Kotlin 치트시트

val name = "Alice"   // immutable (preferred)
var age = 30          // mutable
val pi: Double = 3.14159
val isDev: Boolean = true
val nums: List<Int> = listOf(1, 2, 3)
println(name::class)  // class kotlin.String
println(age is Int)   // true

val name = "Alice"
val age = 30
println("Name: $name, Age: $age")  // simple var
println("Length: ${name.length}")  // expression
println("Upper: ${name.uppercase()}")
val multiline = """
  Hello, $name
  Age: $age
""".trimIndent()

var name: String? = null  // nullable type
println(name?.length)     // null (safe call)
name = "Alice"
println(name!!.length)    // 5 (non-null assertion)
val len: Int = name?.length ?: 0  // elvis operator
// Non-null types can never be null
val s: String = "hi"  // not nullable

val n: Int = 42
val d: Double = n.toDouble()
val s: String = n.toString()
val fromStr: Int? = "100".toIntOrNull()
val invalid: Int? = "abc".toIntOrNull()  // null
println(d, s, fromStr, invalid)

fun describe(x: Any): String =
    when (x) {
        is Int -> "Int: ${x + 1}"  // x smart-cast to Int
        is String -> "String of length ${x.length}"
        is List<*> -> "List with ${x.size} items"
        else -> "Unknown"
    }
println(describe(42))      // Int: 43
println(describe("hello")) // String of length 5

val s = "Hello, World"
println(s.length)          // 12
println(s.uppercase())     // HELLO, WORLD
println(s.lowercase())     // hello, world
println(s.split(", "))     // [Hello, World]
println(s.replace("o", "0"))  // Hell0, W0rld
println(s.reversed())      // dlroW ,olleH
println(s.startsWith("Hello"))  // true

val text = """
    |Hello,
    |World!
""".trimMargin()  // | marks line start
println(text)
val json = """{"name": "Alice", "age": 30}"""
println(json)  // no escape needed for quotes
// trimIndent() removes common leading whitespace

val parts = listOf("apple", "banana", "cherry")
println(parts.joinToString(", "))  // apple, banana, cherry
println(parts.joinToString(prefix="[", postfix="]"))
val sb = StringBuilder()
for (p in parts) sb.append(p).append(" ")
println(sb.toString().trim())
// buildString for concise building
val result = buildString {
    parts.forEach { append(it).append(";") }
}

val n = "42".toInt()           // 42 (throws if invalid)
val d = "3.14".toDouble()      // 3.14
val safe = "abc".toIntOrNull() // null
val withDefault = "x".toIntOrNull() ?: 0  // 0
val bool = "true".toBooleanStrict()  // true
println(n, d, safe, withDefault)

val email = Regex("[\w.]+@[\w]+\.[a-z]+")
val text = "Contact: [email protected]"
val match = email.find(text)
println(match?.value)  // [email protected]
println(email.matches("[email protected]"))  // true
val replaced = text.replace(Regex("\d+"), "#")
println(email.containsMatchIn(text))  // true

val nums = listOf(1, 2, 3)  // immutable
val mutable = mutableListOf(1, 2, 3)
mutable.add(4)
mutable[0] = 0
println(nums.size)         // 3
println(nums.first())      // 1
println(nums.last())       // 3
println(nums.contains(2))  // true
println(nums.indexOf(2))   // 1

val ages = mapOf("Alice" to 30, "Bob" to 25)
println(ages["Alice"])          // 30
println(ages.getOrDefault("Eve", 0))  // 0
println(ages.containsKey("Alice"))    // true
for ((k, v) in ages) println("$k: $v")
val mutable = mutableMapOf("a" to 1)
mutable["b"] = 2  // add/update

val a = setOf(1, 2, 3)
val b = setOf(3, 4, 5)
println(a union b)        // [1,2,3,4,5]
println(a intersect b)    // [3]
println(a subtract b)     // [1,2]
println(a.contains(2))    // true
val mutable = mutableSetOf(1, 2)
mutable.add(3)  // true if added

val arr = arrayOf(1, 2, 3)  // Array<Int>
arr[0] = 10
println(arr.size)  // 3
// Specialized arrays for primitives (no boxing)
val ints = intArrayOf(1, 2, 3)
val doubles = doubleArrayOf(1.0, 2.0)
// Array constructor
val squares = Array(5) { it * it }  // [0,1,4,9,16]
println(squares.toList())

val pair = "Alice" to 30  // Pair<String, Int>
println(pair.first)   // Alice
println(pair.second)  // 30
val (name, age) = pair  // destructuring
val triple = Triple(1, "a", 2.0)
println(triple.first, triple.third)
// Useful for returning two values
fun minMax(list: List<Int>): Pair<Int, Int> =
    Pair(list.min(), list.max())

val score = 85
val grade = if (score >= 90) "A"
            else if (score >= 80) "B"
            else if (score >= 70) "C"
            else "F"
println(grade)  // B
// if returns a value—no ternary needed
// Both branches must have compatible types

val n = 2
val label = when (n) {
    0 -> "zero"
    1, 2, 3 -> "small"
    in 4..10 -> "medium"
    !in 1..100 -> "out of range"
    else -> "large"
}
println(label)  // small
// When without subject = multi-condition if
when {
    n > 0 -> println("positive")
    n < 0 -> println("negative")
    else -> println("zero")
}

for (i in 0 until 5) print(i)  // 01234 (excludes 5)
for (i in 1..5) print(i)      // 12345 (includes 5)
for (i in 5 downTo 1) print(i)  // 54321
for (i in 1..10 step 2) print(i)  // 13579
val list = listOf("a", "b", "c")
for ((index, value) in list.withIndex()) {
    println("$index: $value")
}

