Unit testing is the single most important thing that separates an average developer from a good developer. Programmers often are given excuses for not writing unit tests, but the most common excuse for avoiding unit testing is lack of experience and knowledge of popular unit testing libraries, including JUnit, Mockito, and PowerMock.
I always have a goal to improve my knowledge of unit testing and integration testing libraries, like JUnit 5, Cucumber, Robot framework, and a few others.
If you want to learn JUnit and Mockito and you need a resource, I highly recommend Learn Java Unit Testing with Junit & Mockito in 30 Steps by Ranga Karnam on Udemy. It’s a great, hands-on course to learn these useful libraries.