[marcosdumay]

What do you use to implement interfaces then?

Generic interfaces are the one single feature that made OO popular for GUI programming; what by its turn is the one application that made OO popular. And it's still the most powerful of the OO concepts (what is shame really).

[uryga]

Are you referring to the fact that in Java interfaces are basically pure virtual classes? That always felt like an implementation detail to me. For example, both Go and Rust have interfaces without inheritance.

[marcosdumay]

I'm referring to a pattern on OO languages where you specify an interface using a superclass¹, with hooks that you can specialize on subclasses, reusing most of the code on the superclass and keeping the code around it generic.

Go interfaces have the part about keeping the code around it generic, but I don't think it gets the easy reuse of code by just writing the hooks. (But I can easily be wrong here - I'm not a proficient Go programmer.) In that, it looks very like Java's interfaces (the literal Interface thing).

1 - In Java you would not use a literal Interface for that, since you can't provide default code for the subclasses to extend.

[chillacy]

Sometimes (hard to say without a specific example) the superclass can be represented as its own class and included in each subclass as composition. This has several advantages, including less code per class and therefore smaller interface surface area, more explicit interface / information hiding, and easier testability.

[marcosdumay]

The most obvious examples would be the widget classes all GUI frameworks export. Java has some interesting classes for networking, like its AbstractHTTPServer, and Applet. Most OO languages have some variant of Thread or Runnable that is built this way too.

[dragonwriter]

With just interfaces you can produce a variation of the pattern you describe by using an interface that specifies the hooks, having the concrete types implement the hook interface, and using a proxy that implements the shared behavior by consuming the hook interface.