Is it allowed for a

public class

to implement an

internal interface

?

That is not an error, it's a warning. It's saying that the

MyClass

is never used elsewhere. Either use it somewhere or add

@Suppress("UNUSED")

on top of the class. Note: the warning can be there for a good reason, suppressing it only makes sense for libraries and you do not want to add test for that class.

The screenshot referred to my original question:

MyClass

should be underscored squiggly red, as it is a

public class

which extends an

internal interface

.

internal

just means it's not visible outside the current module/project, if it's in the same module/project it's compile fine I would say (it might fail if you return/expose the interface as a type in some method)

I would think that you should not be able even to create such class, as a

public

class would then be able to expose an

internal

interface to outside code

That's the whole point, you do not expose the interface (it's members) outside your module. https://discuss.kotlinlang.org/t/what-is-the-purpose-of-an-internal-interface/25384/12

Ah, so the methods of an

internal

interface are still publicly visible...