var count = 0
while (count < 3) {
    println(count)
    count++
}
var j = 0
do {
    println(j)
    j++
} while (j < 3)
// do-while runs at least once
// Prefer for loops when possible

for (i in 1..5) {
    if (i == 3) continue  // skip 3
    if (i == 5) break     // stop at 5
    println(i)  // 1, 2, 4
}
// Labeled breaks for nested loops
outer@ for (i in 1..3) {
    for (j in 1..3) {
        if (i == 2 && j == 2) break@outer
        println("$i,$j")
    }
}

fun add(a: Int, b: Int): Int = a + b  // expression body
fun greet(name: String, greeting: String = "Hello"): String {
    return "$greeting, $name!"
}
println(add(3, 4))           // 7
println(greet("Alice"))      // Hello, Alice!
println(greet("Bob", greeting = "Hi"))  // named arg
fun log(msg: String): Unit = println(msg)  // Unit = void

val square: (Int) -> Int = { x -> x * x }
println(square(5))  // 25
val nums = listOf(1, 2, 3)
println(nums.map { it * 2 })      // [2, 4, 6]
println(nums.filter { it > 1 })   // [2, 3]
println(nums.reduce { a, b -> a + b })  // 6
// 'it' is the implicit single parameter

fun String.shout(): String = this.uppercase() + "!"
fun Int.isEven(): Boolean = this % 2 == 0
println("hello".shout())  // HELLO!
println(4.isEven())       // true
// Extensions don't modify the class—they're resolved statically
// Use to add utility methods to any type

inline fun measure(block: () -> Unit) {
    val start = System.currentTimeMillis()
    block()
    println("Took ${System.currentTimeMillis() - start}ms")
}
measure { println("working...") }
// Infix functions for readable calls
infix fun Int.times(str: String): String = str.repeat(this)
println(3 times "ab")  // ababab

fun sum(vararg nums: Int): Int = nums.sum()
println(sum(1, 2, 3, 4))  // 10
// Spread operator to pass array
val arr = intArrayOf(1, 2, 3)
println(sum(*arr))  // 6
// Named/default params can follow vararg
fun greet(vararg names: String, greeting: String = "Hi") =
    names.joinToString { "$greeting, $it!" }
println(greet("A", "B"))

class Person(val name: String, val age: Int) {
    fun greet(): String = "Hi, I'm $name"
    fun isAdult(): Boolean = age >= 18
}
val p = Person("Alice", 30)
println(p.greet())   // Hi, I'm Alice
println(p.name)      // Alice (val = property)
println(p.isAdult()) // true
// val/var in constructor = auto property

data class Point(val x: Int, val y: Int)
val p1 = Point(3, 4)
val p2 = Point(3, 4)
println(p1 == p2)        // true (value equality)
println(p1.copy(x = 5))  // Point(x=5, y=4)
println(p1)              // Point(x=3, y=4)
// Auto: equals, hashCode, toString, copy, componentN
val (x, y) = p1  // destructuring via componentN()

sealed class Result
data class Success(val value: Int) : Result()
data class Failure(val error: String) : Result()
object Loading : Result()
fun handle(r: Result): String = when (r) {
    is Success -> "Got ${r.value}"
    is Failure -> "Error: ${r.error}"
    Loading -> "Loading..."
}
// when is exhaustive—compiler enforces all cases

object Config {  // singleton
    val version = "1.0"
    fun load() = mapOf("key" to "value")
}
println(Config.version)  // 1.0
class User(val name: String) {
    companion object {
        fun create(name: String) = User(name)
        const val MAX = 100
    }
}
val u = User.create("Alice")  // like static method

open class Animal(val name: String) {
    open fun speak() = "..."
}
class Dog(name: String) : Animal(name) {
    override fun speak() = "Woof"
}
interface Greetable {
    val name: String
    fun greet(): String  // can have default impl
}
class Person(override val name: String) : Greetable {
    override fun greet() = "Hi, $name"
}

var name: String? = "Alice"
println(name?.length)      // 5
name = null
println(name?.length)      // null
println(name?.length ?: 0) // 0 (elvis: default if null)
println(name?.length ?: return)  // early return
// Safe call chain
val city: String? = user?.address?.city

var name: String? = "Alice"
name?.let {
    println("Name is $it")  // only runs if not null
    println("Length: ${it.length}")  // it is non-null
}
// Common for null-guarded blocks
val result = name?.let {
    process(it)  // it: String (non-null)
} ?: "default"

val list = mutableListOf<Int>().apply {
    add(1); add(2); add(3)  // 'this' = the list
    println("Built $size items")
}
val config = Config().also {
    it.timeout = 30  // 'it' = config
    println("Configured")
}
// apply: configure and return object (this)
// also: side effects, return object (it)

val result = "Hello".run {
    length  // 'this' = string, returns last expr
}  // 5
val r2 = with(StringBuilder()) {
    append("a"); append("b")
    toString()  // returns this
}
// run: object.run { } - returns block result
// with: with(obj) { } - returns block result (not chainable)

val age = 25
val valid = age.takeIf { it >= 18 }  // 25 (if true)
val invalid = age.takeIf { it < 18 }  // null (if false)
val adult = age.takeUnless { it < 18 }  // 25
// Useful for filtering in chains
val email = getUser()?.email?.takeIf { it.contains("@") }
println(valid, invalid, adult)

val nums = listOf(1, 2, 3, 4, 5)
println(nums.map { it * 2 })          // [2,4,6,8,10]
println(nums.filter { it % 2 == 0 })  // [2,4]
println(nums.reduce { a, b -> a + b }) // 15
println(nums.fold(0) { a, b -> a + b }) // 15
println(nums.sum())                   // 15
println(nums.joinToString(", "))      // 1, 2, 3, 4, 5

