The Java language's core API includes numerous built-in libraries.
There are certainly many useful classes in it, but they still don't do everything for you!
To get the most out of Java, you will need to work with open-source projects at some point.
Wicked Cool Java describes some of these projects that can help you program faster,
or better, or do interesting and unusual things.
This website is the companion site for the book, and has errata, code samples, and links to projects mentioned in the text. There is a forum here too, for discussing subjects related to the book, and for organizing new open source projects. I hope that the book will encourage you to look at more than just the Java core APIs, to experiment with open source tools, to continue learning new techniques, and explore new ideas for open source projects.