Eclipse

Eclipse is an incredibly powerful and extensible Java development environment available at http://www.eclipse.org. Most of the tools made available by Eclipse are in the form of plugins that are bundled with Eclipse, and many more plugins are available on the web. DOC, too, is written as an Eclipse plugin, so it is helpful to understand how Eclipse plugins works.

I won't attempt to explain the inner workings of Eclipse--there simply isn't enough space or time--but I will recommend a book that I've found extremely valuable in understanding Eclipse. The book is "The Java Developer's Guide to Eclipse," by Shavor et al. Unfortunately, Eclipse is constantly updated, so some material in the book may be out of date, but it is still worth having nearby. In addition, the Eclipse javadoc documentation is well-written and very helpful.