val nested = listOf(listOf(1, 2), listOf(3, 4))
println(nested.flatten())         // [1,2,3,4]
println(nested.flatMap { it.map { n -> n * 2 } })  // [2,4,6,8]
val words = listOf("apple", "bat", "ant")
val byFirst = words.groupBy { it.first() }
// {a=[apple, ant], b=[bat]}
println(byFirst)

val nums = listOf(3, 1, 4, 1, 5)
println(nums.sorted())           // [1,1,3,4,5]
println(nums.sortedDescending()) // [5,4,3,1,1]
val people = listOf("Alice" to 30, "Bob" to 25)
val byAge = people.sortedBy { it.second }  // by age
println(byAge)
val byNameDesc = people.sortedByDescending { it.first }
println(byNameDesc)

val nums = (1..1000000).toList()
// Eager: creates intermediate lists
val eager = nums.filter { it % 2 == 0 }.map { it * 2 }.take(5)
// Lazy: no intermediate collections
val lazy = nums.asSequence()
    .filter { it % 2 == 0 }
    .map { it * 2 }
    .take(5)
    .toList()  // forces evaluation
println(lazy)  // [4, 8, 12, 16, 20]

val nums = listOf(1, 2, 3, 4, 5)
val (evens, odds) = nums.partition { it % 2 == 0 }
println(evens)  // [2, 4]
println(odds)   // [1, 3, 5]
val chunked = nums.chunked(2)
println(chunked)  // [[1, 2], [3, 4], [5]]
val windowed = nums.windowed(3)
println(windowed)  // [[1,2,3], [2,3,4], [3,4,5]]

import kotlinx.coroutines.*
fun main() = runBlocking {
    launch {
        delay(1000)
        println("World!")  // after 1s
    }
    println("Hello")  // immediately
    // launch doesn't return a result
    // Use for side-effect coroutines
}
// Output: Hello, then World! after 1s

import kotlinx.coroutines.*
fun main() = runBlocking {
    val deferred = async {
        delay(1000)
        42  // return value
    }
    val result = deferred.await()
    println(result)  // 42
    // Parallel execution
    val a = async { computeA() }
    val b = async { computeB() }
    println(a.await() + b.await())  // concurrent
}

import kotlinx.coroutines.*
suspend fun fetchUser(id: Int): String {
    delay(500)  // simulates network
    return "User $id"
}
suspend fun fetchAll(): List<String> = coroutineScope {
    val a = async { fetchUser(1) }
    val b = async { fetchUser(2) }
    listOf(a.await(), b.await())
}
// suspend functions can only be called from coroutines

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun numbers(): Flow<Int> = flow {
    for (i in 1..5) {
        delay(100)
        emit(i)  // produce value
    }
}
fun main() = runBlocking {
    numbers().collect { println(it) }  // 1,2,3,4,5
    // Transform
    numbers().map { it * 2 }.filter { it > 4 }.collect {
        println(it)  // 6, 8, 10
    }
}

import kotlinx.coroutines.*
fun main() = runBlocking {
    launch(Dispatchers.Main) { /* UI thread */ }
    launch(Dispatchers.IO) {
        // Network/file I/O (thread pool)
        val data = fetchData()
    }
    launch(Dispatchers.Default) {
        // CPU-intensive work
        val result = heavyCompute()
    }
    // withContext switches dispatcher
    val data = withContext(Dispatchers.IO) { readFile() }
}

val result = try {
    "abc".toInt()
} catch (e: NumberFormatException) {
    0  // fallback
} finally {
    println("cleanup")
}
println(result)  // 0
// try is an expression—returns a value
// Kotlin doesn't have checked exceptions

fun parse(s: String): Result<Int> =
    runCatching { s.toInt() }
val r = parse("42")
println(r.getOrNull())    // 42
println(r.getOrElse { 0 }) // 42
r.onSuccess { println("OK: $it") }
    .onFailure { println("Err: ${it.message}") }
val r2 = parse("abc")
println(r2.getOrNull())   // null

class InvalidAgeException(message: String) : Exception(message)
fun setAge(age: Int) {
    if (age < 0) throw InvalidAgeException("Age cannot be negative: $age")
}
try {
    setAge(-5)
} catch (e: InvalidAgeException) {
    println("Caught: ${e.message}")
} catch (e: Exception) {
    println("Other: ${e.message}")
}

import java.io.File
// Write
File("test.txt").writeText("Hello, File!")
// Read
val content = File("test.txt").readText()
println(content)  // Hello, File!
// Append
File("log.txt").appendText("new line\n")
// Line by line
File("test.txt").useLines { lines ->
    lines.forEach { println(it) }
}

import kotlinx.serialization.*
import kotlinx.serialization.json.*
@Serializable
data class User(val name: String, val age: Int)
val user = User("Alice", 30)
val json = Json.encodeToString(user)
println(json)  // {"name":"Alice","age":30}
val decoded = Json.decodeFromString<User>(json)
println(decoded.name)  // Alice
// Requires @Serializable annotation

import kotlinx.coroutines.*
fun main() = runBlocking {
    // coroutineScope waits for all children
    coroutineScope {
        launch { delay(1000); println("A") }
        launch { delay(500); println("B") }
    }
    println("Done")  // after both complete
    // If a child fails, parent cancels siblings
    // Never use GlobalScope unless necessary
}

import kotlinx.coroutines.*
fun main() = runBlocking {
    val job = launch {
        repeat(10) { i ->
            delay(300)
            yield()  // explicit suspension point
            println("Working $i")
        }
    }
    delay(700)
    job.cancelAndJoin()  // cancels and waits
    println("Cancelled")
}
// delay/yield check for cancellation
// isActive checks cancellation state

