> with a mock you have to explicitly specify "when called with this return that".

Riiiight, no I've always hated that Mockito way of doing things.

I find wrapping a hashmap avoids the need for explicit when-this-then-that bindings, because a hashmap already does the expected behaviour natively.

You can even 'integrate' your tests as deeply as you like, all standing on top of your poor hashmap. I.e. Http tests with unit test speed.

var controller = new Controller(new Service(new Repo(new HashMap()))) controller.POST(...); assert(controller.GET(...));