Test Classes in Isolation and in Collaboration

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.

If the individual classes are well tested, I can focus my collaboration testing specifically on wiring and agreements. And if the individual classes are tested well, collaboration test failures tell me about disagreements and improper wiring.

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.