Why does `Foo : Bar<Quax>` require `Quax` to...

# getting-started

[ursus]

Why does Foo : Bar<Quax> require Quax to be public, i.e. internal not allowed?

[Dominaezzz]

I think it's because inheritance is public.

[Casey Brooks]

Is Foo public? I would guess that the type parameter needs to have the same level of access as the class itself.

[ursus]

Yes foo is public

[ursus]

Inheritance is public? I dont get what you mean

[Dominaezzz]

In C++, you can have private inheritance. Where only the class know it has implemented an interface.

[ursus]

I dont think thats the issue here, issue is the visibility of the generic param, and also that its different from java

[Dominaezzz]

Inheritance in Kotlin/Java is public. So anything that has access to Foo will see that it inherits Bar. Since Bar has a generic parameter, the parameter would have to be visible to anything that has access to Foo. So if Foo is public, anything it exposes has to be public.
If something has access to Foo, it won't know what Quax is, since it's internal.
I hope this makes sense.

[ursus]

I dont think you are correct, because package private Quax works in java

[Dominaezzz]

And Foo can be used outside the package?

[ursus]

yep

[ursus]

works np

public class Rah extends Meh<Ohe> {
}

class Ohe {
}

public abstract class Meh<T> {
}

[Dominaezzz]

Yup. Sounds like I'm wrong. Still weird to see. Perhaps Kotlin handles generics differently or something.

[ursus]

yea idk; thanks anyways

[ursus]

although, by the same logic private Ohe should work too, but doesnt, something is fishy

[thanksforallthefish]

@Dominaezzz you are actually right, Roman explains the reason for the choice in Kotlin (Java behaves differently) here https://discuss.kotlinlang.org/t/why-does-kotlin-prohibit-exposing-restricted-visibility-types/7047/6

[thanksforallthefish]

though, truth be told, it looks a bit inconsistent to me

Does not compile:

abstract class Bar<T>
internal class Quax
class Foo : Bar<Quax>

compiles:

interface Bar<T> //visibility here does not matter, it always compiles
internal class Quax
class Foo : Bar<Quax>

[ursus]

huh really? weird

[corneil]

When you extend a class you need access to the internals when you implement interface there are no internals you need access to. The compiler isn't going to wait for your extended class to access something it can't have access to before complaining so it complains when you try to extend in a way that may cause problems.