By running a doclet on the JDK source code, you can put the resulting Vim help files in your help path and read Javadoc from directly within Vim. :help String and :help java.util.List bring up the javadoc for those files, within Vim. Everything's hyperlinked and the entire javadoc is present.
If you set keywordprg to blank, you can do a K on any Java keyword and bring up the javadoc quickly. This has really cut down on the amount of time I spend outside Vim while coding.
Go to vimdoclet.sourceforge.net to download the doclet and see what the output looks like. There is a sample download of javadoc for java.lang and java.util if you'd like to try it out before running the doclet.