I think this is the common understanding of what 'abstraction' and 'generality' mean in the over-engineering sense, and I think it's wrong. You make things mor

I think this is the common understanding of what 'abstraction' and 'generality' mean in the over-engineering sense, and I think it's wrong. You make things more general/abstract/future-proof by removing things, not adding things. ShoppingCart is a collection. We have libraries and libraries of code which work over collections. So List<Item> is better. Once you start hacking on the various ShoppingCart-related methods a bit more, you'll probably realise you don't depend on Item. Many of those methods should start operating on List<A> instead. And then chances are they'll be pass-through methods that just forward the behaviour to some List implementation, at which point you can delete the passthrough methods, and directly call the List methods directly.