28. Inheritance & Overriding

Classes and members are final by default. Mark a class open to allow subclassing and a function open to allow overriding. A subclass calls its base constructor in the header and reaches the parent’s implementation with super..

open class Animal(val name: String) {
    open fun sound(): String = "..."
}

class Dog(name: String) : Animal(name) {
    override fun sound(): String = super.sound() + " Woof"
}

fun main() {
    val a: Animal = Dog("Rex")
    println("${a.name} says ${a.sound()}")
}

The override keyword is mandatory, and Dog passes name up to Animal(name).

Running it:

$ kotlin run
Rex says ... Woof

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