import kotlinx.coroutines.*
fun main() = runBlocking {
    val handler = CoroutineExceptionHandler { _, e ->
        println("Caught: $e")
    }
    val job = launch(handler) {
        throw RuntimeException("Boom!")
    }
    job.join()
    // async exceptions surface on await()
    val deferred = async { throw IllegalStateException() }
    try { deferred.await() } catch (e: Exception) {
        println("Async failed: $e")
    }
}

import kotlinx.coroutines.*
fun main() = runBlocking {
    // supervisorScope: child failures don't cancel siblings
    supervisorScope {
        launch { delay(100); throw RuntimeException("A fails") }
        launch { delay(200); println("B still runs") }
    }
    // vs coroutineScope: A's failure cancels B
    // Use SupervisorJob for long-lived services
    val scope = CoroutineScope(SupervisorJob())
}

import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun main() = runBlocking {
    val channel = Channel<Int>(3)  // buffered capacity 3
    launch {
        for (i in 1..5) {
            channel.send(i)
            println("Sent $i")
        }
        channel.close()
    }
    // receive() blocks until available
    for (x in channel) println("Got $x")
    // Produce-consume pattern
    // Rendezvous (cap 0) syncs sender/receiver
}

import kotlinx.coroutines.flow.*
fun nums() = flow {
    for (i in 1..5) emit(i)
}
suspend fun main() {
    nums().map { it * it }           // 1,4,9,16,25
        .filter { it > 5 }          // 9,16,25
        .take(2)                    // 9,16
        .collect { println(it) }
    // transform: emit multiple values
    nums().transform { x ->
        emit(x)
        emit(x * 10)
    }.collect { println(it) }
    // flatMapConcat / flatMapMerge / flatMapLatest
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun events() = flow {
    for (i in 1..3) {
        delay(100); emit(i)
    }
}
fun main() = runBlocking {
    // Without buffer: producer & consumer alternate (slow)
    events().buffer()  // decouple producer/consumer
        .collect { delay(200); println(it) }
    // conflate: drop intermediate values
    events().conflate().collect { println(it) }
    // collectLatest: cancel previous collection
    events().collectLatest { delay(200); println(it) }
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    // StateFlow: holds one value, stateful
    val state = MutableStateFlow(0)
    launch {
        state.collect { println("State: $it") }
    }
    state.value = 1  // update synchronously
    state.value = 2
    // SharedFlow: broadcasts to multiple collectors
    val events = MutableSharedFlow<String>()
    launch { events.collect { println("A: $it") } }
    launch { events.collect { println("B: $it") } }
    events.emit("Hello")  // both receive
}

import kotlinx.coroutines.flow.*
fun numbers() = flow {
    emit(1)
    throw RuntimeException("fail")
    emit(2)  // never reached
}
suspend fun main() {
    // catch operator (upstream only)
    numbers().catch { e -> emit(-1) }
        .collect { println(it) }  // 1, -1
    // retry on failure
    numbers().retry(3) { e ->
        println("Retry: $e"); true
    }.collect { println(it) }
    // retryWhen for custom logic
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun diskFlow() = flow {
    for (i in 1..3) {
        Thread.sleep(100)  // blocking I/O
        emit(i)
    }
}
suspend fun main() {
    // flowOn changes upstream context
    diskFlow().flowOn(Dispatchers.IO)
        .collect { println(it) }
    // Without flowOn, runs on collector's dispatcher
    // flowOn applies to all operators above it
    diskFlow().map { it * 2 }
        .flowOn(Dispatchers.IO)
        .filter { it > 2 }
        .collect { println(it) }
}

sealed class Result<out T> {
    data class Success<T>(val value: T) : Result<T>()
    data class Failure(val error: String) : Result<Nothing>()
    object Loading : Result<Nothing>()
}
fun handle(r: Result<Int>) = when (r) {
    is Result.Success -> println("Got ${r.value}")
    is Result.Failure -> println("Error: ${r.error}")
    Result.Loading -> println("Loading...")
}
// All subclasses defined in same file/module

sealed interface Shape {
    fun area(): Double
}
data class Circle(val r: Double) : Shape {
    override fun area() = Math.PI * r * r
}
data class Square(val side: Double) : Shape {
    override fun area() = side * side
}
// Sealed interfaces allow multiple inheritance
sealed interface Clickable { fun click() }
class Button : Shape, Clickable {
    override fun area() = 0.0
    override fun click() = println("Clicked")
}

sealed class NetworkState {
    object Loading : NetworkState()
    data class Success(val data: String) : NetworkState()
    data class Error(val message: String) : NetworkState()
}
fun render(state: NetworkState): String = when (state) {
    NetworkState.Loading -> "Spinner"
    is NetworkState.Success -> "Data: ${state.data}"
    is NetworkState.Error -> "Error: ${state.message}"
    // No else needed—compiler enforces all cases
}
// Adding a new subclass causes compile error
// until you handle it

sealed class Either<out L, out R> {
    data class Left<out L>(val value: L) : Either<L, Nothing>()
    data class Right<out R>(val value: R) : Either<Nothing, R>()
}
fun <L, R> Either<L, R>.fold(
    ifLeft: (L) -> Unit,
    ifRight: (R) -> Unit
) = when (this) {
    is Either.Left -> ifLeft(value)
    is Either.Right -> ifRight(value)
}
fun divide(a: Int, b: Int): Either<String, Int> =
    if (b == 0) Either.Left("Divide by zero")
    else Either.Right(a / b)

sealed class Expr {
    data class Num(val value: Int) : Expr()
    data class Add(val left: Expr, val right: Expr) : Expr()
    data class Mul(val left: Expr, val right: Expr) : Expr()
}
fun eval(e: Expr): Int = when (e) {
    is Expr.Num -> e.value
    is Expr.Add -> eval(e.left) + eval(e.right)
    is Expr.Mul -> eval(e.left) * eval(e.right)
}
val expr = Expr.Add(Expr.Num(2), Expr.Mul(Expr.Num(3), Expr.Num(4)))
fun main() = println(eval(expr))  // 14

