To test your code you're often be given testing code and you may sometimes develop your own tests. When dode tests individual methods in your class, these tests are called unit tests and so you need to use the standard JUnit unit-testing library with the appropriate Java class files to test your classes.
To choose Run as JUnit test first use the Run As option in the Run menu as shonw on the left below. You have to select the JUnit option as shown on the right below. Most of you will have that as the only option, I have two "run as" options on my machine.
You may often be given several testing methods in the JUnit test code provided as part of an assignment. Each testing method is preceeded by the annotation @Test as you can see in testing code provided as part of both the DNA assignment and the Boggle assignment
If the JUnit tests pass, you'll get all green as shown on the left
below. That screenshot was taken using the testing class
provided as part the DNA assignment with
Otherwise you'll get red -- on the right below -- and an indication of
the first test to fail. Fix that, go on to more tests. The red was
obtained from a an implemrentation of the DNA assignment
LinkStrand that had nothing but code-stubs generated by
Eclipse (with a constructor added).