Sun has online reference documentation for the Java API that you may find useful in conjunction with this quick reference handbook. Visit http://www.javasoft.com/ to view or download this API documentation and other useful documents.
There are many other sites on the Web that contain useful Java information. One of the most well-known is http://www.gamelan.com/, also known as http://java.developer.com/. For discussion (in English) about Java, try the various comp.lang.java.* newsgroups.