val heavyConfig: Config by lazy {
    println("Initializing...")
    loadConfigFromFile()  // runs once, first access
}
fun main() {
    println("Before access")
    println(heavyConfig)  // initializes here
    println(heavyConfig)  // cached, no re-init
}
// lazy is thread-safe by default (LazyThreadSafetyMode.SYNCHRONIZED)
// Use LazyThreadSafetyMode.NONE for single-threaded

import kotlin.properties.Delegates
var count: Int by Delegates.observable(0) { _, old, new ->
    println("Changed: $old -> $new")
}
var age: Int by Delegates.vetoable(0) { _, old, new ->
    new >= 0  // reject negative values
}
fun main() {
    count = 1   // prints "Changed: 0 -> 1"
    count = 2   // prints "Changed: 1 -> 2"
    age = 25    // accepted
    age = -5    // rejected, stays 25
}

import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty
class TrimmedString : ReadWriteProperty<Any?, String> {
    private var value: String = ""
    override fun getValue(thisRef: Any?, prop: KProperty<*>) = value
    override fun setValue(thisRef: Any?, prop: KProperty<*>, value: String) {
        this.value = value.trim()
    }
}
class User {
    var name: String by TrimmedString()
}
fun main() {
    val u = User()
    u.name = "  Alice  "
    println(u.name)  // "Alice"
}

class User(map: Map<String, Any?>) {
    val name: String by map
    val age: Int by map
    val email: String? by map
}
fun main() {
    val u = User(mapOf(
        "name" to "Alice",
        "age" to 30,
        "email" to "[email protected]"
    ))
    println(u.name)  // Alice
}
// Mutable version: MutableMap
class MutableUser(map: MutableMap<String, Any?>) {
    var name: String by map
}

import kotlin.properties.Delegates
class Service {
    // Late-init for non-null, set once
    var config: String by Delegates.notNull()
    // Throws before initialization
}
// Singleton via object (no delegate needed)
object Database {
    val connection = connect()
}
// Or lazy singleton
val db: Database by lazy { Database() }
fun main() {
    val s = Service()
    // println(s.config)  // IllegalStateException
    s.config = "prod"
    println(s.config)  // prod
}

class Table {
    private val rows = mutableListOf<Row>()
    fun row(init: Row.() -> Unit) {
        val r = Row(); r.init(); rows.add(r)
    }
    fun build() = rows
}
class Row {
    private val cells = mutableListOf<String>()
    fun cell(text: String) { cells.add(text) }
    fun build() = cells
}
fun table(init: Table.() -> Unit) = Table().apply(init).build()
val t = table {
    row { cell("A"); cell("B") }
    row { cell("C"); cell("D") }
}

@DslMarker
annotation class HtmlDsl
@HtmlDsl
class HTML { fun body(b: Body.() -> Unit) { /*...*/ } }
@HtmlDsl
class Body { fun p(text: String) { /*...*/ } }
fun html(init: HTML.() -> Unit) = HTML().apply(init)
// Without @DslMarker, inner 'this' could call outer methods
html {
    body {
        // p() is unambiguous—Body's method
        // body() would be an error (not Body's method)
        p("Hello")
    }
}

infix fun Int.times(str: String) = str.repeat(this)
fun main() {
    println(3 times "ab")  // "ababab"
    // Equivalent to: 3.times("ab")
    // No dot, no parentheses—reads like natural language
}
// Common in testing
infix fun <T> T.shouldEqual(expected: T) =
    assert(this == expected)
fun test() { 5 shouldEqual 5 }
// Also: to, until, step, in, etc.

data class Vec(val x: Int, val y: Int) {
    operator fun plus(o: Vec) = Vec(x + o.x, y + o.y)
    operator fun minus(o: Vec) = Vec(x - o.x, y - o.y)
    operator fun times(s: Int) = Vec(x * s, y * s)
    operator fun unaryMinus() = Vec(-x, -y)
    operator fun get(i: Int) = when (i) { 0 -> x; 1 -> y; else -> throw IndexOutOfBoundsException() }
}
fun main() {
    val a = Vec(1, 2)
    val b = Vec(3, 4)
    println(a + b)        // Vec(x=4, y=6)
    println(a * 2)        // Vec(x=2, y=4)
    println(-a)           // Vec(x=-1, y=-2)
    println(a[0])         // 1
}

// Lambda with receiver: A.(B) -> C
val greet: String.(Int) -> String = { times -> repeat(times) { this + "!" } }
fun main() {
    println("Hello".greet(3))  // Hello!!!Hello!!!Hello!!!
    // 'this' is the String receiver
}
// Build DSLs with receiver
class StringBuilder {
    private val parts = mutableListOf<String>()
    fun add(s: String) { parts.add(s) }
}
fun build(init: StringBuilder.() -> Unit) =
    StringBuilder().apply(init)
val sb = build { add("a"); add("b") }

import org.junit.jupiter.api.*
import org.junit.jupiter.api.Assertions.*
class CalculatorTest {
    lateinit var calc: Calculator
    @BeforeEach fun setup() { calc = Calculator() }
    @Test fun `add two numbers`() {
        assertEquals(5, calc.add(2, 3))
    }
    @Test fun `divide by zero throws`() {
        assertThrows<ArithmeticException> { calc.divide(10, 0) }
    }
    @ParameterizedTest
    @ValueSource(ints = [1, 2, 3])
    fun `positive numbers`(n: Int) { assertTrue(n > 0) }
    @Disabled("TODO") @Test fun skip() {}
}

