiple hashing strategies.