Hey guys I’m stuck with a kotlin generics problem:...

# announcements

[Jay]

Hey guys I’m stuck with a kotlin generics problem: How to inherit from same generic interface twice with two different types?

interface A<T> {}

class B: A<Apple> {}

class MyClass: B, A<Orange> {}

Kotlin gave me an error: Type parameter T of ‘A’ has inconsistent values: Apple, Orange

[diesieben07]

Simple answer: you can't.

[hho]

And it's not just a Kotlin generics problem, it's a JVM generics problem due to type erasure – https://en.wikipedia.org/wiki/Generics_in_Java#Problems_with_type_erasure

[louiscad]

@Jay Because of type erasure, you can only implement an interface once with the same type parameter

[Jay]

got it, Thx!

[marstran]

I thought you could do something like class MyClass<T> : B, A<T> where T : Apple, T : Orange if Apple and Orange were interfaces, but it gives an error. Anyone know what the problem is with that?

[marstran]

The error is Type parameter T of 'A' has inconsistent values: Apple, T#1 (type parameter of MyClass). Why can't it infer that T#1 is an Apple?

[petersommerhoff]

@marstran Because you want it to be an A<Apple> and an A<Apple and Orange> at the same time which is not possible due to type erasure (of course in this case it's just redundant so you can just get rid of B)