import io.mockk.*
interface UserService { fun find(id: Int): String?; fun save(name: String): Int }
class Test {
    val mock = mockk<UserService>()
    @Test fun test() {
        every { mock.find(1) } returns "Alice"
        every { mock.find(any()) } returns null
        every { mock.save(any()) } throws RuntimeException("fail")
        verify { mock.find(1) }              // called once
        verify(exactly = 2) { mock.find(any()) }
        confirmVerified(mock)
    }
}
// relaxUnitFun / relaxed = true for void methods

import app.cash.turbine.test
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.runTest
import kotlin.test.*
class FlowTest {
    @Test fun testFlow() = runTest {
        flowOf(1, 2, 3).test {
            assertEquals(1, awaitItem())
            assertEquals(2, awaitItem())
            assertEquals(3, awaitItem())
            awaitComplete()
        }
    }
    @Test fun testError() = runTest {
        flow<Int> { throw RuntimeException("x") }.test {
            assertTrue(awaitError() is RuntimeException)
        }
    }
}

import kotlinx.coroutines.test.*
import kotlinx.coroutines.*
import kotlin.test.*
class RepoTest {
    @Test fun fetch() = runTest {
        // Virtual time: delay(1000) completes instantly
        val result = async { delay(1000); "data" }.await()
        assertEquals("data", result)
    }
    @Test fun withDispatcher() = runTest {
        val scheduler = StandardTestDispatcher()
        val scope = CoroutineScope(scheduler)
        // advanceUntilIdle() runs pending coroutines
        scope.launch { /*...*/ }
        scheduler.advanceUntilIdle()
    }
}

import io.kotest.core.spec.style.StringSpec
import io.kotest.matchers.shouldBe
import io.kotest.matchers.collections.shouldContain
import io.kotest.matchers.string.shouldStartWith
class MyTest : StringSpec({
    "addition works" {
        1 + 1 shouldBe 2
    }
    "list contains" {
        listOf(1, 2, 3) shouldContain 2
    }
    "string prefix" {
        "hello" shouldStartWith "he"
    }
})
// Other styles: FunSpec, BehaviorSpec, ShouldSpec

data class User(var name: String, var age: Int)
fun main() {
    val u = User("A", 1)
    // apply: configure & return this (builders)
    val configured = u.apply { age = 30 }
    // let: transform, it = receiver, returns lambda result
    val name = u.let { it.name.uppercase() }
    // run: transform, this = receiver, returns result
    val s = u.run { "$name:$age" }
    // with: like run but called as function
    val str = with(u) { "$name:$age" }
    // also: side effects, returns this (logging/chaining)
    u.also { println("Created $it") }
}

fun main() {
    // Eager: List processes each step fully
    val eager = (1..10).toList()
        .map { it * 2 }
        .filter { it > 5 }
    // Lazy: Sequence processes element-by-element
    val lazy = (1..10).asSequence()
        .map { println("map $it"); it * 2 }
        .filter { println("filter $it"); it > 5 }
        .take(2)
        .toList()  // only processes until 2 found
    // Use Sequence for large/pipelined data
    // generateSequence for infinite streams
    val naturals = generateSequence(1) { it + 1 }
}

// inline: copies bytecode at call site (no lambda overhead)
inline fun measure(block: () -> Unit): Long {
    val start = System.currentTimeMillis()
    block()
    return System.currentTimeMillis() - start
}
// non-local return: inline lambdas can return from outer fun
inline fun forEach(list: List<Int>, f: (Int) -> Unit) {
    for (i in list) f(i)
}
// reified: access generic type at runtime
inline fun <reified T> List<*>.filterIsInstance() =
    filter { it is T } as List<T>
val strs = listOf(1, "a", 2, "b").filterIsInstance<String>()

fun <T, R> List<T>.mapTo(transform: (T) -> R): List<R> {
    val result = mutableListOf<R>()
    for (item in this) result.add(transform(item))
    return result
}
fun compose(f: (Int) -> Int, g: (Int) -> Int): (Int) -> Int =
    { x -> f(g(x)) }
fun main() {
    val double = { x: Int -> x * 2 }
    val inc = { x: Int -> x + 1 }
    val doubleThenInc = compose(inc, double)
    println(doubleThenInc(3))  // 7
    listOf(1, 2, 3).mapTo { it * it }
}

// Regular recursion: stack overflow on large n
fun factorial(n: Int): Long =
    if (n <= 1) 1 else n * factorial(n - 1)
// tailrec: compiler optimizes to a loop (no stack growth)
tailrec fun factorialTail(n: Int, acc: Long = 1): Long =
    if (n <= 1) acc else factorialTail(n - 1, acc * n)
// Fibonacci with tailrec
tailrec fun fib(n: Int, a: Long = 0, b: Long = 1): Long =
    when (n) { 0 -> a; 1 -> b; else -> fib(n - 1, b, a + b) }
fun main() {
    println(factorialTail(10000))  // no stack overflow
}

class Box<T>(val value: T) {
    fun get(): T = value
}
fun <T> singletonList(item: T): List<T> = listOf(item)
fun main() {
    val intBox = Box(42)        // Box<Int> inferred
    val strBox = Box<String>("hi")
    val nums = singletonList(1)
}
// Generic constraints
class Repository<T : Entity> {
    fun save(item: T) { /* T is Entity subtype */ }
}
interface Entity { val id: Int }

// out (covariant): Producer<Sub> is Producer<Super>
interface Source<out T> { fun next(): T }
// in (contravariant): Consumer<Super> is Consumer<Sub>
interface Sink<in T> { fun put(item: T) }
// invariant (default): neither
class MutableList<T> {
    fun add(item: T) {}
    fun get(): T = TODO()
}
open class Animal
class Dog : Animal()
val src: Source<Animal> = Source<Dog>()  // OK (out)
val sink: Sink<Dog> = Sink<Animal>()     // OK (in)

