fun main() {\n val set = setOf(1, 7, 53)\n val list = listOf(1, 7, 53)\n val map = mapOf(1 to "one", 7 to "seven", 53 to "fifty-three")\n \n println(set.javaClass)\n println(list.javaClass)\n println(map.javaClass)\n }\n fun <T> joinToString(\n collection: Collection<T>,\n separator: String,\n prefix: String,\n postfix: String\n ): String {\n \n val result = StringBuilder(prefix)\n \n for ((index, element) in collection.withIndex()) {\n if (index > 0) result.append(separator) // 첫 원소 앞에는 구분자를 붙이면 안된다.\n result.append(element)\n }\n \n result.append(postfix)\n return result.toString()\n }\n 자바의 일부 코딩스타일에서는 method 호출 시, 파라미터 이름을 주석에 넣으라고 요구하기도 한다.
\n자바에서는 일부 클래스에서 오버로딩한 메서드가 너무 많아질 수 있다.
\n fun <T> joinToString(\n collection: Collection<T>,\n separator: String = ", ",\n prefix: String = "",\n postfix: String = ""\n ): String\n join.kt -> JoinKtvar opCount = 0\nval UNIX_LINE_SEPARATOR = "\\n"\nconst val UNIX_LINE_SEPARATOR = "\\n" // java code에 public static final 필드로 노출하고 싶다면 const 변경자 추가\n 확장 함수
\nfun String.lastChar(): Char = this.get(this.length - 1)\n import strings.lastChar as last\n\nval c = "Kotlin".last()\n char c = StringUtilKt.lastChar("Java");\n fun <T> Collection<T>.joinToString( // Collection<T>에 대한 확장 함수 선언\n separator: String = ", ",\n prefix: String = "",\n postfix: String = ""\n): String {\n val result = StringBuilder(prefix)\n\n for ((index, element) in this.withIndex()) {\n if (index > 0) result.append(separator)\n result.append(element)\n }\n\n result.append(postfix)\n return result.toString()\n}\n joinToString을 클래스의 멤버인 것처럼 호출할 수 있다.fun View.showOff() = println("I'm a view")\nfun Button.showOff() = println("I'm a button")\n\nfun main() {\n val view: View = Button()\n view.showOff() // 확장함수는 정적으로 결정된다.\n // I'm a view\n}\n val String.lastChar: Char\n get() = get(length - 1)\n val list = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)\n public fun <T> listOf(vararg elements: T): List<T> = if (elements.size > 0) elements.asList() else emptyList()\n fun main(args: Array<String>) {\n val list = listOf("args: ", *args)\n}\n val map = mapOf(1 to "one", 2 to "two", 3 to "three")\n infix fun Any.to(other: Any): Pair(this, other)\n val (number, name) = 1 to "one"\n fun main() {\n println("12.345-6.A".split("\\\\.|-".toRegex()))\n}\n fun main() {\n println("12.345-6.A".split('.', '-'))\n}\n fun parsePath(path: String) {\n val directory = path.substringBeforeLast("/")\n val fullName = path.substringAfterLast("/")\n val fileName = fullName.substringBeforeLast(".")\n val extension = fullName.substringAfterLast(".")\n \n println("Dir: $directory, name: $fileName, ext: $extension")\n}\n fun parsePathRegax(path: String) {\n val regex = """(.+)/(.+)\\.(.+)""".toRegex()\n val matchResult = regex.matchEntire(path)\n if (matchResult != null) {\n val (directory, filename, extension) = matchResult.destructured\n println("Dir: $directory, name: $fileName, ext: $extension")\n }\n}\n trimIndent 확장 함수를 호출하면 문자열의 모든 줄에서 가장 짧은 공통 들여쓰기를 찾아 각 줄의 첫 부분에서 제거하고, 공백만으로 이뤄진 첫 번째 줄과 마지막 줄을 제거해준다.DRY: Don't Repeat Yourself(반복하지 말라)
