The Java Cookbook: Solutions and Examples for Java Developers - Book by Ian F. Darwin provides solutions to many problems that Java-developers need to solve regularily. The authors site provides errata, download of all code samples and further information about Java. - http://javacook.darwinsys.com/
The Design Patterns Java Companion - Free online book by James W. Cooper. Also it can be download in PDF format. Examples. - http://www.patterndepot.com/put/8/JavaPatterns.htm
Sun's Java Tutorial - On-line version of book from Addison-Wesley. - http://java.sun.com/docs/books/tutorial/