A quick-n-dirty example of Mockito using @Mock to automatically inject dependencies and @InjectMocks to fill in the class under test automatically.

Mockito docs at http://mockito.org/

There's CoolService, which calls java.util.Random to get a random number. And then there's CoolServiceTest which injects a mock for java.util.Random so you can return whatever specific number you want, to make CoolService actually testable.

Note that in this case I declared CoolService.random as package-local so I could override it, you could do the same thing with constructor dependencies or with get/set methods if you wanted to. Lots of ways to skin these cats.