copy()와 부분 재정의로 복사하기

copy()를 사용한 불변 업데이트

기본 copy()

data class Person(val name: String, val age: Int, val city: String)
val alice = Person("Alice", 30, "Berlin")
val olderAlice = alice.copy(age = 31)
println(olderAlice)  // Person(name=Alice, age=31, city=Berlin)

여러 필드 복사

data class Config(val host: String, val port: Int, val ssl: Boolean, val timeout: Int)
val prod = Config("prod.server", 443, true, 30)
val test = prod.copy(host = "test.server", port = 8080, ssl = false)
println(test)

copy는 원본을 수정하지 않습니다

data class Point(val x: Int, val y: Int)
val origin = Point(0, 0)
val moved = origin.copy(x = 5)
println(origin)  // Point(x=0, y=0) — unchanged
println(moved)   // Point(x=5, y=0)

깊은 복사의 주의점

data class Team(val name: String, val members: MutableList<String>)
val t1 = Team("A", mutableListOf("Alice"))
val t2 = t1.copy(name = "B")
t2.members.add("Bob")       // modifies SHARED list!
println(t1.members)  // [Alice, Bob] ← also changed!

얕은 복사 문제 피하기

data class Team(val name: String, val members: List<String>)  // immutable list
val t1 = Team("A", listOf("Alice"))
val t2 = t1.copy(name = "B", members = t1.members + "Bob")
println(t1.members)  // [Alice]
println(t2.members)  // [Alice, Bob]

상태 관리에서의 copy

data class UiState(val loading: Boolean, val items: List<String>, val error: String?)
fun reduce(state: UiState, action: Action) = when (action) {
    is Loading -> state.copy(loading = true)
    is Success -> state.copy(loading = false, items = action.data)
    is Error -> state.copy(loading = false, error = action.msg)
}

기본 매개변수와 함께 사용하는 copy

data class Article(
    val id: Long, val title: String, val body: String,
    val published: Boolean = false, val views: Int = 0
)
val draft = Article(1L, "Hello", "Content...")
val published = draft.copy(published = true)
println(published.title)  // Hello

테스트에서 copy 사용하기

val baseUser = User(id = 1, name = "Test", email = "test@test.com", active = true)
val inactiveUser = baseUser.copy(active = false)
val renamed = baseUser.copy(name = "Renamed")

copy와 빌더 패턴 비교

// Java Builder:
User.builder().name("A").age(30).role("admin").build()
// Kotlin:
val u = User("A", 30, "admin")  // or copy from base

레코드 의미론

data class RGB(val r: Int, val g: Int, val b: Int)
val white = RGB(255, 255, 255)
println(white)  // RGB(r=255, g=255, b=255)

빠른 확인

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