Why doesn't Map extend Collection?
Reson 1)
Collection assume elements of one value. Map assumes entries of key/value pairs. They could have been engineered to re-use the same common interface however some methods they implement are incompatible e.g.
Collection.remove(Object) - removes an element.
Map.remove(Object) - removes by key, not by entry.
You could model a Map as a collection of entries, which is what Map.entrySet() does.
There are some methods in common; size(), isEmpty(), clear(), putAll/addAll() but these are unlikely to have much value as a stand alone interface. (Again Map.entrySet() can be used instead)
Reson 2)
This was by design. We feel that mappings are not collections and collections are not mappings. Thus, it makes little sense for Map to extend the Collection interface (or vice versa).
