Unit testing is when you (a programmer) write test code to verify units of code.
The size of a unit isn’t precisely defined, so we’ll view a unit as a small bit of
code that exhibits some useful behavior in your system. A unit on its own
usually doesn’t represent complete end-to-end behavior. It instead represents
some small subset of that end-to-end-behavior.
We’re coding in Java, so we write our unit tests in Java, too. We run these
unit tests through JUnit, a tool that marks our tests as passing or failing.
