Both are generic classes and require the argument to implement interface COMPARABLE (which is true about STRING and INTEGER in Eiffel. In addition they offer you search capabilities that may be useful.
Returning VOID from feature
Result variable is VOID by default for all complex types (try declaring MY_MAP[STRING, STRING] and test what is returned by get feature
Result is set to 0 for all numerical types (e.g. INTEGER)
If Result is complex (e.g. LIST or SET) and you want to per