\nclass User(val id: Int, val name: String, val address: String)\n\nfun saveUser(user: User) {\n if (user.name.isEmpty()) {\n throw IllegalArgumentException("Can't save user ${user.id}: empty name")\n }\n if (user.address.isEmpty()) {\n throw IllegalArgumentException("Can't save user ${user.id}: empty address")\n }\n}\n fun saveUser(user: User) {\n fun validate(user: User, value: String, fieldName: String) {\n if (value.isEmpty()) {\n throw IllegalArgumentException("Can't save user ${user.id}: empty $fieldName")\n }\n }\n validate(user, user.name, "Name")\n validate(user, user.address, "Address")\n}\n class User(val id: Int, val name: String, val address: String)\n\nfun saveUser(user: User) {\n fun validate(value: String, fieldName: String) {\n if (value.isEmpty()) {\n throw IllegalArgumentException("Can't save user ${user.id}: " + "empty $fieldName")\n }\n }\n validate(user.name, "Name")\n validate(user.address, "Address")\n}\n class User(val id: Int, val name: String, val address: String)\n\nfun User.validateBeforeSave() {\n fun validate(value: String, fieldName: String) {\n if (value.isEmpty()) {\n throw IllegalArgumentException("Can't save user ${id}: empty $fieldName")\n }\n }\n validate(name, "Name")\n validate(address, "Address")\n}\n button.setOnClickListener(object: OnClickListener {\n override fun onClick(v: View) {\n println("I was clicked!")\n }\n})\n button.setOnClickListener {\n println("I was clicked!")\n}\n fun findTheOldest(people: List<Person>) {\n var maxAge = 0\n var theOldest: Person? = null\n for (person in people) {\n if (person.age > maxAge) {\n maxAge = person.age\n theOldest = person\n }\n }\n println(theOldest)\n}\n people.maxByOrNull { it.age }\n maxByOrNull 함수를 이용하면 편한다.fun main() {\n val sum = {x: Int, y: Int -> x + y}\n println(sum)\n}\n fun main() {\n { println(42) }\n}\n fun main() {\n run{ println(42) }\n}\n people.maxByOrNull({p: Person -> p.age })\n people.maxByOrNull() {p: Person -> p.age }\n people.maxByOrNull {p: Person -> p.age }\n fun main() {\n val people = listOf(Person("Dmitry", 4), Person("Eve", 4))\n val names = people.joinToString(\n separator = " ",\n transform = { p: Person -> p.name }\n )\n println(names)\n}\n people.joinToString(" ") { p: Person -> p.name }\n people.maxByOrNull { p: Person -> p.age }\n people.maxByOrNull { p -> p.age } // 파라미터 타입을 컴파일러가 추론\n val getAge = { p: Person -> p.age }\n people.maxByOrNull { getAge}\n fun main() {\n val sum = { x: Int, y: Int -> \n println("Computing $x and $y")\n x + y\n }\n}\n fun printMessagesWithPrefix(messages: Collection<String>, prefix: String) {\n messages.forEach { \n println("$prefix $it")\n }\n}\n forEach 람다는 자신을 둘러싼 영역에 정의된 prefix 변수와 다른 변수에 접근할 수 있다.\nfun printProblemCounts(responses: Collection<String>) {\n var clientErrors = 0\n var serverErrors = 0\n responses.forEach {\n if (it.startsWith("4")) {\n clientErrors++\n } else if (it.startsWith("5")) {\n serverErrors++\n }\n }\n}\n val getAge = Person::age\n fun salute() = println("Salute")\n\nfun main() {\n run { ::salute }\n}\n fun main() {\n val createPerson = ::Person\n}\n fun main() {\n val seb = Person("Sebastian", 26)\n val personsAgeFunction = Person::age // 사람이 주어지면 나이를 돌려주는 멤버 참조\n println(personsAgeFunction(seb)) // 사람을 인자로 받음\n \n val sebsAgeFunction = seb::age // 특정 사람의 나이를 돌려주는, 값과 엮인 호출 가능 참조\n println(sebsAgeFunction) // 파라미터 지정하지 않아도 됨\n}\n 함수형 인터페이스 or 단일 추상 메서드 인터페이스
