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.
Yeah, more information would be good. Creating an instance of a subclass from the super class sounds very odd to me, and it couples your classes. The situation Paul Clapham mentioned (a factory method) is the only obvious reason for it, to my mind. Maybe there is a different approach for what you are trying to do that's a little cleaner?