val and var

Kotlin gives you two ways to introduce a name, and it very much wants you to pick the boring one.

fun main() {
    val name = "Ada"
    var visits = 0

    visits = visits + 1
    visits += 1

    println("$name has visited $visits times")
}

//> Ada has visited 2 times

Try to reassign a val and the program does not build:

val name = "Ada"
name = "Grace"   // error: val cannot be reassigned

That is a compile error, not a runtime one. The bug never reaches anybody.

What does val actually freeze?

Hold the right mental model from the start, because this is where beginners go wrong later: val freezes the name, not the thing the name points at.

A val is a label glued to one object. The label cannot be moved to a different object. If the object itself has moving parts, those parts still move. A String and an Int have no moving parts, so the distinction never bites here — it matters enormously for collections, which is where the Lists and Arrays lesson picks it up.

Types: inferred, but still there

Kotlin is statically typed. Every name has a type fixed at compile time; you just rarely have to write it down.

val city = "Oslo"      // String
val year = 2016        // Int
val ratio = 0.75       // Double
val ready = true       // Boolean
val initial = 'K'      // Char — single quotes, exactly one character

There is one case where you must write the type: declaring a name before you have a value for it, because there is nothing to infer from.

val label: String
label = "ready"

The core types are Int, Long, Double, Float, Boolean, Char and String. Kotlin has no int versus Integer split like Java's — Int is one type, and the compiler uses a machine primitive underneath wherever it can.

✓ / ✗

✓ Reach for val first and let the compiler tell you when you actually need var:

val total = 3 + 4

✗ Declaring everything var "just in case" throws away help you were being given for free:

var total = 3 + 4    // nothing ever reassigns it

Reading input, and that !!

Every exercise here reads from standard input with readLine(). It hands back a String? — a string or nothing at all, because the input may have run out. The !! in the starters means "I promise a line is there; blow up if I am wrong":

val a = readLine()!!.toInt()

readLine()!! gives you a plain String, and .toInt() turns the text "7" into the number 7. Skip the conversion and you still hold text — and + on text glues rather than adds. The Null Safety lesson takes !! apart properly and shows the safer alternatives; for now read it as the starter's way of saying "the input is guaranteed".

Your exercise

Sum Two Numbers hands you a main that already reads two lines and converts each to Int. Add one statement that prints their sum.

The first visible test feeds 7 then 5 and expects 12. The mistake the grader catches is dropping the .toInt() conversions: readLine()!! + readLine()!! concatenates text, so that test prints 75 and fails. A hidden test feeds -3 and 3 and expects 0, so do not assume the values are positive — and print the number on its own, with no Sum: in front of it.