Why don't sub classes have access to nested interfaces in Kotlin?

Question

In Java, I can define an interface nested inside MainClass, and I can still reference that interface via any SubClass, like this:

public class MainClass {
  interface MyInterface {
    public void printName();
  }
}

public class SubClass extends MainClass {
}

// Notice here that I use `SubClass.MyInterface` so the client doesn't know MainClass exists.
public void handleInterface(SubClass.MyInterface implementation) {
}

However, if I try to do the same in Kotlin, it does not work:

open class MainClass {
  interface MyInterface {
    fun printName()
  }
}

class SubClass : MainClass()

// This will not compile unless I do `MainClass.MyInterface`. See the Java notes about why I might want that.
fun handleInterface(implementation: SubClass.MyInterface) {
}

I have already had the debate that we don't need SubClass.MyInterface and can just reference it from the base class where it was defined, but I really am curious why Kotlin doesn't support this but Java seems to allow it.

The fact that you can reference `SubClass.Interface` in Java is considered to have been a bug (similar to being able to access static members via a reference variable), but for backward-compatibility it wasn't banned. — chrylis -cautiouslyoptimistic-, Mar 18 '19 at 20:39
@chrylis Neat! You should post that as an answer. Do you happen to recall any YouTrack or GitHub issue that called that out explicitly? — AdamMc331, Mar 18 '19 at 21:04
I don't have a citation to the subtype thing, but see https://stackoverflow.com/a/34709162/1189885 — chrylis -cautiouslyoptimistic-, Mar 18 '19 at 23:12
@chrylis do you have any pointers to why it's considered a bug? — Ray Tayek, Mar 19 '19 at 00:30
Because in both cases it's not *really* referring to that thing--it's accidentally referring to it indirectly. In particular, code changes could break the Java examples (by introducing a real `SubClass.MyInterface`, for example). — chrylis -cautiouslyoptimistic-, Mar 19 '19 at 00:52

Why don't sub classes have access to nested interfaces in Kotlin?

0 Answers0