This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
That link is about the Rhino JavaScript engine, which is written in Java, and not to my knowledge used in any browser. I assume you're talking about JavaScript in the context of a browser. In that case you're looking for a technology called LiveConnect, which enables two-way communication between Java applets and JavaScript within a web page. Unfortunately, LiveConnect is not supported by all browsers. Here is a test page you can use to check your browser.