Java Developers Almanac 1.4 - Includes code examples from book organized by package. - http://www.exampledepot.com/
Java Precisely - A concise quick reference to the Java 2 Platform, available for free in PDF and Postscript format (including printable A5 format, and hyperlinked PDF). - http://www.dina.kvl.dk/~sestoft/javaprecisely/
The Java Class Libraries: Volume 2 - Comprehensive API reference to java.applet, java.awt, and java.beans. From Addison-Wesley. - http://java.sun.com/docs/books/chanlee/second_edition/
The Java Class Libraries: Volume 1 - Comprehensive API reference to java.io, java.lang, java.math, java.net, java.reflect, java.security, java.text, and java.util. From Addison-Wesley. - http://java.sun.com/docs/books/chanlee/second_edition/vol1/