hey! I have an Interface MyInt and its implementation MyIntImp. I'm trying to define a custom annota...

# getting-started

[nadi]

hey!
I have an Interface MyInt and its implementation MyIntImp. I'm trying to define a custom annotation(\@MyAnnotation) which accepts one argument of type Class<MyInt>.
When I'm trying to use the annotation like this \@MyAnnotation(MyIntImp::class it gives me an error that it expects Class<MyInt> and not Class<MyIntImp>

what am i missing here?

[Joffrey]

Class<T> is invariant in its type parameter, so by default Class<MyIntImp> is not a subtype of Class<MyInt> even if MyIntImp is a subtype of MyInt.

You probably want Class<out MyInt> (with the out modifier) in your annotation declaration, not just Class<MyInt>. You can learn more about generics and variance here:
https://kotlinlang.org/docs/generics.html

[nadi]

opsy, right

[nadi]

10x buddy

[nadi]

hey again @Joffrey!
will it work if MyInt is parameterzied? MyInt<T>

[Joffrey]

You will have the same problem transitively 🙂 Does MyInt have an in or out modifier on its own type parameter? Because that is what decides whether the subtyping relationship between MyInt<T> and MyInt<U> depends on the subtyping relationship between T and U.

[Joffrey]

Oh wait, but if we stick to the specific Class<T> example, you might have an issue. Class<MyInt<A>> doesn't really carry any meaning in itself, because the Class instance of a generic class/interface doesn't care about the type parameter.