Class Delegation

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 inherit from an interface Base and delegate all of its public methods 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(args: Array<String>) {
    val b = BaseImpl(10)
    Derived(b).print() // prints 10

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.

Note that overrides work as you might expect: The compiler will use your override implementations instead of those in the delegate object. If we were to add override fun print() { print("abc") } to Derived, the program would print "abc" instead of "10".

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