Kotlin使用可能空変数および空値チェック

package com.hbk

//             
fun parseInt(str: String): Int? {
    return str.toIntOrNull()
}

fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)
    //      x*y                  
    if (x != null && y != null) {
        // x   y                 
        println(x * y)
    } else {
        println("either '$arg1' or '$arg2' is not a number")
    }
}

fun main(args: Array<String>) {
    printProduct("6", "7")
    printProduct("a", "7")
    printProduct("a", "b")
}

42
either 'a' or '7' is not a number
either 'a' or 'b' is not a number