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.
I added 2 additional public and 2 additional private methods to my implementing class. No problems with that at all. I would NOT add these method declarations to the supplied interface, though. Leave that untouched. Hope this helps.