Java Developers Almanac 1.4
Includes code examples from book organized by package.
Java in a Nutshell
Comprehensive API and language reference for the full Java 2 Platform. From O'Reilly.
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).
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.
The Java Class Libraries: Volume 2
Comprehensive API reference to java.applet, java.awt, and java.beans. From Addison-Wesley.