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 fun strLen(s: String) = s.length\n fun main() {\n strLen(null) // error\n}\n fun strLenSafe(s: String?) = s.length() // error\n fun main() {\n val x: String? = null\n var y: String = x // error\n}\n fun main() {\n val x: String? = null\n strLen(x) // error\n}\n fun strLenSafe(s: String?): Int = if (s != null) s.length else 0\n ?.는 null 검사와 메서드 호출을 한 연산으로 수행class Employee(val name: String, val manager: Employee?)\n\nfun managerName(employee: Employee): String? = employee.manager?.name\n\nfun main() {\n val ceo = Employee("Da Boss", null)\n val developer = Employee("Bob Smith", ceo)\n println(managerName(developer))\n println(managerName(ceo))\n}\n class Address(val streetAddress: String, val zipCode: Int, val city: String, val country: String)\n\nclass Company(val name: String, val address: Address?)\n\nclass Person(val name: String, val company: Company?)\n\nfun Person.countryName(): String {\n val country = this.company?.address?.country\n return if (country != null) country else "Unknown"\n}\n\nfun main() {\n val person = Person("Dmitry", null)\n println(person.countryName())\n // Unknown\n}\n fun greet(name: String?) {\n val recipient: String = name ?: "unnamed"\n println("Hello, $recipient")\n}\n fun strLenSafe(s: String?): Int = s?.length ?: 0\n class Address(val streetAddress: String, val zipCode: Int, val city: String, val country: String)\n\nclass Company(val name: String, val address: Address?)\n\nclass Person(val name: String, val company: Company?)\n\nfun printShippingLabel(person: Person) {\n val address = person.company?.address\n ?: throw IllegalArgumentException("No address")\n with(address) {\n println(streetAddress)\n println("$zipCode $city, $country")\n }\n}\n ClassCastException이 발생class Person(val firstName: String, val lastName: String) {\n override fun equals(o: Any?): Boolean {\n val otherPerson = o as? Person ?: return false\n\n return otherPerson.firstName == firstName && otherPerson.lastName == lastName\n }\n\n override fun hashCode(): Int = firstName.hashCode() * 37 + lastName.hashCode()\n}\n !!을 사용하면 어떤 값이든 널이 아닌 타입으로 바꿀 수 있다.!!를 사용하면 NPE 발생fun ignoreNulls(str: String?) {\n val strNotNull: String = str!! // 예외는 이 지점을 가리킨다.\n println(strNotNull.length)\n}\n !!는 컴파일러에게 null이 아님을 알고 있으며, 예외가 발생해도 감수하겠다는 것을 말하는 것이다.class SelectableTextList(\n val contents: List<String>,\n var selectedIndex: Int? = null\n)\n\nclass CopyRowAction(val list: SelectableTextList) {\n fun isActionEnabled(): Boolean = list.selectedIndex != null\n fun executeCopyRow() {\n val index = list.selectedIndex!!\n val value = list.contents[index]\n }\n}\n !!를 사용하여 발생하는 에러 스택 트레이스에는 몇번째 줄인지에 대한 정보가 들어있지 않다.\n!! 단언문을 한 줄에 함께 쓰는 일을 피하는 것이 좋다.fun sendEmailTo(email: String) {\n println("Sending email to $email")\n}\n\nfun main() {\n var email: String? = "yole@gmail.com"\n email.let { sendEmailTo(it) }\n email = null\n email?.let { sendEmailTo(it) }\n}\n class MyService {\n fun performAction(): String = "Action Done!"\n}\n\nclass MyTest {\n private var myService: MyService? = null\n \n @BeforeAll fun setUp() {\n myService = MyService()\n }\n \n @Test fun testAction() {\n assertEquals("Action Done!", myService!!.performAction())\n }\n}\n class MyTest {\n private lateinit var myService: MyService\n\n @BeforeAll fun setUp() {\n myService = MyService()\n }\n\n @Test fun testAction() {\n assertEquals("Action Done!", myService.performAction())\n }\n}\n lateinit 변경자를 붙이면 프로퍼티를 나중에 초기화할 수 있다.fun verifyUserInput(input: String?) {\n if (input.isNullOrBlank()) {\n println("Please fill in the required fields")\n }\n}\n\nfun main() {\n verifyUserInput(" ")\n verifyUserInput(null) // 예외 발생하지 않음.