委任模式已被證明為繼承實作的良好替代方案
如下Derived這個類別可以透過委任特定物件的方式去實作介面Base

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 b = BaseImpl(10)
    Derived(b).print()
}

• 覆寫介面成員
  如下這個範例想覆寫函式printMessage() { print("abc") }到Derived
  當我們去呼叫printMessage時，則會列印abc而不是10

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 b = BaseImpl(10)
    Derived(b).printMessage()
    Derived(b).printMessageLine()
}