1. Right click on the file that you wish to test -> Tools -> Create JUnit Tests (shortcut key: Ctrl + shift +U)
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3. Modify the class however you want it. I recommend reading the following JUnit tutorial. You may refer to my SudokuTest.java to see how I implemented it.
4. Once you're done, right click on the test file -> Test File (shortcut key: Ctrl + F6) or right click on the file that you're testing -> Test File. The difference between the two methods is that the latter will run all test files associated with the file being tested.