Mocking & Unit Testing- Why check that something was called only once?

Question

I know that many mocking libraries let the programmer check that a method was called only once. But why is that useful?

Also, why is it useful to verify the parameters to a mock's method?

Answer 1

In general, the answers to both of those questions are the same.

It's useful if the requirements of the unit/method you're testing specify that that behavior is required. If that behavior is required, then that's what you need to verify is actually happening.

If it's important to ensure that a particular method is only called once, then you can do that. If it doesn't matter that a method is called more than once, then don't test for it explicitly. Note that the default for the Mockito "verify" method is "times(1)", which means that it confirms that the method was called once and only once.

Concerning the second question, you might want to verify the parameters if it's entirely possible the method could be called with different parameters, and you don't want to count those occurrences, you only care about a specific set of parameter values.

Answer 2

When doing unit tests you are testing a isolated method: all other methods are supposed to work correctly, and you test only that your method behaves in the expected (specified...) way.

But in many occasions the expected way implies calling methods of classes you depend on (via dependency injection, if you want to do unit testing).

For these reason you need to assure that this calls are really done... and of course they are called with the expected parameters.

Example:

In your real application you have a repository class that stores all your changes in the database (and does only this!). But for unit test your "business" class (where all your business rules are defined), you should mock that "repository" class: then you must check that this mocked class receives the correct updated data.