You could fill pages on the subject of abstraction. While on the subject it would've been apt for the Author to quote or give a nod to the one of the pioneers i

You could fill pages on the subject of abstraction. While on the subject it would've been apt for the Author to quote or give a nod to the one of the pioneers in this field : Barbara Liskov, and the Liskov substitution principle. Not that I've anything against Joel Spolsky - but I'd read Liskov any day over Joel's. So easy to get wrapped up in all the syntactic sugar.