How do I call extension methods from outside the class they are defined in?

Question

Here is a minimal example that demonstrates the problem:

abstract class Base {
    abstract fun String.extension(x: Char)
}

class Derived : Base() {
    ove         


        

Answer 1

Your extension function is defined only inside of Base/Derived class. See Declaring Extensions as Members.

abstract class Base {
    abstract fun String.extension(x: Char)
}

class Derived : Base() {
    override fun String.extension(x: Char) {
        // Calling lots of methods on String, hence extension method
        println("${first()} $length ${last()} ${firstOrNull { it == x }} ...")
    }

    fun callExtension(c: Char) {
        "hello".extension(c)
    }
}

fun main(args: Array) {
    val base = Derived()
    base.callExtension('h')
}