\n}\n fun sendEmailTo(email: String) {\n println("Sending email to $email")\n}\n\nfun main() {\n val recipient: String? = null\n recipient.let { sendEmailTo(it) } // 안전한 호출을 사용하지 않아 널이 될 수 있는 타입으로 취금\n}\n ?.를 사용해야 한다.fun <T> printHashCode(t: T) {\n println(t?.hashCode()) // 안전한 호출 사용\n}\n\nfun main() {\n printHashCode(null)\n // null\n}\n printHashCode 호출에서 타입 파라미터 T에 대해 추론한 타입은 널이 될 수 있는 Any? 타입이다.fun <T: Any> printHashCode(t: T) {\n println(t.hashCode()) // 안전한 호출 사용\n}\n\nfun main() {\n printHashCode(null) // error\n}\n data class Person(val name: String, val age: Int)\n fun main() {\n val list = listOf(1, 2, 3, 4)\n println(list.filter { it % 2 == 0 })\n}\n fun main() {\n val list = listOf(1, 2, 3, 4)\n println(list.map { it * it })\n}\n fun main() {\n val numbers = listOf(1, 2, 3, 4, 5, 6, 7)\n val filtered = numbers.filterIndexed { index, element -> index % 2 == 0 && element > 3 }\n\n println(filtered)\n\n val mapped = numbers.mapIndexed { index, element -> index + element }\n println(mapped)\n}\n fun main() {\n val list = listOf(1, 2, 3, 4, 5, 6, 7)\n println(list.reduce { acc, element -> acc + element })\n}\n fun main() {\n val people = listOf(\n Person("Alice", 29),\n Person("Bob", 31),\n )\n val folded = people.fold("") { acc, person -> acc + person.name }\n println(folded)\n}\n runningReduce, runningFold를 사용하면 중간 단계의 모든 누적 값을 뽑아낼 수 있다.\nval canBeInClub27 = { p: Person -> p.age <= 27 }\n\nfun main() {\n val people = listOf(\n Person("Alice", 27),\n Person("Bob", 31),\n )\n println(people.all(canBeInClub27))\n}\n val canBeInClub27 = { p: Person -> p.age <= 27 }\n\nfun main() {\n val people = listOf(\n Person("Alice", 27),\n Person("Bob", 31),\n )\n println(people.count(canBeInClub27))\n}\n val canBeInClub27 = { p: Person -> p.age <= 27 }\n\nfun main() {\n val people = listOf(\n Person("Alice", 27),\n Person("Bob", 31),\n )\n println(people.find(canBeInClub27))\n}\n fun main() {\n val people = listOf(\n Person("Alice", 26),\n Person("Bob", 29),\n Person("Carol", 31),\n )\n val comeIn = people.filter(canBeInClub27)\n val stayOut = people.filterNot(canBeInClub27)\n println(comeIn)\n println(stayOut)\n}\n val (comeIn, stayOut) = people.partition(canBeInClub27)\n println(comeIn)\n println(stayOut)\n fun main() {\n val people = listOf(\n Person("Alice", 31),\n Person("Bob", 29),\n Person("Carol", 31),\n )\n println(people.groupBy { it.age })\n}\n fun main() {\n val people = listOf(\n Person("Joe", 22),\n Person("Mary", 31),\n )\n val nameToAge = people.associate { it.name to it.age }\n println(nameToAge)\n}\n associate 함수를 사용fun main() {\n val people = listOf(\n Person("Joe", 22),\n Person("Mary", 31),\n Person("Jamie", 22)\n )\n val personToAge = people.associateWith { it.age }\n println(personToAge)\n val ageToPerson = people.associateBy { it.age }\n println(ageToPerson)\n}\n associateWith는 컬렉션의 원래 원소를 키로 사용associateBy는 컬렉션의 원래 원소를 맵의 값으로 하고, 람다가 만들어내는 값을 맵의 키로 사용fun main() {\n val names = mutableListOf("Martin", "Samuel")\n println(names)\n\n names.replaceAll { it.uppercase() }\n println(names)\n\n names.fill("(redacted)")\n println(names)\n}\n replaceAll함수를 가변 컬렉션에 적용하면 지정한 람다로 모든 원소를 변경한다.fill함수를 쓸 수 있다.fun