Divide by Zero

Final

Another evening another article. Tonight I want to talk about the rarely used but extremely useful final keyword in many C-like languages.

The final keyword guarantees that the given object or method cannot be extended. I remember when I first started programming I would always use the protected keyword for methods just in case someone wanted to modify it through inheritance but over the years I’ve learnt that not understanding how your code should be modified means you don’t have a clear design. So let’s start there then move in to final.

By using protected it obviously means you allow overriding in children. Initially I misunderstood the difference between allowing and expecting. Rather, you should only use the protected keyword for things you expect to be overriden and use private everywhere else. If your design necessitates that a child class calls a protected method on the parent then you should rethink your design. You would likely be better served by calling an abstract method in the parent class that the child class then implements. Why? Because it allows you to refactor the internals of your objects safely without fear of breaking things outside of a very narrow scope. Realistically, protected should only be used in abstract classes.

The final keyword when it comes to classes forces composition over inheritance. By having a flat hierarchy you get the benefit of creating new objects that take in other objects when you want to extend behavior. See my post on SOLID to get a clearer view of the open/closed principle. It also naturally moves your design towards dependency inversion since you’ll need to start injecting behaviour in order to reuse it.

Finally, it clarifies intent. By saying a class is either abstract or final, it means other developers know exactly what they should and shouldn’t extend.

Written by Matthew Hotchen on