There are many mocking frameworks for Java classes like PowerMock, JMock but I personally like Mockito for its simple API, great documentation, and a lot of examples.
Mocking is one of the essential techniques of modern-day unit testing, as it allows you to test your code in isolation without any dependency and that's why I encourage every Java developer to learn a mocking framework along with JUnit.
My favorite mocking framework is Mockito but if you like you can also explore PowerMock or JMock.
If you choose to stick with Mockito and decide to learn this framework then this JUnit and Mockito Crash Course on Udemy is a good course to start with.