Caching hashes in Java collections?

Question

When I implement a collection that uses hashes for optimizing access, should I cache the hash values or assume an efficient implementation of hashCode()?

On

Answer 1

On the other hand, when I implement a class that overrides hashcode(), should I assume that the collection (i.e. HashSet) caches the hash?

No, you should not make any assumptions beyond the scope of the class you are writing.

Of course you should try to make your hashCode cheap. If it isn't, and your class is immutable, create the hashCode on initialization or lazily upon the first request (see java.lang.String). If your class is not immutable, I don't see any other option than to re-calculate the hashCode every time.