I think you need to go back to the manual and understand WHEN and WHY you would want to use a mock. In short, you use a mock to simulate what a collaborator would do. It is appropriate to mock a collaborator if setting it up is expensive or too involved or if the real collaborator crosses system boundaries to do its work, such as access the file system or hit a web service or a database. With something like a Map, there is hardly ever any good reason to mock that. A Map resides in memory, so why don't you just use a real one instead of mocking it?