delegation.md

The Delegation pattern has proven to be a good alternative to implementation inheritance, and Kotlin supports it natively requiring zero boilerplate code.

A class Derived can implement an interface Base by delegating all of its public members to a specified object:

interface Base {
    fun print()
}

class BaseImpl(val x: Int) : Base {
    override fun print() { print(x) }
}

class Derived(b: Base) : Base by b

fun main() {
    val base = BaseImpl(10)
    Derived(base).print()
}

{kotlin-runnable="true"}

The by-clause in the supertype list for Derived indicates that b will be stored internally in objects of Derived and the compiler will generate all the methods of Base that forward to b.

Overriding a member of an interface implemented by delegation

Overrides work as you expect: the compiler will use your override implementations instead of those in the delegate object. If you want to add override fun printMessage() { print("abc") } to Derived, the program would print abc instead of 10 when printMessage is called:

interface Base {
    fun printMessage()
    fun printMessageLine()
}

class BaseImpl(val x: Int) : Base {
    override fun printMessage() { print(x) }
    override fun printMessageLine() { println(x) }
}

class Derived(b: Base) : Base by b {
    override fun printMessage() { print("abc") }
}

fun main() {
    val base = BaseImpl(10)
    Derived(base).printMessage()
    Derived(base).printMessageLine()
}

{kotlin-runnable="true"}

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:

interface Base {
    val message: String
    fun print()
}

class BaseImpl(x: Int) : Base {
    override val message = "BaseImpl: x = $x"
    override fun print() { println(message) }
}

class Derived(b: Base) : Base by b {
    // This property is not accessed from b's implementation of `print`
    override val message = "Message of Derived"
}

fun main() {
    val b = BaseImpl(10)
    val derived = Derived(b)
    derived.print()
    println(derived.message)
}

{kotlin-runnable="true"}

Learn more about delegated properties.