sealed classとsealed interface：使い分け

Kotlin の sealed 型

sealed class の基本

sealed class Shape
class Circle(val radius: Double) : Shape()
class Rectangle(val w: Double, val h: Double) : Shape()
object Unknown : Shape()

sealed interface：より柔軟

sealed interface Result<out T>
data class Success<T>(val value: T) : Result<T>
data class Error(val ex: Exception) : Result<Nothing>

主な違い

// sealed class subtype extends another class:
sealed interface Command
class MoveCommand(val dx: Int, val dy: Int) : Command, Serializable
class ResizeCommand(val scale: Double) : Command, Cloneable

網羅的な when：else は不要

fun area(shape: Shape): Double = when (shape) {
    is Circle -> Math.PI * shape.radius * shape.radius
    is Rectangle -> shape.w * shape.h
}  // no 'else' — compiler guarantees completeness

サブタイプの追加で既存の when が壊れる

// Add Triangle to Shape:
class Triangle(val base: Double, val height: Double) : Shape()
// Now the area() function above won't compile until
// a Triangle branch is added — safety net!

共通プロパティを持つ sealed class

sealed class NetworkResult {
    abstract val timestamp: Long
}
data class SuccessResult(val data: String, override val timestamp: Long) : NetworkResult()
data class FailureResult(val error: String, override val timestamp: Long) : NetworkResult()

ネストされた sealed 型

sealed interface UiState
object Loading : UiState
sealed interface Loaded : UiState
data class Content(val items: List<String>) : Loaded
data class Empty(val hint: String) : Loaded
data class Failure(val error: String) : UiState

状態を持たないバリアントには object サブタイプ

sealed class Direction
object North : Direction()
object South : Direction()
object East : Direction()
object West : Direction()

fun move(d: Direction) = when (d) {
    North -> y++
    South -> y--
    East -> x++
    West -> x--
}

sealed、data class、object の組み合わせ

sealed class LoadState
object Idle : LoadState()
object Loading : LoadState()
data class Success(val count: Int) : LoadState()
data class Error(val msg: String) : LoadState()

sealed class と interface の選び方

クイックチェック

まとめ