If you're worried about the J2ME section of the Sun Certified Java Associate Exam (SCJA), I'd say you're going way too in depth if you're exploring the packages. The exam will ping you with three or four questions where J2ME is either the right or wrong answer. But don't worry too much about the APIs and the core packages. You won't be asked to troubleshoot any J2ME code snippets. Concentrate on your J2SE basic, and you will rule on the coding section!