Kotlin: How can I create a “static” inheritable function?

Question

For example, I want to have a function example() on a type Child that extends Parent so that I can use the function on both.

Answer 1

Since companion objects play by the same rules as any others, it is easy to use normal means of code reuse on them:

open class Example {
    fun example() {}
}

class Parent {
    companion object : Example()
}

class Child {
    companion object : Example()
}

fun main(args: Array) {
    Child.example()
    Parent.example()
}

Alternatively with class delegation you can reuse the implementation from Parent directly:

interface Example {
    fun example()
}

class Parent {
    companion object : Example {
        override fun example() {}
    }
}

class Child {
    companion object : Example by Parent.Companion
}