2. 가독성

//case A if (moveStrategy.movable()) { racingCar.moveStraight() } else { racingCar.stop() } //case B racingCar.takeIf { moveStrategy.movable() } ?.run(racingCar::moveStraight) ?: racingCar.stop()

//case A if (moveStrategy.movable()) { racingCar.moveStraight() observers.notifyMove(racingCar) } else { racingCar.stop() observers.notifyStop() } //case B racingCar.takeIf { moveStrategy.movable() } ?.run { it.moveStraight() observers.notifyMove(it) }?: run { racingCar.stop() observers.notifyStop() }

data class RacingCar(val distance: Distance) var racingCar: RacingCar? = null fun moveCar() { racingCar?.let { car -> //car는 RacingCar? 에서 RacingCar로 스마트 캐스팅 된다. // logic... } }

val abc = "A" { "B" } and "C" print(abc) //ABC operator fun String.invoke(f: ()-> String): String = this + f() infix fun String.and(s: String) = this + s

fun Int.factorial(): Int = (1..this).product() fun Iterable<Int>.product(): Int = fold(1) { acc, i -> acc * i}

print(10 * 6.factorial()) // 7200

10 * 6!

operator fun Int.not() = factorial() print(10 * !6) // 7200

data class Car(private var distance: Distance) { fun move() { distance += SPEED } fun getDistance(): Distance = Distance(distance.value) } val racingCar = Car(distance = Distance(0)) 3 * { racingCar.move() } print(racingCar.getDistance()) // Distance(value: 3)

infix fun Int.timesRepeated(operation: () -> Unit) = { repeat(this) { operation() } } val racingCar = Car(distance = Distance(0)) 3 timesRepeated {racingCar.move() } print(racingCar.getDistance()) // Distance(value: 3)

body { div { +"Some text" } }

fun verifyKey(key: String): Unit? = //... verifyKey(key) ?: return

fun verifyKey(key: String): Boolean = //... if(!verifyKey(key) { return; }

val num = 10 val name = "catsbi" val ids = listOf(1, 2, 3, 4, 5)

val data = getSomeData()

class Node(val name: String) { fun makeChild(childName: String) = create("$name.$childName") .apply { print("Created ${name}") } fun create(name: String): Node? = Node(name) } fun main() { val node = Node("parent") node.makeChild("child") }

class Node(val name: String) { fun makeChild(childName: String) = create("$name.$childName") .apply { print("Created ${this?.name}") } // <-- 요기에 리시버 명시 fun create(name: String): Node? = Node(name) } fun main() { val node = Node("parent") node.makeChild("child") // Created parent.child }

class Node(val name: String) { fun makeChild(childName: String) = create("$name.$childName") .also { print("Created ${it?.name}") } // <-- 요기에 리시버 명시 fun create(name: String): Node? = Node(name) } fun main() { val node = Node("parent") node.makeChild("child") // Created parent.child }

class Node(val name: String) { fun makeChild(childName: String) = create("$name.$childName") .apply{ print("Created ${this?.name} in ${this@Node.name}") } fun create(name: String): Node? = Node(name) } fun main() { val node = Node("parent") node.makeChild("child") // Created parent.child in parent }

table { tr { td { + "Column 1" } td { + "Column 2" } } tr { td { + "Value 1" } td { + "Value 2" } tr { td { + "Column 1" } td { + "Column 2" } } } }

@DslMarker annotation class HtmlDsl fun table(f: TableDsl.() -> Unit) { /*..*/ } @HtmlDsl class TableDsl { /*..*/ }

table { tr { td { + "Column 1" } td { + "Column 2" } } tr { td { + "Value 1" } td { + "Value 2" } tr { // Compile Error td { + "Column 1" } td { + "Column 2" } } } }

table { tr { td { + "Column 1" } td { + "Column 2" } } tr { td { + "Value 1" } td { + "Value 2" } this@table.tr { td { + "Column 1" } td { + "Column 2" } } } }

//kotlin var name: String? = null //java String name = null;

//java @AllArgsConstructor class Foo { String name; }

class Foo(name: String?) { var name: String? = name } fun main() { var foo = Foo("catsbi") println(foo.name) // ?? }

var date: Date get() = Date(millis) set(value) { millis = value.time }

interface Person { val name: String } class Catsbi : Person { override val name: String get() = TODO("Not yet implemented") } open class Parent { open val firstName: String = "KIM" } class Child: Parent() { override val firstName: String = "LEE" }

val db: Database by lazy { connectToDb() }

val Developer.language: DevelopLanguage get() = DevelopLanguage.JAVA val Developer.salary: Long get() = SalaryAnalysticsApi.findByLevel("JUNIOR") val Developer.keyboard: Keyboard get() = Keyboard(name = "moonlander", switches = KeySwitches.MX_BROWN)

//worst use case val Tree<Int>sum: Int get() = when (this) { is Leaf -> value is Node -> left.sum + right.sum }

//good use case val Tree<Int>sum: Int = when (this) { is Leaf -> value is Node -> left.sum + right.sum }

data class NutritionalInformation( private val calorie: Int, //칼로리 - 필수 private val sodium: Int = 0, //나트륨 private val carbohydrate: Int, //탄수화물 - 필수 private val sugars: Int = 0, //당류 private val fat: Int, //지방 - 필수 private val transFat: Int, //트랜스지방 private val saturatedFat: Int = 0, //포화지방 private val cholesterol: Int = 0, //콜레스테롤 private val protein: Int //단백질 - 필수 )

//Java 였다면.... NutritionalInformation info = new NutritionalInformation( 377, 115, 15, 4, 11, 0, 4, 0, 2 )

int calorie = 377; int sodium = 115; int carbohydrate =15; int sugars = 4; int fat = 11; int transFat = 0; int saturatedFat = 4; int cholesterol =0; int protein =2; //local variable 사용 NutritionalInformation info = new NutritionalInformation( calorie,sodium,carbohydrate,sugars,fat,transFat,saturatedFat,cholesterol,protein ) //builder pattern 사용 NutritionalInformation info = NutritionalInformation.builder() .calorie(377) .sodium(115) .carbohydrate(15) .sugars(4) .fat(11) .transFat(0) .saturatedFat(4) .cholesterol(0) .protein(2) .build();

//kotlin named parameter val info = NutritionalInformation( calorie = 377, sodium = 115, carbohydrate = 15, sugars = 4, fat = 11, transFat = 0, saturatedFat = 4, cholesterol = 0, protein = 2 )

fun sendEmail(to: String, message: String)

sendEmail( to = "catsbi@midasin.com" message = "hello, ..." )

repeat(5) { println("hello,") } //hello,hello,hello,hello,hello,

observable.getUsers() .subscribeBy( onNext = { users: List<User> -> ...}, onError = {throwable: Throwable -> ...}, onCompleted = { ...} )