// Function types are variant by nature
// (T) -> R is contravariant in T, covariant in R
val dogHandler: (Dog) -> Unit = { println(it) }
val animalHandler: (Animal) -> Unit = dogHandler  // OK
// Covariant return types
interface Repository<out T> { fun find(id: Int): T }
class DogRepo : Repository<Dog> {
    override fun find(id: Int): Dog = Dog()
}
val repo: Repository<Animal> = DogRepo()  // OK
// PECS rule: Producer extends, Consumer super
// Kotlin: out = producer, in = consumer

fun copy(from: Array<out Any>, to: Array<in Any>) {
    // from: only read (covariant projection)
    // to: only write (contravariant projection)
    for (i in from.indices) to[i] = from[i]
}
// Star projection: unknown type, read-only
fun printSize(list: List<*>) {
    println(list.size)  // OK (no type needed)
    // list[0]  // type is Any? — limited use
}
val mixed: List<*> = listOf(1, "a", 3.0)
// Use when type is irrelevant or unknown

// reified requires inline
inline fun <reified T> List<*>.filterIsInstance(): List<T> =
    filter { it is T } as List<T>
inline fun <reified T> Any.castTo(): T = this as T
inline fun <reified T> typeOf() = T::class.simpleName
fun main() {
    val list = listOf(1, "a", 2, "b", 3)
    val strs: List<String> = list.filterIsInstance()
    println(strs)  // [a, b]
    println(typeOf<String>())  // String
    // Without reified, T is erased at runtime
    // reified makes T::class and 'is T' available
}

var name: String = "Alice"  // Non-null
// name = null  // Error
var nickname: String? = null  // Nullable
// Safe call
println(nickname?.length)  // null if nickname is null
// Elvis operator
val len = nickname?.length ?: 0  // 0 if null
// Not-null assertion (use sparingly)
val len2 = nickname!!.length  // NPE if null

val name: String? = "Alice"
name?.let {
    println("Length: ${it.length}")  // Only if not null
}
// run: object + block
val result = "Hello".run {
    length  // Returns 5
}
// apply: configure object
val list = mutableListOf<Int>().apply {
    add(1); add(2); add(3)
}

class Service {
    lateinit var dependency: Database
    fun init() {
        dependency = connectDatabase()
    }
    fun use() {
        if (::dependency.isInitialized) {
            dependency.query()
        }
    }
}

val list: List<String?> = listOf("a", null, "b")
val filtered = list.filterNotNull()  // ["a", "b"]
val lengths = list.map { it?.length }  // [1, null, 1]
val first = list.firstOrNull()  // "a" or null
val firstNonEmpty = list.firstOrNull { it?.isNotEmpty() == true }

// Java interop: platform type (no null info)
val name: String = javaObject.getName()  // May be null!
// Kotlin does not know if Java returns null
// Fix: explicit nullable type
val name2: String? = javaObject.getName()
// Or @Nullable annotation in Java

class StringBuilder {
    private val parts = mutableListOf<String>()
    fun line(s: String) { parts.add(s) }
    fun build() = parts.joinToString("\n")
}
fun buildString(init: StringBuilder.() -> Unit): String {
    val sb = StringBuilder()
    sb.init()  // Execute lambda with sb as receiver
    return sb.build()
}

fun html(init: HTML.() -> Unit): HTML {
    val h = HTML(); h.init(); return h
}
class HTML {
    fun body(init: Body.() -> Unit) { /* ... */ }
}
html {
    body {
        // this: Body
        p("Hello")
    }
}

@DslMarker
annotation class HtmlDsl
@HtmlDsl
class HTML { fun body(...) {} }
@HtmlDsl
class Body { fun p(...) {} }
html {
    body {
        // p()  // OK: in Body scope
        // body()  // Error: in Body scope, not HTML
    }
}

plugins {
    kotlin("jvm") version "1.9.0"
}
dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.3")
    testImplementation(kotlin("test"))
}
// All are function calls with lambda receivers

fun users(where: SqlExpressionBuilder.() -> Op<Boolean>) {
    // DSL for SQL queries
}
users {
    (Users.age greater 18) and (Users.name like "A%")
}
// Type-safe SQL: compiler checks column types

val list = listOf(1, 2, 3, 4, 5)
val doubled = list.map { it * 2 }
val evens = list.filter { it % 2 == 0 }
val sum = list.reduce { acc, n -> acc + n }
val grouped = list.groupBy { it % 2 }  // {0=[2,4], 1=[1,3,5]}
val chunked = list.chunked(2)  // [[1,2],[3,4],[5]]

val result = (1..1000000).asSequence()
    .map { it * 2 }
    .filter { it > 100 }
    .take(10)
    .toList()
// Lazy: only computes what is needed
// No intermediate collections

val (a, b) = Pair(1, "hello")
val (x, y, z) = Triple(1, 2, 3)
for ((index, value) in list.withIndex()) {
    println("$index: $value")
}
data class Point(val x: Int, val y: Int)
val (px, py) = Point(1, 2)

val mutable = mutableListOf(1, 2, 3)
mutable.add(4)  // OK
val immutable = listOf(1, 2, 3)
// immutable.add(4)  // Error: no add method
// Convert
val imm = mutable.toList()
val mut = imm.toMutableList()

val list = listOf("a" to 1, "b" to 2)
val map = list.toMap()  // {a=1, b=2}
val byLength = list.associate { it.first to it.second }
val (evens, odds) = list.partition { it.second % 2 == 0 }
// evens: [(a,1)?] odds: [(b,2)]
val flat = listOf(listOf(1), listOf(2, 3)).flatten()  // [1,2,3]

