Inheritance and Composition

The question

A Car is permanently a Vehicle; that is not a role it plays sometimes. But a Car's pricing rule, logging destination and fuel strategy are all things that could vary independently, and none of them should be in its type.

Why inheritance multiplies

Suppose notifications vary by channel — email, SMS, push — and by urgency — normal, batched, immediate. With inheritance you need a class per combination: EmailNormal, EmailBatched, EmailImmediate, SmsNormal, and so on. Three channels and three urgencies is nine classes, and a fourth channel makes twelve.

With composition, a Notifier holds a Channel and a DeliveryStrategy. Three plus three is six classes, a fourth channel makes seven, and any combination is a constructor call rather than a new class.

Inheritance is the tightest coupling there is

A subclass can see its parent's protected state and depends on the order in which the parent calls its own methods. Change a parent's internal call order — a refactor that looks entirely private — and a subclass that overrode one of those methods can break, silently, without either file looking wrong.

Polymorphism without a hierarchy

Polymorphism — calling the same method and getting different behaviour — is the genuinely valuable part, and it does not require inheritance. An interface gives you exactly that with none of the coupling, which is why most modern designs have wide, shallow interface hierarchies rather than deep class trees.