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

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Use an interface if you want to define a contract. I.e. X must take Y and return Z. It doesn’t care how the code is doing that. A class can implement multiple interfaces.

Use an abstract class if you want to define default behaviour in non-abstract methods so that the endusers can reuse it without rewriting it again and again. A class can extend from only one other class. An abstract class with only abstract methods can be as good definied as an interface. An abstract class without any abstract method is recognizeable as the Template Method pattern (see this answer for some real world examples).

An abstract class in turn can perfectly implement an interface whenever you want to provide the enduser freedom in defining the default behaviour.

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