This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
The JVM needs some memory to do its work, and how much it uses is unspecified (and differs from one JVM version to the next). It depends on the implementation of the JVM. So your question can't be answered with a simple number.