Composion with multiple delegation

Language Design
1

I found delegation very useful for modularization… but I want more.

I want that composed interface (CarPlane) is implemented by each delegation(CarImp, PlaneImpl)
so 1. composed implementation(CarPlaneImpl) needs to care only composition(fantastic) and 2. users doesn’t have to know about each interface(Car, Plane)

If composition(fantastic) part is an issue, then that can be extracted into another interface so CarPlane doesn’t have its own method(pure composition).

This is an example for explanation. What I actually have is a sole CarPlane interface with many methods which I want to split into several interfaces(Car, Plane).

interface Car {
    fun drive()
}

private class CarImpl(private val wheel: String): Car {
    override fun drive() {}
}

interface Plane {
    fun fly()
}

private class PlaneImpl(private val wing: String): Plane {
    override fun fly() {}
}

interface CarPlane : Plane, Car {
    fun fantastic()
}

// Suggestion
private class CarPlaneImpl1(private val c1: CarImpl, private val p1: PlaneImpl) : CarPlane by (c1, p1) {
    override fun fantastic() {
        // commnicate with delegations(c1 and p1)
    }
}
fun factory(private val wheel: String, private val wing: String) =
      CarPlaneImpl1(CarImpl(wheel), PlaneImpl(wing))

// Suggestion 2(no factory)
private class CarPlaneImpl2(private val wheel: String, private val wing: String) :
    CarPlane by (private val c1:CarImpl(wheel), private val p1:PlaneImpl(wing)) {
    override fun fantastic() {
        // communicate with delegations(c1 and p1)
    }
}
1 Like
2

It’s already possible. You just need to delegate each interface separately.

private class CarPlaneImpl1(private val c1: CarImpl, private val p1: PlaneImpl) : CarPlane, Car by c1, Plane by p1 {
    override fun fantastic() {
        // co-work between delegations(c1 and p1)
    }
}
5 Likes
3

Thank @nickallendev. You saved me.
I never thought adding CarPlane, Car, Plane together would be possible.
Problem solved.

4

You are right, it was not mentioned in the official documentation page: Delegation | Kotlin

1 Like