One of the objectives for the Certified Java Developer exam states: "The examinee has to run your completed project against a test harness". What do they mean by this?
To make things easier for the person judging your project, Sun also delivers you a test harness, which basically runs your application and tests it to ensure that it completely fullfills the requirements.
You should send in your code back to the testing company only when the application pass the test harness without any errors. You can run your application against this test harness as many times as you want until it shows no more errors.