Mock, Stub, Fake




Some authors[1] draw a distinction between fake and mock objects. Fakes are the simpler of the two, simply implementing the same interface as the object that they represent and returning pre-arranged responses. Thus a fake object merely provides a set of method stubs.

In the book “The Art of Unit Testing”[2] mocks are described as a fake object that helps decide if a test failed or passed, by verifying if an interaction on an object occurred or not. Everything else is defined as a stub. In that book, “Fakes” are anything that is not real. Based on their usage, they are either stubs or mocks.

Mock objects in this sense do a little more: their method implementations contain assertions of their own. This means that a true mock, in this sense, will examine the context of each call— perhaps checking the order in which its methods are called, perhaps performing tests on the data passed into the method calls as arguments.