companion object: Kotlin의 정적 멤버와 유사한 요소

static 없이 companion object 사용하기

companion object 선언하기

class MathUtils {
    companion object {
        fun square(x: Int) = x * x
    }
}
fun main() {
    println(MathUtils.square(5)) // 25
}

companion object의 상수

class HttpClient {
    companion object {
        const val DEFAULT_TIMEOUT = 5000
        const val USER_AGENT = "MyApp/1.0"
    }
}
fun main() {
    println(HttpClient.DEFAULT_TIMEOUT)
    println(HttpClient.USER_AGENT)
}

이름이 지정된 companion object

class Database {
    companion object Factory {
        fun create() = Database()
    }
}
fun main() {
    val db1 = Database.create()       // works
    val db2 = Database.Factory.create() // also works
    println("$db1 / $db2")
}

팩토리 메서드 패턴

class User private constructor(val name: String, val email: String) {
    companion object {
        fun create(name: String, email: String): User {
            require(email.contains("@")) { "invalid email" }
            return User(name, email)
        }
    }
}
fun main() {
    val u = User.create("Ada", "ada@example.com")
    println("${u.name} <${u.email}>")
}

인터페이스 구현

interface Loadable<T> { fun load(): T }
class Config(val data: String) {
    companion object : Loadable<Config> {
        override fun load() = Config("loaded data")
    }
}
fun main() {
    println(Config.load().data)
}

Java 연동을 위한 @JvmStatic

class Util {
    companion object {
        @JvmStatic
        fun greet(name: String) = "Hello, $name!"
    }
}
fun main() {
    println(Util.greet("Kotlin"))
    // In Java: Util.greet("Java")
}

동반 객체 자체에 접근하기

class Counter {
    companion object {
        fun greet() = "hi from companion"
    }
}
fun main() {
    val ref = Counter.Companion
    println(ref.greet())
}

동반 객체와 최상위

// File-level — usually cleaner:
fun greet(name: String) = "Hi, $name!"
// Versus:
class Greeter { companion object { fun greet(n: String) = "Hi, $n!" } }
fun main() {
    println(greet("Ada"))
    println(Greeter.greet("Ada"))
}

클래스마다 동반 객체 하나

class Service {
    companion object {
        val version = "1.0"
    }
    object InternalHelper { fun log() = println("internal") }
}
fun main() {
    println(Service.version)
    Service.InternalHelper.log()
}

동반 객체 확장 함수

class Lib {
    companion object
}
fun Lib.Companion.helper() = "added later"
fun main() {
    println(Lib.helper())
}

빠른 확인

요약