Why not allow an external interface to provide hashCode/equals for a HashMap?

Question

With a TreeMap it\'s trivial to provide a custom Comparator, thus overriding the semantics provided by Comparable objects added to the

Answer 1

A bit late for you, but for future visitors, it might be worth knowing that commons-collections has an AbstractHashedMap (in 3.2.2 and with generics in 4.0). You can override these protected methods to achieve your desired behaviour:

protected int hash(Object key) { ... }
protected boolean isEqualKey(Object key1, Object key2) { ... }
protected boolean isEqualValue(Object value1, Object value2) { ... }
protected HashEntry createEntry(
    HashEntry next, int hashCode, Object key, Object value) { ... }

An example implementation of such an alternative HashedMap is commons-collections' own IdentityMap (only up to 3.2.2 as Java has its own since 1.4).

This is not as powerful as providing an external "Hasharator" to a Map instance. You have to implement a new map class for every hashing strategy (composition vs. inheritance striking back...). But it's still good to know.