\n void postponeComputation(int delay, Runnable computation) {}\n postponeComputation(1000) { println(42) }\n postponeComputation(1000, object : Runnable) {\n override fun run() {\n println(42)\n }\n }\n fun createAllDoneRunnable(): Runnable {\n return Runnable { println("All done!") }\n}\n\nfun main() {\n createAllDoneRunnable().run()\n}\n val listener = OnClickListener {view -> // 람다를 사용해 SAM 생성자 호출\n val text = when (view.id) {\n button1.id -> "First button"\n button2.id -> "Second button"\n else -> "Unknown button"\n }\n toast(text)\n}\n this가 없다.\nthis는 그 람다를 둘러싼 클래스의 인스턴스를 가리킨다.fun interface IntCondition {\n fun check(i: Int): Boolean\n fun checkString(s: String) = check(s.toInt())\n fun checkChar(c: Char) = check(c.digitToInt())\n}\n fun interface라고 정의된 타입의 파라미터를 받는 함수가 있을 때 람다 구현이나 람다에 대한 참조를 직접 넘길 수 있다.fun checkCondition(i: Int, condition: IntCondition): Boolean {\n return condition.check(i)\n}\n\nfun main() {\n checkCondition(1) { it % 2 != 0} // 람다 직접 사용\n\n val isOdd: (Int) -> Boolean = { it % 2 != 0 }\n checkCondition(1, isOdd) // 시그니처가 일치하는 람다에 대한 참조 사용\n}\n fun alphabet(): String {\n val result = StringBuilder()\n for (letter in 'A'..'Z') {\n result.append(letter)\n }\n result.append("\\nNow I know the alphabet!")\n return result.toString()\n}\n fun alphabet(): String {\n val stringBuilder = StringBuilder()\n return with(stringBuilder) { // 메서드를 호출하려는 수신 객체를 지정\n for (letter in 'A'..'Z') {\n this.append(letter)\n }\n this.append("\\nNow I know the alphabet!")\n this.toString()\n }\n}\n fun alphabet(): String {\n val stringBuilder = StringBuilder()\n return with(stringBuilder) { // 메서드를 호출하려는 수신 객체를 지정\n for (letter in 'A'..'Z') {\n append(letter)\n }\n append("\\nNow I know the alphabet!")\n toString()\n }\n}\n fun alphabet() = with(StringBuilder()) {\n for (letter in 'A'..'Z') {\n append(letter)\n }\n append("\\nNow I know the alphabet!")\n toString()\n}\n fun alphabet() = StringBuilder().apply {\n for (letter in 'A'..'Z') {\n append(letter)\n }\n append("\\nNow I know the alphabet!")\n}.toString()\n fun createViewWithCustomAttributes(context: Context) =\n TextView(context).apply {\n text = "Sample Text"\n textSize = 20.0\n setPadding(10, 0, 0, 0)\n }\n fun alphabet() = buildString {\n for (letter in 'A'..'Z') {\n append(letter)\n }\n append("\\nNow I know the alphabet!")\n}\n buildString 함수는 StringBuilder를 활용해 String을 만드는 경우 사용할 수 있는 우아한 해법buildList, buildSet, buildMapfun main() {\n val fruits = listOf("Apple", "Banana", "Cherry")\n val uppercaseFruits = mutableListOf<String>()\n val reversedLongFruits = fruits\n .map { it.uppercase() }\n .also { uppercaseFruits.addAll(it) }\n .filter { it.length > 5 }\n .also { println(it) }\n .reversed()\n}\n interface Clickable {\n fun click()\n}\n class Button : Clickable {\n override fun click() = println("I was clicked")\n}\n\nfun main() {\n Button().click()\n}\n interface Clickable {\n fun click()\n fun showOff() = println("I'm clickable!")\n}\n interface Focusable {\n fun setFocus(b: Boolean) = println("I ${if (b) "got" else "lost"} focus.")\n \n fun showOff() = println("I'm focusable!")\n}\n class Button : Clickable, Focusable {\n override fun click() = println("I was clicked")\n \n override fun showOff() {\n super<Clickable>.showOff()\n super<Focusable>.showOff()\n }\n}\n open class RichButton : Clickable {\n fun disable() {}\n open fun animate() {}\n override fun click() {}\n}\n open class RichButton : Clickable {\n final override fun click() {}\n}\n abstract class Animated { // 추상클래스의 인스턴스 만들 수 없음.