[mcv]

I don't understand why he added all those exceptions to the abstract class. That's not what abstract classes are for. The initial code example shows an implementation class inheriting from the abstract class; that seems like the perfect place for exceptions. Why not simply use that most basic feature of OOP? Override the parts of your abstract class that you need to override.

[duderific]

I guess he wanted to make sure that the subclass code would be as simple as possible. So when he wanted to add a new entity to his model, as he mentioned, he only had to extend the abstract class, without writing any new business logic.

Clearly what you're saying is correct, however, it would mess up his original intent, which was frictionless extensibility.