Edit: see also the follow up article on how to Test log4j with JUnit if you’re interested in specifically testing log output.
Just occasionally, it can be useful to verify output to System.out in a unit test. For example, if you’re testing application logging or if you’re using log output to sense some other behaviour. It can be tricky to properly mock behaviour of System.out but fortunately, it is possible to test System.out with JUnit.