Delegation with by

Kotlin language Mid-level Kotlin 2.0+ API 24+ Reviewed 31 Oct 2026 By Best Latest Apps

Predict the output

What does this print?

interface Greeter { fun hello(): String; fun bye(): String }

class Plain : Greeter {
    override fun hello() = "hi"
    override fun bye() = "bye"
}

class Loud(inner: Greeter) : Greeter by inner {
    override fun hello() = "HI"
}

fun main() {
    val g = Loud(Plain())
    println(g.hello() + " " + g.bye())
}

OptionsHI byehi byeHI BYEIt doesn't compile: bye isn't implemented

Hint 1 of 1

Which methods does by inner generate?

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Show answer

Answer: A HI bye

Correct. hello is overridden; bye is forwarded to the inner Greeter.

Why not the others?

B hi bye. Loud overrides hello.
C HI BYE. bye isn't overridden, so it's delegated.
D It doesn't compile: bye isn't implemented. by inner generates the missing methods.

30-second answer

Class delegation (class Logged(private val inner: Repo) : Repo by inner) generates every interface method to forward to inner, so you override only what you change. That's composition without boilerplate. Property delegation (val x by lazy { }, by mutableStateOf(), by viewModels()) hands a property's get and set to a delegate object.

Practise saying it · 30 s. No recording, just a timer

Explain delegation in Kotlin.

Say your answer out loud, then check which of these points you covered:

Class delegation forwards interface methods
Override only what changes
Property delegates handle get/set
lazy, viewModels, mutableStateOf are delegates

Deep dive

Write your own property delegate with ReadOnlyProperty / ReadWriteProperty or getValue/setValue operators.

Delegates.observable runs a callback on every change; vetoable can reject changes.

by map reads properties from a map by name.

Class delegation calls the delegate's methods, not your overrides, from inside the delegate.

Common mistakes

Expecting the delegate to call your override internally.
Writing manual forwarding methods for a large interface.
Hiding heavy work in a custom getter delegate.

Senior take & follow-ups

Explain both kinds and name a familiar Android delegate.

Follow-up questions

Think of your answer first, then open the hint.

How does by viewModels() work?
Show hint
It's a property delegate that looks up or creates the ViewModel on first access.
When would you use Delegates.observable?
Show hint
To react to changes of a simple property, like logging.

Suggest a fix if something here is wrong or out of date.