Then a Map would be a ReadableMap, but a ReadableMap wouldn't be a Map. If a method expects a Map, you would't be able to put a ReadableMap (or a WriteableMap)

Then a Map would be a ReadableMap, but a ReadableMap wouldn't be a Map. If a method expects a Map, you would't be able to put a ReadableMap (or a WriteableMap) into it. My solution so far is to avoid to return a map or to expect a map as argument.