Question

I have a basic understanding of mock and fake objects, but I’m not sure I have a feeling about when/where to use mocking – especially as it would apply to this scenario here.

Answer 1

A unit test should test a single codepath through a single method. When the execution of a method passes outside of that method, into another object, and back again, you have a dependency.

When you test that code path with the actual dependency, you are not unit testing; you are integration testing. While that’s good and necessary, it isn’t unit testing.

If your dependency is buggy, your test may be affected in such a way to return a false positive. For instance, you may pass the dependency an unexpected null, and the dependency may not throw on null as it is documented to do. Your test does not encounter a null argument exception as it should have, and the test passes.

Also, you may find its hard, if not impossible, to reliably get the dependent object to return exactly what you want during a test. That also includes throwing expected exceptions within tests.

A mock replaces that dependency. You set expectations on calls to the dependent object, set the exact return values it should give you to perform the test you want, and/or what exceptions to throw so that you can test your exception handling code. In this way you can test the unit in question easily.

TL;DR: Mock every dependency your unit test touches.