sealed class vs. sealed interface: Wann welche Variante verwendet wird

Sealed-Typen in Kotlin

Grundlagen von sealed class

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

sealed interface: Mehr Flexibilität

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

Wichtige Unterschiede

// 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

Vollständiges when: Kein else erforderlich

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

Das Hinzufügen eines Subtyps macht vorhandene when-Ausdrücke ungültig

// 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!

Sealed classes mit gemeinsamen Eigenschaften

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()

Verschachtelte sealed-Typen

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

object-Subtypen für Varianten ohne Zustand

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 zusammen verwenden

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

Wann sealed class und wann interface?

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Zusammenfassung