\n abstract val animationSpeed: Double // 추상 프로퍼티: 하위 클래스는 반드시 값이나 접근자 제공\n val keyframes: Int = 20\n open val frames: Int = 60\n \n abstract fun animate() // 추상함수: 하위 클래스는 반드시 오버라이드해야함.\n open fun stopAnimating() {}\n fun animateTwice() {}\n}\n internal open class TalkativeButton {\n private fun yell() = println("Hey!")\n protected fun whisper() = println("Let's talk!")\n}\n\nfun TalkativeButton.giveSpeech() { // 오류\n yell() // 오류\n \n whisper() // 오류\n}\n interface State : Serializable\n\ninterface View {\n fun getCurrentState(): State\n fun restoreState(state: State) {}\n}\n\nclass Button : View {\n override fun getCurrentState(): State = ButtonState()\n override fun restoreState(state: State) {}\n class ButtonState : State {}\n}\n class Outer {\n inner class Inner {\n fun getOuterReference(): Outer = this@Outer\n }\n}\n interface Expr\nclass Num(val value: Int) : Expr\nclass Sum(val left: Expr, val right: Expr) : Expr\n\nfun eval(e: Expr): Int =\n when (e) {\n is Num -> e.value\n is Sum -> eval(e.left) + eval(e.right)\n else -> throw IllegalArgumentException("Unknown expression")\n }\n sealed class Expr\nclass Num(val value: Int) : Expr()\nclass Sum(val left: Expr, val right: Expr) : Expr()\n\nfun eval(e: Expr): Int =\n when (e) {\n is Num -> e.value\n is Sum -> eval(e.left) + eval(e.right)\n }\n sealed interface Toggleable {\n fun toggle()\n}\n\nclass LightSwitch : Toggleable {\n override fun toggle() = println("Lights!")\n}\n\nclass Camera: Toggleable {\n override fun toggle() = println("Camera!")\n}\n class User(val nickname: String)\n class User constructor(_nickname: String) {\n val nickname: String\n \n init {\n nickname = _nickname\n }\n}\n class User(val nickname: String, val isSubscribed: Boolean = true)\n open class User(val nickname: String)\n\nclass SocialUser(nickname: String) : User(nickname)\n class Secretive private constructor(private val agentName: String) {}\n open class Downloader {\n constructor(url: String?) {}\n \n constructor(uri: URI?) {}\n}\n class MyDownloader : Downloader() {\n constructor(url: String?) : this(URI(url))\n constructor(uri: URI?) : super(uri)\n}\n interface User {\n val nickname: String\n}\n class PrivateUser(override val nickname: String) : User\n\nclass SubscribingUser(val email: String) : User {\n override val nickname: String\n get() = email.substringBefore('@')\n}\n\nclass SocialUser(val accountId: Int) : User {\n override val nickname: getFacebookName(accountId)\n}\n\nfun getNameFromSocialNetwork(accountId: Int) = "bottleh$accountId"\n interface EmailUser {\n val email: String\n val nickname: String\n get() = email.substringBefore('@')\n}\n class User(val name: String) {\n var address: String = "unspecified"\n set(value: String) {\n println(\n """\n Address was changed for $name:\n "$field" -> "$value".\n """.trimIndent())\n field = value\n }\n}\n class LengthCounter {\n var counter: Int = 0\n private set // 클래스 밖에서 이 프로퍼티의 값을 바꿀 수 없다.