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 val i: Int = 1\nval list: List<Int> = listOf(1, 2, 3)\n | 타입 | \n비트 크기 | \n표현 범위 | \n
|---|---|---|
UByte | \n8비트 | \n0 ~ 255 | \n
UShort | \n16비트 | \n0 ~ 65,535 | \n
UInt | \n32비트 | \n0 ~ 4,294,967,295 | \n
ULong | \n64비트 | \n0 ~ 18,446,744,073,709,551,615 | \n
data class Person(val name: String, val age: Int? = null) {\n fun isOlderThan(other: Person): Boolean? {\n if (age == null || other.age == null) {\n return null\n }\n return age > other.age\n }\n}\n val i = 1\nval i: Long = b.toLong() // 자동 변환 X\n val x = 1\nval list = listOf(1L, 2L, 3L)\nx in list // false\n fun main() {\n println(Int.MAX_VALUE + 1)\n // 음수 최솟값\n \n println(Int.MIN_VALUE - 1)\n // 양수 최댓값\n}\n val answer: Any = 42\n fun f(): Unit {}\nfun f() {}\n return Unit을 넣어준다.fun fail(message: String): Nothing {\n throw IllegalArgumentException(message)\n}\n val address = company.address ?: fail("No address")\n fun readNumbers(text: String): List<Int?> {\n val result = mutableListOf<Int?>()\n for (line in text.lineSequence()) {\n val numberOrNull = line.toIntOrNull()\n result.add(numberOrNull)\n }\n return result\n}\n List<Int?>\nList<Int>?\nfun readNumbers2(text: String): List<Int?> = text.lineSequence().map { it.toIntOrNull() }.toList()\n List<Int?>?로 표현할 수 있다.\nfun addValidNumbers(numbers: List<Int?>) {\n var sumOfValidNumbers = 0\n var invalidNumbers = 0\n for (number in numbers) {\n if (number != null) {\n sumOfValidNumbers += number\n } else {\n invalidNumbers++\n }\n }\n println("Sum of valid numbers: $sumOfValidNumbers")\n println("Invalid numbers: $invalidNumbers")\n}\n\nfun main() {\n val input = """\n 1\n abc\n 42\n """.trimIndent()\n val numbers = readNumbers(input)\n addValidNumbers(numbers)\n // Sum of valid numbers: 43\n // Invalid numbers: 1\n}\n filterNotNull을 사용해 간단하게 만들 수 있다.fun addValidNumbers(numbers: List<Int?>) {\n val validNumbers = numbers.filterNotNull()\n println("Sum of valid numbers: ${validNumbers.sum()}")\n println("Invalid numbers: ${numbers.size - validNumbers.size}")\n}\n 코틀린의 컬렉션 인터페이스는 읽기 전용(read-only) 과 변경 가능(mutable) 두 가지 버전이 있다.
\nfun <T> copyElements(source: Collection<T>, target: MutableCollection<T>) {\n for (item in source) {\n target.add(item)\n }\n}\n fun <T> copyElements(source: Collection<T>, target: MutableCollection<T>) {\n for (item in source) {\n target.add(item)\n }\n}\n\nfun main() {\n val source: Collection<Int> = arrayListOf(3, 5, 7)\n val target: Collection<Int> = arrayListOf(1)\n copyElements(source, target) // 컴파일 에러 발생\n}\n 모든 코틀린 컬렉션은 그에 상응하는 자바 컬렉션 인터페이스의 인스턴스이다.
\njava.util 패키지에 있는 자바 컬렉션 인터페이스의 구조와 같다.| 컬렉션 타입 | \n읽기 전용 타입 | \n변경 가능 타입 | \n
|---|---|---|
List | \nlistOf, List | \nmutableListOf, MutableList | \n
Set | \nsetOf | \nmutableSetOf, MutableSet 등 | \n
Map | \nmapOf | \nmutableMapOf, MutableMap 등 | \n
public class CollectionUtils {\n public static List<String> upperCaseAll(List<String> items) {\n for (int i = 0; i < items.size(); i++) {\n items.set(i, items.get(i).toUpperCase());\n }\n return items;\n }\n}\n fun printInUpperCase(list: List<String>) {\n println(CollectionUtils.upperCaseAll(list))\n println(list.first())\n}\n\nfun main() {\n val list = listOf("a", "b", "c")\n printInUpperCase(list)\n}\n Collection을 넘겨도 코틀린 컴파일러가 이를 막을 수 없다.public interface FileContentProcessor {\n void processContents(File path, byte[] binaryContents, List<String> textContents);\n}\n class FileIndexer : FileContentProcessor {\n override fun processContents(path: File, binaryContents: ByteArray?, textContents: MutableList<String>?) {\n TODO("Not yet implemented")\n }\n}\n interface DataParser<T> {\n void parseData(\n String input,\n List<T> output,\n List<String> errors\n );\n}\n class PersonParser : DataParser<Person> {\n override fun parseData(input: String, output: MutableList<Person>, errors: MutableList<String?>) {\n TODO("Not yet implemented")\n }\n}\n fun main(args: Array<String>) {\n for (i in args.indices) {\n println("Argument $i is: ${args[i]}")\n }\n}\n fun main() {\n val letters = Array<String>(26) { i -> ('a' + i).toString() }\n println(letters.joinToString(""))\n}\n fun main() {\n val letters = Array(26) { i -> ('a' + i).toString() }\n println(letters.joinToString(""))\n}\n fun main() {\n val strings = listOf("a", "b", "c")\n println("%s/%s/%s".format(*strings.toTypedArray())) // 스프레드 연산자 사용\n}\n val fiveZeros = IntArray(5)\nval fiveZerosToo = intArrayOf(0, 0, 0, 0, 0)\n\nfun main() {\n val squares = IntArray(5) { i -> (i + 1) * (i + 1) }\n println(squares.joinToString())\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