Intro to Hamcrest Matchers
First things first, I should quickly explain what a Hamcrest Matcher is. When conducting unit tests, the built-in assertion types that come with the testing framework are generally pretty limited. They make it very easy for a person to end up with multiple asserts to essentially check one thing. Even if it doesn't contain multiple asserts, those asserts aren't the most fluent to read and don't tell exactly what you're checking.
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That's where Hamcrest Matchers come in (and other assertion libraries, but we're looking at Hamcrest right now). They allow you to define your own more robust and more fluent assertions, essentially. For example, if you were testing whether a method correctly returns an empty String, that test might look something like this: