Refactoring

Refactoring means changing the structure of the code by rearranging its internal representation without modifying its external behavior.

One example would be if you identify a class that has lots of responsibilities and very long methods, and then decide to change it by using smaller methods, creating new internal collaborators, and distributing responsibilities into new, smaller objects. As you do that, you're careful not to change the original interface of that class, keep all its public methods as before, and not change any signature. To an external observer of that class, it might look like nothing happened (but we know otherwise).

Refactoring is a critical activity in software maintenance, yet something that can't be done (at least not correctly) without having unit tests. This is because, as each change gets made, we need to know that our code is still correct. In a sense, you can think of our unit tests as the "external observer" for...