Josh Bloch:
In one sense, return values should have the opposite behavior of input parameters: It’s best to return the most specific applicable collection interface rather than the most general. For example, if you’re sure that you’ll always return a SortedMap, you should give the relevant method the return type of SortedMap rather than Map. SortedMap instances are more time-consuming to build than ordinary Map instances and are also more powerful. Given that your module has already invested the time to build a SortedMap, it makes good sense to give the user access to its increased power. Furthermore, the user will be able to pass the returned object to methods that demand a SortedMap, as well as those that accept any Map.
Method parameter types should be as general as possible, and method return types should be as specific as possible.