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 b = BaseImpl(10)
Derived(b).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.
by keyword in Kotlin allows one class to delegate some of its behavior to another class.
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 b = BaseImpl(10)
Derived(b).printMessage()
Derived(b).printMessageLine()
}
// Output:
abc
10
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(val 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)
}
// Output:
BaseImpl: x = 10
Message of Derived