Why isn't it possible to pass an object of the type `ZusammenfassungABC` , even if it inherits from ...

# announcements

[Jakob K]

Why isn't it possible to pass an object of the type ZusammenfassungABC , even if it inherits from ZusammenfassungAC (line 22)?

Unbenannt

[Jakob K]

The warning message IntelliJ gives:

[diesieben07]

Generics are invariant by default. The docs explain this: https://kotlinlang.org/docs/reference/generics.html

[Zach Klippenstein (he/him) [MOD]]

Add the right out and in modifiers and it should work I think.

[Jakob K]

So, this is the "fix"?

Unbenannt

[Nir]

Yes though you want to make sure that the in and out modifiers are actually appropriate

[Nir]

Sometimes in Kotlin, given the way inheritance work, there's no alternative except to simply duplicate the code

[Tobias Berger]

the best fix would probably be this:

open class ErstTyp<out T : GrundTyp>

This out-projection allows the compiler to use instance with a more specific generic type to be used in places where a superclass would be expected as the generic parameter.

[Tobias Berger]

it basically means you can safely cast ErstTyp<T> to ErstTyp<U> if U inherits from T

[Tobias Berger]

And refering to your original question: you're not passing an object of type ZusammenfassungABC, but an object of type ErstTyp<ZusammenfassungABC>. That's an important difference.

[Jakob K]

Thank you for adding that!