Basics

Hello World

fun main() {
    println("Hello, World!")
}

Variables

val name = "Kotlin"        // immutable (prefer)
var count = 0               // mutable
val pi: Double = 3.14159   // explicit type
const val MAX = 100         // compile-time constant

Basic Types

Int, Long	32-bit / 64-bit signed integers
Double, Float	64-bit / 32-bit floating point
Boolean	`true` / `false`
Char	Single Unicode character
String	Immutable text, supports templates
Unit	Equivalent to `void` (single value)
Nothing	Function never returns (e.g., throws)

String Templates

val name = "World"
println("Hello, $name!")
println("Length: ${name.length}")
val raw = """line 1
    |line 2""".trimMargin()

Functions

Function Declaration

fun add(a: Int, b: Int): Int {
    return a + b
}
fun add(a: Int, b: Int) = a + b  // single expression

Default & Named Arguments

fun greet(name: String, greeting: String = "Hello") {
    println("$greeting, $name!")
}
greet("Alice")                    // Hello, Alice!
greet("Bob", greeting = "Hi")     // Hi, Bob!

Higher-Order Functions

fun operate(a: Int, b: Int, op: (Int, Int) -> Int): Int {
    return op(a, b)
}
val sum = operate(3, 4) { a, b -> a + b }

Varargs

fun sum(vararg nums: Int): Int = nums.sum()
sum(1, 2, 3)
val arr = intArrayOf(1, 2, 3)
sum(*arr)                         // spread operator

Classes

Class Definition

class Person(val name: String, var age: Int) {
    fun greet() = "Hi, I'm $name"
}
val p = Person("Alice", 30)
println(p.name)

Inheritance

open class Shape(val sides: Int) { open fun area(): Double = 0.0 }
class Circle(val r: Double) : Shape(0) {
    override fun area() = Math.PI * r * r
}

Visibility Modifiers

public	Visible everywhere (default)
private	Visible within the class / file
protected	Class and subclasses
internal	Same module only

Abstract & Interfaces

interface Drawable { fun draw() }
abstract class Widget : Drawable { abstract val label: String }
class Button(override val label: String) : Widget() {
    override fun draw() = println("Drawing $label")
}

Null Safety

Nullable Types

var name: String? = null     // nullable
val len = name?.length       // safe call: null
val len2 = name?.length ?: 0 // Elvis operator: 0
val len3 = name!!.length     // assert non-null (throws)

Safe Operations

?.	Safe call — returns null if receiver is null
?:	Elvis — default value when null
!!	Non-null assertion (throws if null)
?.let { }	Execute block only if non-null
as?	Safe cast — returns null on failure

Smart Casts

if (obj is String) println(obj.length)  // auto-cast
when (obj) {
    is Int    -> println(obj + 1)
    is String -> println(obj.uppercase())
}

Collections

Creating Collections

val list = listOf(1, 2, 3)         // immutable
val mList = mutableListOf(1, 2, 3) // mutable
val map = mapOf("a" to 1, "b" to 2)
val set = setOf("x", "y", "z")

Collection Operations

val nums = listOf(1, 2, 3, 4, 5)
nums.filter { it > 2 }             // [3, 4, 5]
nums.map { it * 2 }                // [2, 4, 6, 8, 10]
nums.firstOrNull { it > 3 }        // 4
nums.sumOf { it }                  // 15

Common Operations

.filter { }	Keep elements matching predicate
.map { }	Transform each element
.flatMap { }	Map and flatten
.groupBy { }	Group by key into Map
.sortedBy { }	Sort by selector
.associate { }	Transform to Map (key-value pairs)
.any { } / .all { }	Check if any/all match predicate
.fold(init) { }	Reduce with initial accumulator

Coroutines

Basic Coroutine

import kotlinx.coroutines.*
fun main() = runBlocking {
    launch { delay(1000); println("World") }
    println("Hello")
}

Async / Await

val deferred = async { fetchData() }
val result = deferred.await()
// parallel: launch multiple async, await all
val (a, b) = awaitAll(async { fetchA() }, async { fetchB() })

Coroutine Builders

launch { }	Fire-and-forget coroutine (returns Job)
async { }	Returns Deferred with result
runBlocking { }	Bridges blocking and suspending code
withContext(dispatcher)	Switch coroutine context
coroutineScope { }	Structured concurrency scope

Dispatchers

Dispatchers.Default	CPU-intensive work (thread pool)
Dispatchers.IO	Blocking I/O operations
Dispatchers.Main	Main/UI thread (Android, Swing)
Dispatchers.Unconfined	Starts in caller thread, resumes in any

Extensions

Extension Functions

fun String.isPalindrome(): Boolean {
    return this == this.reversed()
}
println("racecar".isPalindrome())  // true

Extension Properties

val String.wordCount: Int
    get() = this.split("\\s+".toRegex()).size
println("hello world".wordCount)   // 2

Operator Overloading

data class Vec(val x: Double, val y: Double) {
    operator fun plus(other: Vec) = Vec(x + other.x, y + other.y)
}
val v = Vec(1.0, 2.0) + Vec(3.0, 4.0)  // Vec(4.0, 6.0)

Data Classes

Data Class

data class User(val name: String, val age: Int)
val u1 = User("Alice", 30)
val u2 = u1.copy(age = 31)        // non-destructive copy
val (name, age) = u1               // destructuring

Auto-Generated Members

equals()	Structural equality based on properties
hashCode()	Consistent with `equals()`
toString()	`User(name=Alice, age=30)`
copy()	Create modified copy
componentN()	Destructuring support

Enum Classes

enum class Direction { NORTH, SOUTH, EAST, WEST }
val dir = Direction.NORTH
when (dir) { Direction.NORTH -> "up"; else -> "other" }

Sealed Classes

Sealed Class Hierarchy

sealed class Result {
    data class Success(val data: T) : Result()
    data class Error(val message: String) : Result()
    data object Loading : Result()
}

Exhaustive When

fun handle(result: Result): String = when (result) {
    is Result.Success -> result.data
    is Result.Error   -> "Error: ${result.message}"
    is Result.Loading -> "Loading..."
}   // no else needed — compiler checks exhaustiveness

Sealed vs Enum

Sealed class	Subclasses can hold different state
Sealed interface	Allows multiple inheritance
Enum class	Fixed set of singleton instances
data object	Singleton with `toString()` override

Scope Functions

Scope Function Comparison

let	Context as `it`, returns lambda result
run	Context as `this`, returns lambda result
with(obj)	Context as `this`, returns lambda result
apply	Context as `this`, returns context object
also	Context as `it`, returns context object

let & apply

val name: String? = "Alice"
name?.let { println("Name is $it") }
val person = Person("Bob", 25).apply {
    age = 26                     // configure object
}

run & with

val result = "Hello".run { uppercase() + " WORLD" }
val info = with(person) { "$name is $age years old" }

also

val numbers = mutableListOf(1, 2, 3)
    .also { println("Original: $it") }
    .also { it.add(4) }
// also is useful for side effects (logging, validation)

KOTLIN QUICK REFERENCE