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.
As far as I know you are not allowed to change the given interface. But you can of course create your own interface which extends the DBMain interface and define a few new methods. That's just fine. I would definitely not change the given interface.