The JIndex plugin enables jEdit to show the JavaDoc API html documentation on the currently selected word, with a single key press, in a browser of your choice. You can index the class, method and field names of all the libraries that you frequently use, including classes.zip, rt.jar and swingall.jar. After you have created an index, you can execute the 'JIndex on current word' command and JIndex will search in the index for the current word, that the cursor is on. If an entry is found, it will display the appropiate JavaDoc URL in your browser. If there are more than one entries on this word, you will be presented a choose dialog.
The index creation and other options may be controlled through a dialog under "File -> PlugIn Options". For help, see "Help Contents -> Plugins -> JIndex".