A map whose values exist in a list. Great initial demo, has actual use in many projects. Similar to Apache Commons but not exactly the same.

* Map<KeyType, List<KeyType>>
* put(key, value)
* get returns a list of KeyType objects
* throw exception on null key
* answers size (# of keys)
* answers total # of values (across all keys)
* answers isEmpty

Can add nuances such as disallows duplicate values, allows or disallows null values.