Writing unit tests is an important part of the application development process. Unit tests help you catch bugs in your application at very early stages; they also help you catch any regression because of subsequent code changes. JUnit and Eclipse provide an easy way to integrate unit tests into your development workflow. Eclipse also creates a nice report in the JUnit view, which makes it easy to identify the failed tests and jump to the line in the code where the test failed.
Unit tests are meant to be executed without any external dependencies. Libraries such as Mockito help you to mock any external dependencies.
Use coverage tools such as JaCoCo to find out the quality of the unit tests that you have written. Coverage tools tell you the percentage of the application code that is covered by your unit tests. You can also see in each class which lines are covered by your unit tests and which are not. Such a report can help you to decide whether you need to write more unit test cases...