How to solve violation of Finite Bound Restriction in Kotlin? [duplicate]

Question

Assume that I have this declaration in Java, it's okay.

abstract class Start<T extends End> {
    public T end;
}

abstract class End<T extends Start> {
    public T start;
}

However, it's not okay in Kotlin, since Kotlin has restriction for "cyclic" type parameter.

abstract class Start<T : End<*>> {
    lateinit var end: T
}

abstract class End<T : Start<*>> {
    lateinit var start: T
}

Is there any approach to solve this in Kotlin, so that I can have generic types that depend on each other?

Answer 1

It is impossible to use just one type parameter. Introducing Self type, which is natively supported in some other languages, is necessary. However, in kotlin you will have to introduce the Self type by yourself, because JetBrains officially turned down the request of adding self type.

abstract class Start<Self: Start<Self, T>, T: End<T, Self>> {
    lateinit var end: T
}

abstract class End<Self: End<Self, T>, T: Start<T, Self>> {
    lateinit var start: T
}

PS: This Self may later induce tediously long type. Proceed with caution.