When to implement an interface and when to extend a superclass?

Question

I\'ve been reading a lot about interfaces and class inheritance in Java, and I know how to do both and I think I have a good feel for both. But it seems that nobody ever rea

Answer 1

I think that interfaces work best when you use them to express that the object has a certain property or behavior, that spans multiple inheritance trees, and is only clearly defined for each class.

For example think of Comparable. If you wanted to create a class Comparable to be extended by other classes, it would have to be very high on the inheritance tree, possible right after Object, and the property it expresses is that two objects of that type can be compared, but there's no way to define that generally (you can't have an implementation of compareTo directly in the Comparable class, it's different for every class that implements it).

Classes work best when they define something clear, you know what properties and behaviors they have, and have actual implementations for methods, that you want to pass down to the children.

So classes work when you need to define a concrete object like a human, or a car, and interfaces work better when you need more abstract behavior that's too general to belong to any inheritance tree, like the ability to be compared (Comparable) or to be run (Runnable).