JUnit: Basic 
🏆 Can use simple JUnit tests
JUnit 4 with IntelliJ: A quick introduction -- by DrBFraser
- JUnit cookbook - a short tutorial from JUnit creators
- JUnit tutorial - a more detailed tutorial from a developer Lars Vogel
- How to test private methods in Java? [ short answer ] [ long answer ]
JUnit: Intermediate 
🏆 Can use intermediate features of JUnit
Some intermediate JUnit techniques that may be useful:
- It is possible for a JUnit test case to verify if the SUT throws the right exception.
- JUnit Rules are a way to add additional behavior to a test. e.g. to make a test case use a temporary folder for storing files needed for (or generated by) the test.
- It is possible to write methods thar are automatically run before/after a test method/class. These are useful to do pre/post cleanups for example.
- Testing private methods is possible, although not always necessray