Minor Description
MultiKeyMap as designed can only accept keys of the same type, if made to support generics, i.e.
MultiKeyMap<MultiKey<String>,Integer> map = ...;
map.put("foo", "bar", 10);
// not possible
//MultiKeyMap<MultiKey<String,Integer>,Double> map = ...;
Attached is a proposal that may support multi-key maps with keys of different types. I do not believe varying numbers of typed keys can be supported in a single interface, thus this proposal includes
BinaryKey<K1,K2>
BinaryKeyMap<K1,K2,V> extends Map<BinaryKey<K1,K2>,V>
TertiaryKey<K1,K2,K3>
TertiaryKeyMap<K1,K2,K3,V> extends Map<TertiaryKey<K1,K2,K3>,V>
QuaternaryKey<K1,K2,K3,K4>
QuaternaryKeyMap<K1,K2,K3,K4,V> extends Map<QuaternaryKey<K1,K2,K3,K4>,V>
To prevent unnecessary object creation, I also propose to support MapIterator-style iterators:
BinaryKeyMapIterator<K1,K2,V> extends Iterator<V>
TertiaryKeyMapIterator<K1,K2,K3,V> extends Iterator<V>
QuaternaryKeyMapIterator<K1,K2,K3,K4,V> extends Iterator<V>
Current multi-map MapIterator support is implemented as Iterator<K1> but I find that Iterator<V> makes more sense here. It is not possible to support MapIterator<K,V> without object creation, e.g. MapIterator<BinaryKey<K1,K2>,V> must create BinaryKey objects for each call to next().