Kotlin extension functions

extfun.kt

package extfun

import java.util.List
import java.util.ArrayList

fun main(args : Array<String>) : Unit {
    swapEm()
    /*
    Output is
    [3, 2, 1]
    [x, s, b]
    */
}

// Extension function on List<T>
fun <T> List<T>.swap(x : Int, y : Int) {
    val tmp = this[x]
    this[x] = this[y]
    this[y] = tmp
}

fun swapEm() {
    val l = ArrayList<Int>()
    l.add(1)
    l.add(2)
    l.add(3)
    l.swap(0, 2)
    println(l)

    val s = ArrayList<String>()
    s.add("s")
    s.add("x")
    s.add("b")
    s.swap(0, 1)
    println(s)
}

Reference:  http://confluence.jetbrains.net/display/Kotlin/Extension+functions