Kotlin Help


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() }

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() }

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) }

Learn more about delegated properties.

Last modified: 29 May 2024