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.
You may not access a variable before it's initialized. Member variables get initialized to a default value automatically, if you don't assign a value to them. Local variables do not, you always have to assign a value to them explicitly.