How can I get a file.class from a directory(for example desktop) and use it as an Object?

Hi I want to be able to run a JunitTest Class that is not inside my eclipse project,but in a directory , thats is what I mean

        Class<?> clazz = How can I get a file.class("my/dir");
        Result result =  JUnitCore.runClasses(clazz);
        for (Failure failure : result.getFailures()) {
                 System.out.println(failure.toString());
              }
              System.out.println(result.wasSuccessful());
           }