Kotlin 2 cheatsheet

Kotlin 2: Basics and null safety

7 common Kotlin 2 tasks.

Declare variables

val is read-only and var is mutable. Types are inferred.

val name = "Ada"        // read-only
var score = 0            // mutable
score += 10

Use $name or ${expression} inside a string.

println("$name scored ${score * 2} points")

A type with ? can be null. Use ?. to call safely, ?: for a default, and !! only when you are certain.

val nickname: String? = null
val length = nickname?.length ?: 0

when is a flexible switch that can be an expression.

val label = when (score) {
    in 0..49 -> "low"
    in 50..89 -> "ok"
    else -> "high"
}

Parameters can have defaults and be passed by name. Single-expression functions drop the braces.

fun greet(name: String, greeting: String = "Hello") = "$greeting, $name!"

greet("Ada", greeting = "Hi")

After an is check, Kotlin smart-casts the value to that type.

fun describe(x: Any) = when (x) {
    is String -> "text of length ${x.length}"
    is Int -> "number ${x + 1}"
    else -> "something else"
}

require checks arguments and check verifies state, both throwing on failure.

fun withdraw(amount: Int) {
    require(amount > 0) { "amount must be positive" }
    check(balance >= amount) { "insufficient funds" }
}