Kotlin Cheat Sheet

Variables & Types

val name = "Ada": Read-only variable (prefer by default)
var score = 0: Mutable variable
val price: Double = 9.99: Explicit type
const val MAX_USERS = 100: Compile-time constant (top level or in an object)
"42".toInt() / "42".toIntOrNull(): Convert — throwing vs null-returning

"Hello, $name! You are ${age + 1}": String templates
s.length / s.isEmpty() / s.isBlank(): Length and emptiness
s.uppercase() / s.lowercase(): Change case
s.trim() / s.split(","): Trim / split
s.startsWith("He") / "ll" in s: Prefix / substring check
val text = """ ... """.trimIndent(): Raw multi-line string (no escapes processed)

val max = if (a > b) a else b: if is an expression
when (x) { 1, 2 -> "low"; in 3..9 -> "mid"; else -> "high" }: when expression
for (i in 1..5) { }: Inclusive range
for (i in 0..<n) { } / for (i in 10 downTo 0 step 2) { }: Exclusive range / reverse with step
for ((i, v) in list.withIndex()) { }: Index and value

val nums = listOf(1, 2, 3): Read-only list
val items = mutableListOf<String>(): Mutable list
val ages = mapOf("ada" to 36): Map (key to value)
ages.getOrDefault("bob", 0): Lookup with a default
nums.map { it * 2 }.filter { it > 2 }: Transform and filter
people.groupBy { it.city } / sortedBy { it.age }: Group / sort by a property

fun add(a: Int, b: Int = 0): Int = a + b: Default argument, expression body
greet(name = "Ada", loud = true): Named arguments
val double = { n: Int -> n * 2 }: Lambda
fun String.shout() = uppercase() + "!": Extension function
user.apply { age = 37 }.also { log(it) }: Scope functions

class User(val name: String, var age: Int): Class with a primary constructor
data class Point(val x: Int, val y: Int): Data class: equals, toString, copy
open class Animal / class Dog : Animal(): Inheritance (classes are final by default)
object Config { val debug = true }: Singleton object
sealed interface Result { data class Ok(val v: Int) : Result }: Sealed hierarchy for exhaustive when
enum class Color { RED, GREEN }: Enum class