\n\n fun addWord(word: String) {\n counter += word.length\n }\n}\n class Customer(val name: String, val postalCode: Int)\n class Customer(val name: String, val postalCode: Int) {\n override fun toString() = "Customer(name=$name, postalCode=$postalCode)"\n}\n class Customer(val name: String, val postalCode: Int) {\n override fun equals(other: Any?): Boolean {\n if (other == null || other !is Customer) \n return false\n return name == other.name && postalCode == other.postalCode\n }\n}\n class Customer(val name: String, val postalCode: Int) {\n override fun equals(other: Any?): Boolean {\n if (other == null || other !is Customer)\n return false\n return name == other.name && postalCode == other.postalCode\n }\n override fun hashCode(): Int = name.hashCode() * 31 + postalCode\n}\n data class Customer(val name: String, val postalCode: Int)\n class Customer(val name: String, val postalCode: Int) {\n fun copy(name: String = this.name, postalCode: Int = this.postalCode): Customer = Customer(name, postalCode)\n}\n Decorator 패턴
\nclass DelegatingCollection<T>: Collection<T> {\n private val innerList = arrayListOf<T>()\n \n override val size: Int get() = innerList.size\n override fun isEmpty(): Boolean = innerList.isEmpty()\n override fun contains(element: T): Boolean = innerList.contains(element)\n override fun containsAll(elements: Collection<T>): Boolean = innerList.containsAll(elements)\n override fun iterator(): Iterator<T> = innerList.iterator()\n}\n class DelegatingCollection<T>(innerList: Collection<T> = mutableListOf<T>()) : Collection<T> by innerList\n object 키워드를 쓰는 상황
\nobject Payroll {\n val allEmployees = arrayListOf<Person>()\n \n fun calculateSalary() {\n for (person in allEmployees) {\n \n }\n }\n}\n object CaseInsensitiveFileComparator : Comparator<File> {\n override fun compare(file1: File, file2: File): Int {\n return file1.path.compareTo(file2.path, ignoreCase = true)\n }\n}\n\nfun main() {\n println(\n CaseInsensitiveFileComparator.compare(\n File("/User"), File("/user")\n )\n )\n}\n class MyClass {\n companion object {\n fun callMe() {\n println("Companion object called")\n }\n }\n}\n\nfun main() {\n MyClass.callMe()\n}\n fun main() {\n val myObject = MyClass()\n myObject.callMe() // error\n}\n class User {\n val nickname: String\n \n constructor(email: String) {\n nickname = email.substringBefore('@')\n }\n \n constructor(socialAccountId: Int) {\n nickname = getSocialNetworkName(socialAccountId)\n }\n}\n class User private constructor(val nickname: String) {\n companion object {\n fun newSubscribingUser(email: String) = User(email.substringBefore('@'))\n fun newSocialUser(accountId: Int) = User(getNameFromSocialNetwork(accountId))\n }\n}\n class Person(val name: String) {\n companion object Loader {\n fun fromJSON(jsonText: String): Person = /* */\n }\n}\n interface JSONFactory<T> {\n fun fromJSON(jsonText: String): T\n}\n\nclass Person(val name: String) {\n companion object : JSONFactory<Person> {\n override fun fromJSON(jsonText: String): Person = /* */\n }\n}\n class Person(val firstName: String, val lastName: String) {\n companion object {\n }\n}\n\nfun Person.Companion.fromJSON(json: String): Person {\n}\n fromJSON 함수를 정의한 것처럼 호출할 수 있다.interface MouseListener {\n fun onEnter()\n fun onClick()\n}\n\nclass Button(private val listener: MouseListener) {\n \n}\n fun main() {\n Button(object : MouseListener {\n override fun onEnter() {}\n override fun onClick() {}\n })\n}\n fun addExpense(expense: Int) {\n // 비용을 미국 달러의 센트 단위로 저장\n}\n class UsdCent(val amount: Int)\n\nfun addExpense(expense: UsdCent) {\n // 비용을 미국 달러의 센트 단위로 저장\n}\n @JvmInline\nvalue class UsdCent(val amount: Int)\n