val a = Integer(127)
val b = Integer(127)
a == b  // true (value equality, calls equals)
a === b  // true (Integer caches -128 to 127)
val c = Integer(128)
val d = Integer(128)
c == d  // true
c === d  // false (not cached)

class MyClass {
    companion object {
        const val CONSTANT = 42
        fun create() = MyClass()
    }
}
MyClass.CONSTANT  // 42
MyClass.create()  // Static-like access
// companion object is a real object, not static

data class User(val name: String, val age: Int)
val alice = User("Alice", 30)
val older = alice.copy(age = 31)  // New instance
// alice is unchanged
// copy uses named arguments for changed fields

sealed class Result
data class Success(val value: Int) : Result()
data class Failure(val error: String) : Result()
fun handle(r: Result) = when (r) {
    is Success -> r.value
    // Warning: Missing Failure branch
}
// Fix: handle all cases
fun handle(r: Result) = when (r) {
    is Success -> r.value
    is Failure -> 0
}  // No else needed

class Foo {
    fun bar() = "member"  // Member function
}
fun Foo.bar() = "extension"  // Extension function
Foo().bar()  // "member" - members win!
// Extensions are resolved statically
// Members are resolved dynamically (virtual)

// commonMain/src/Platform.kt
expect fun getPlatformName(): String
// androidMain/src/Platform.kt
actual fun getPlatformName(): String = "Android"
// iosMain/src/Platform.kt
actual fun getPlatformName(): String = "iOS"
// Shared business logic
class Greeting {
    fun greet() = "Hello from ${getPlatformName()}"
}

// build.gradle.kts (shared module)
kotlin {
    androidTarget()
    iosX64(); iosArm64(); iosSimulatorArm64()
    sourceSets {
        val commonMain by getting {
            dependencies { implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.3") }
        }
    }
}

// Shared HTTP client
suspend fun fetchUser(): User {
    return client.get("https://api.example.com/user").body()
}
// Platform-specific engine
// Android: OkHttp or Android
// iOS: Darwin
// Common code uses the same API

// .sq file: User.sq
CREATE TABLE User (id INTEGER, name TEXT);
selectById: SELECT * FROM User WHERE id = ?;
insertUser: INSERT INTO User (id, name) VALUES (?, ?);
// Generated Kotlin code
val queries: UserQueries = database.userQueries
queries.insertUser(1, "Alice")
val user = queries.selectById(1).executeAsOne()

// Shared UI with Compose Multiplatform
@Composable
fun Greeting(name: String) {
    Text("Hello, $name!")
}
// Android
class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent { Greeting("World") }
    }
}
// Desktop
fun main() = application {
    Window { Greeting("Desktop") }
}

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.Assertions.*
class CalculatorTest {
    @Test
    fun `test addition`() {
        val calc = Calculator()
        assertEquals(5, calc.add(2, 3))
    }
    @Test
    fun `throws on negative`() {
        assertThrows<IllegalArgumentException> { calc.sqrt(-1) }
    }
}

class UserServiceTest {
    @MockK
    lateinit var repo: UserRepository
    @InjectMockKs
    lateinit var service: UserService
    @BeforeEach
    fun setup() {
        MockKAnnotations.init(this)
        every { repo.find(1) } returns User("Alice")
    }
    @Test
    fun `test find`() {
        assertEquals("Alice", service.find(1).name)
        verify { repo.find(1) }
    }
}

import io.kotest.core.spec.style.StringSpec
import io.kotest.matchers.shouldBe
class CalculatorTest : StringSpec({
    "addition should work" {
        Calculator().add(2, 3) shouldBe 5
    }
    "should throw on negative" {
        shouldThrow<IllegalArgumentException> { Calculator().sqrt(-1) }
    }
})

@Test
fun `test async`() = runTest {
    val result = fetchData()
    assertEquals("data", result)
}
// runTest replaces runBlocking with virtual time
// advanceUntilIdle() runs all pending coroutines
// Delay skips instantly

@Test
fun `test flow`() = runTest {
    flowOf(1, 2, 3).test {
        awaitItem() shouldBe 1
        awaitItem() shouldBe 2
        awaitItem() shouldBe 3
        awaitComplete()
    }
}
// Test errors
flow<Int> { throw Exception() }.test {
    awaitError() shouldBe Exception()
}

class MyViewModel : ViewModel() {
    fun fetchData() {
        viewModelScope.launch {
            val data = api.getData()
            _data.value = data
        }
    }
}
// Custom scope
val scope = CoroutineScope(Dispatchers.Main + SupervisorJob())
scope.launch { /* ... */ }
scope.cancel()  // Cancel all children

launch(Dispatchers.Main) { updateUI() }
launch(Dispatchers.IO) { writeFile() }
launch(Dispatchers.Default) { heavyComputation() }
launch(Dispatchers.Unconfined) { runAnywhere() }
// Switch context
withContext(Dispatchers.IO) {
    val data = readDisk()
}

flowOf(1, 2, 3, 4, 5)
    .map { it * 2 }
    .filter { it > 4 }
    .collect { println(it) }  // 6, 8, 10

// Cold flow: emits per collector
// StateFlow: hot, stateful
// SharedFlow: hot, broadcast
val state = MutableStateFlow(0)
state.value = 1

val handler = CoroutineExceptionHandler { _, e ->
    Log.e("TAG", "Caught: $e", e)
}
scope.launch(handler) {
    throw RuntimeException("oops")
}
// try/catch in coroutine
try {
    riskyCall()
} catch (e: Exception) {
    // handle
}

val channel = Channel<Int>(capacity = 10)
launch {
    for (i in 1..5) channel.send(i)
    channel.close()
}
launch {
    for (x in channel) println(x)
}
// Produce pattern
fun numbers() = produce {
    for (i in 1..10) send(i)
}