When I’m talking to programmers about writing tests for their own code, one of the questions that comes up often is: Should we test classes in isolation from each other, or in collaboration with each other?
I like both kinds of tests. Here’s why.
I like tests that isolate classes. When a failure occurs, the tests tell me specifically what class failed, and what method failed. That guides me more directly to the fault—the specific code that is broken—and saves a ton of debugging.
I like tests that exercise collaborations. When a failure occurs, the tests tell me that:
- one class or the other is not fulfilling its responsibilities, or
- the collaborators disagree about each other's responsibilities, or
- some other class (the "electrician" class that connects the collaborators with each other) has wired the collaborators together improperly.
When I test classes in isolation, failures guide me quickly to faults.
When I test classes in collaboration, failures tell me where the classes disagree about each other’s responsibilities.
So I write both kinds of tests.