<https kotlinlang org docs reference delegation html> gt Not kotlinlang #announcements

Note, however, that members overridden in this way do not get called from the members of the delegate object, which can only access its own implementations of the interface members

This behavior is the opposite of what a manual delegation pattern does. What’s the rationale behind this difference?

Andreas Sinz

05/10/2018, 2:05 PM

how do you implement that with manual delegation?

dagguh

05/10/2018, 2:16 PM

interface Named {
        fun name(): String
        fun another()
        fun print() {
            println(name())
        }
    }

    open class Base : Named {
        override fun name() = "Base"
        override fun another() { println("Some complex behavior") }
    }

    class KotlinDelegate(private val delegate: Named) : Named by delegate {
        override fun name(): String = "KotlinDelegate"
    }

    class Inherited : Base() {
        override fun name() = "Inherited"
    }

    class ManualDelegate(private val delegate: Named) : Named {
        override fun name(): String = "ManualDelegate"
        override fun another() = delegate.another()
    }

However, I see the difference is only there when I use default interface methods

If I manually delegate

print

ManualDelegate

, I’ll get the same behavior as in

KotlinDelegate

Andreas Sinz

05/10/2018, 2:31 PM

i don't see a difference between your

ManualDelegate

and

KotlinDelegate

(apart from

another()

implementation). am I missing something?

dagguh

05/10/2018, 2:32 PM

yeah, I’m not delegating

print

, but Kotlin does

I’ll stick to the Kotlin way and rearrange my code so this doesn’t become a problem

Andreas Sinz

05/10/2018, 2:34 PM

Ah, that means inside your

ManualDelegate

it prints

ManualDelegate

and inside

KotlinDelegate

it prints e. g.

Base

dagguh

05/10/2018, 2:36 PM

exactly

Base
Base
Base
Inherited
ManualDelegate

Andreas Sinz

05/10/2018, 2:39 PM

technically there is no way that

delegate

calls something from

KotlinDelegate

, because there is no inheritance involved and

delegate

doesn't know about the other class

so your main problem is that kotlin automatically implements default methods when delegating

dagguh

05/10/2018, 3:12 PM

my original problem was: the calls are early-bound, e.g. not late-bound from the top-level implementor I no longer consider this a Kotlin delegation problem, because it’s just how Java works

I’m only able to get the “late-bind” via default method or inheritance

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