Since it's rare that everything works first time, it's often necessary to develop iteratively, adding progressively more functionality each time. Secondly, sometimes it's necessary to find out what's going on under the covers when trying to fix a bug, particularly if it's hard to track down exceptions such as NullPointerException.

Fortunately, Eclipse comes with excellent debugging support, which can be used for debugging both standalone Java applications as well as Eclipse plug-ins.