This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
The Graphics object that is passed to the paint methods is an instance of some unknown sub class of Graphics. Usually it's also an instance of an unknown subclass of Graphics2D. That's all you need to know; you can use the methods defined in Graphics (and Graphics2D after casting) without having to worry about how and where they are implemented.