Kotlin Generics: Trouble migrating from Java

Question

I'm new to Kotlin. When trying to convert a java application to Kotlin I ran into the following issue so posting this question.

This is a model-view-binder app and here are the java interfaces. Key thing is Model and View are interdependent on each other.

interface Model<T extends View<?>> {}

interface View<T extends Model<?>> {}

class RealModel implements Model<RealView> {}

class RealView implements View<RealModel> {}

class Binder<T extends Model<?>> {

     static <ModelT extends Model<?>> Binder<ModelT> of(View<ModelT> view) {}

}

// Application code
Binder<RealModel> binder = Binder.of(new RealView());
binder.bind(new RealModel());

Can I do this in Kotlin?

I tried the following

interface Model<T : View<*>> {}

ERROR: This type parameter violates the Finite Bound Restriction

interface Model<T : View<Any>> {}

ERROR: Type argument is not within its bounds: should be subtype of 'Model<*>'

interface Model<T : View<Model<*>>> {}

ERROR: This type parameter violates the Finite Bound Restriction

Answer 1

You can't convert it 1:1 in Kotlin since Kotlin restricts the cyclic type parameters.

You can solve it adding a new type parameter which other languages supports natively but Kotlin doesn't, which is the self type parameter (identified with S in my example) and making the self type parameter and the other type parameter as covariant types (using out ):

interface Model<out S : Model<S, T>, out T : View<T, S>>

interface View<out S : View<S, T>, out T : Model<T, S>>

class RealModel : Model<RealModel, RealView>

class RealView : View<RealView, RealModel>

class Binder<T : Model<*, *>> {
    companion object {
        fun <ViewT : View<*, ModelT>, ModelT : Model<ViewT, *>> of(view: ViewT): Binder<ModelT> {
            throw RuntimeException()
        }
    }
}