JuniorTheoryCommon

What does the program print (lines separated by spaces)?

KOTLIN

fun main() {
    val a: String? = null
    val b: String? = "hi"
    println(a?.length ?: -1)
    println(b?.length ?: -1)
}

-1 2

Expecting null 2: forgetting the ?:. a?.length is null, and ?: -1 replaces that null with -1 before it is printed.
Expecting -1 -1: thinking ?: always gives its right side. It does so only when the left side is null; b?.length is 2, so 2 is printed.

JuniorTheoryCommon

KOTLIN

fun main() {
    val a: String? = null
    val b: String? = "hi"
    println(a?.length ?: -1)
    println(b?.length ?: -1)
}

-1 2

Expecting null 2: forgetting the ?:. a?.length is null, and ?: -1 replaces that null with -1 before it is printed.
Expecting -1 -1: thinking ?: always gives its right side. It does so only when the left side is null; b?.length is 2, so 2 is printed.