Testing is an essential skill for any Java developer, particularly unit testing, integration testing, and automation testing. At the bare minimum, every Java developer should be familiar with JUnit and Mockito, two of the most popular Unit testing and Mock library.

If you know these two and know how to use them to effectively create a unit test, you will be a much better Java developer than without them.

There are more advanced libraries that also exist like Cucumber for Business-driven testing, Robot Framework for integration testing, but there is no substitute for JUnit, you will always need that.

When it comes to a mocking library, you have a couple of choices like PowerMock, Mockito, and EasyMock. Still, I strongly suggest you learn Mockito because it’s a vast library, and also many Java developers and companies are doing that. It is slowly becoming the standard library for creating mock objects in Java.

If you want to learn JUnit and Mockito from scratch, then I also suggest you join Learn Java Unit Testing with Junit & Mockito in the 30 Steps course by In28Minutes on Udemy. It’s a practical and hands-on course to get started with both these libraries.

If you need more options than you I have already shared a lot of resources like books and courses, you can check those to learn more.