Maps should do whatever ArrayList does when your index is out of bounds. And so long as null is a value that could be in the collection, lookup should definitel

Maps should do whatever ArrayList does when your index is out of bounds. And so long as null is a value that could be in the collection, lookup should definitely not return null. If a lookup fails, it is often a bug or a violation of a constraint that you shouldn't be handling. Throw an exception. If a lookup is expected to fail in normal execution, you can use an Optional to mash together a check and lookup in a typesafe way. Lookup failures should not be allowed to propagate as nulls only to be caught as NullPointerExceptions in disparate parts of the code.