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

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