class Driver(val vehicle: Vehicle){

    fun honk() {vehicle.honk()}
}

abstract class Vehicle {
    abstract fun honk()
}

class Taxi : Vehicle() {
    override fun honk() {
        println("ba!ba!")
    }
}

class Truck : Vehicle() {
    override fun honk() {
        println("boooom...")
    }
}

fun main(args: Array<String>) {

    val taxi = Taxi()
    val taxiDriver = Driver(taxi)
    taxiDriver.honk()
    // ba!ba!

    val truck = Truck()
    val truckDriver = Driver(truck)
    truckDriver.honk()
    // boooom...

}