How do you unit test a class that is dependent on many other classes?

Question

I have been hearing that with unit testing we can catch most of the bugs in the code and I really believe that this is true. But my question is in large projects where each class is dependent on many other classes how do you go about unit testing the class ? Stubbing out every other class doesn't make much sense because of both complexity and the time required to write the stubs. What is your opinion about it?

Answer 1

Part of the advantage of using tests, is that it forces you to minimize dependencies thereby creating testable code. By minimizing dependencies you will increase maintainability and reusability of your code, both highly desirable traits.

Since you are introducing tests into an existing code-base, you will no doubt encounter many hard-to-test situations which will require refactoring to test properly. This refactoring will increase the testability of your code, decreasing dependencies at the same time.

The reason why its hard to retrofit code with tests is why many advocate following Test-Driven-Development. If you write your tests first and then write the code to pass the tests, your code will by default be much more testable and decoupled.

Answer 2

Use a mocking framework to stub your classes for you. Mocking frameworks (I use Rhino Mocks for C#/.NET) make it pretty easy to stub out your dependencies. Used with dependency injection, your classes can be easily decoupled from other classes and don't take much work to make them so. Note that sometimes it does become easier to "fake" something rather than mock. I do end up faking a few classes, but usually these are pretty easy to write -- they just contain "good" or "bad" data and return it. No logic involved.