(1) Missing contains(Object) function. Without this function, any ArrayList function like retainAll(...) did not work properly: the research in the collection was made with a bad key. KeyExtractor was not used.
(2) Result of a research (function get(...)) must return a copy of the result, since the result is an ArrayList, value of the HashMap used for research. If a copy is not returned, any modification (and particularly remove(...)) can be made on the value of the index (HashMap) used for the research.