main() {\n val empty = emptyList<String>()\n val full = listOf("apple", "orange", "banana")\n \n println(empty.ifEmpty { listOf("no", "values", "here") })\n println(full.ifEmpty { listOf("no", "values", "here") })\n}\n ifEmpty 함수를 사용화면 아무 원소도 없을 때 기본값을 생성하는 람다를 제공할 수 있다.ifBlank 함수는 공백만 들어있는 문자열을 다룰 수 있다.val temperatures = listOf(27.7, 29.8, 22.0, 35.5, 19.1)\n fun main() {\n println(temperatures.windowed(3))\n //[[27.7, 29.8, 22.0], [29.8, 22.0, 35.5], [22.0, 35.5, 19.1]]\n \n println(temperatures.windowed(3) { it.sum() / it.size })\n //[26.5, 29.099999999999998, 25.53333333333333]\n}\n windowed 함수를 사용할 수 있다.fun main() {\n println(temperatures.chunked(2))\n // [[27.7, 29.8], [22.0, 35.5], [19.1]]\n \n println(temperatures.chunked(2) { it.sum() })\n // [57.5, 57.5, 19.1]\n}\n chunked 함수를 사용fun main() {\n val names = listOf("Joe", "Mary", "Jamie")\n val ages = listOf(22, 31, 22, 44, 0)\n println(names.zip(ages))\n // [(Joe, 22), (Mary, 31), (Jamie, 22)]\n \n println(names.zip(ages) { name, age -> Person(name, age) })\n // [Person(name=Joe, age=22), Person(name=Mary, age=31), Person(name=Jamie, age=22)]\n}\n zip 함수를 사용해 두 컬렉션에서 같은 인덱스에 있는 원소들의 쌍으로 이뤄진 리스트를 만들 수 있다.fun main() {\n val countries = listOf("DE", "NL", "US")\n println(names zip ages zip countries)\n // [((Joe, 22), DE), ((Mary, 31), NL), ((Jamie, 22), US)]\n}\n class Book(val title: String, val authors: List<String>)\n\nval library = listOf(\n Book("Kotlin in Action", listOf("Isakova", "Elizarov", "Aigner", "Jemerov")),\n Book("Atomic Kotlin", listOf("Eckel", "Isakova")),\n Book("The Three-Body Problem", listOf("Liu"))\n)\n fun main() {\n val authors = library.map { it.authors }\n println(authors)\n // [[Isakova, Elizarov, Aigner, Jemerov], [Eckel, Isakova], [Liu]]\n}\n fun main() {\n val authors = library.flatMap { it.authors }\n println(authors)\n // [Isakova, Elizarov, Aigner, Jemerov, Eckel, Isakova, Liu]\n println(authors.toSet())\n // [Isakova, Elizarov, Aigner, Jemerov, Eckel, Liu]\n}\n flatMap 함수는 컬렉션의 각 원소를 파라미터로 주어진 함수를 사용해 변환한다.flatten 함수를 사용people.map(Person::name).filter { it.startsWith("A") }\n people\n .asSequence()\n .map(Person::name)\n .filter { it.startsWith("A") }\n .toList()\n fun main() {\n listOf(1, 2, 3, 4)\n .asSequence()\n .map {\n print("map($it) ")\n it * it\n }\n .filter {\n print("filter($it) ")\n it % 2 == 0\n }\n}\n fun main() {\n listOf(1, 2, 3, 4)\n .asSequence()\n .map {\n print("map($it) ")\n it * it\n }\n .filter {\n print("filter($it) ")\n it % 2 == 0\n }.toList()\n}\n fun main() {\n println(\n listOf(1, 2, 3, 4)\n .asSequence()\n .map { it * it }\n .find { it > 3 }\n )\n // 4\n}\n fun main() {\n val people = listOf(\n Person("Alice", 29), Person("Bob", 31),\n Person("Charles", 31), Person("Dan", 21)\n )\n println(\n people\n .asSequence()\n .map(Person::name)\n .filter { it.length < 4 }\n .toList()\n )\n // [Bob, Dan]\n println(\n people\n .asSequence()\n .filter { it.name.length < 4 }\n .map(Person::name)\n .toList())\n // [Bob, Dan]\n}\n fun main() {\n val naturalNumbers = generateSequence(0) { it + 1 }\n val numbersTo100 = naturalNumbers.takeWhile { it <= 100 }\n println(numbersTo100.sum())\n // 5050\n}\n generateSequence 함수는 이전의 원소를 인자로 받아 다음 원소를 계산한다.import java.io.File\n\nfun File.isInsideHiddenDirectory() =\n generateSequence(this) { it.parentFile }.any { it.isHidden }\n\nval file = File("/Users/svtk/.HiddenDir/a.txt")\nprintln(file.isInsideHiddenDirectory())\n// true\n\n