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.
If you uncomment that line (//0) there is no implementation of the method in class Bang. So BigBang would have to implement it. But BigBang does not, so BigBang has to be declared abstract. But then we have a contradiction: BigBang is final and cannot be declared abstract! The final prevents BigBang being extended and method f from being provided with an implementation.