As testing is essential, testability is an important design consideration. Our designs must also support testing and debugging because a class that merely appears to work is of no value. A class that has evidence that it works is much more valuable.

Ideally, we'd like a hierarchy of testing. At the foundation is unit testing. Here, we test each class or function in isolation to be sure that it meets the contractual obligations of the API. Each class or function is a single unit under test. Above this comes integration testing. Once we know that each class and function works individually, we can test groups and clusters of classes. We can test whole modules and whole packages, too. After the integration tests work, we can look at the automated testing of the complete application.

This is not an exhaustive list of the types of tests. We can do performance testing or security vulnerability testing too. We'll focus, however, on automated unit testing because...