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【Kotlin】seale class 與 enum 的比較

差異比較

特性 Sealed Class Enum
使用情景 一組相關但不同的狀態或事件 一組固定的常量值
子類數量 可以有多個不同類型的子類 固定數量的常量
可擴展性 子類必須在相同文件中擴展 無法擴展，固定常量
狀態 可以有狀態（文字和屬性） 通常是不可變的
設計靈活性 更靈活，可以有複雜的層次結構 固定簡單
安全性 編譯期檢查未處理的分支 固定選項無需額外檢查 
enum class MachineState {
    START, SLEEP, RUNNING
}


fun machineStateChecker(state: MachineState) = when (state) {
    MachineState.START -> "Machine is about to start"
    MachineState.SLEEP -> "Machine is sleeping"
    MachineState.RUNNING -> "Machine is running"
}
  
fun main(argv: Array<String>) {
    val machineState = MachineState.SLEEP
    val state =  machineStateChecker(machineState)
    println("current state = $state")
    //current state = Machine is sleeping
}
sealed class WorkingState {
    // data class
    data class Finished(val result: List<String>): WorkingState()
    data class ErrorHappened(val whatHappened: String): WorkingState()
    // object
    object Working: WorkingState()
    object EmptyResult: WorkingState()
}


fun machineStatePrinting(workingState: WorkingState) {
    when (workingState) {
        is WorkingState.Finished -> {
            if (workingState.result.isNotEmpty()) {
                for ((index, text) in workingState.result.withIndex()) {
                    println("the ${index + 1} result is $text")
                }
            }
        }
        is WorkingState.ErrorHappened -> {
            println("""
                Error Occurred: reason is
                ${workingState.whatHappened}
            """.trimIndent())
        }
        WorkingState.Working -> {
            println("Printing machine is working...")
        }
        WorkingState.EmptyResult -> {
            println("It's empty result.")
        }
    }
}

fun main(argv: Array<String>) {
     val machineState =
            WorkingState.Finished(
                mutableListOf(
                    "print doc 1",
                    "print doc 2",
                    "print doc 3",
                    "print doc 4"
                )
            )
        machineStatePrinting(machineState)
  		// the job1 result is print doc 1
		// the job2 result is print doc 2
        // the job3 result is print doc 3
        // the job4 result is print doc 4

        val secondMachineState = WorkingState.ErrorHappened("no papper")
        machineStatePrinting(secondMachineState)
  		// Error Occurred: reason is no papper

        val thirdMachineState = WorkingState.Working
        machineStatePrinting(thirdMachineState)
  		// Printing machine is working...
}

來源：https://louis383.medium.com/kotlin-sealed-classes-%E7%9A%84%E5%9F%BA%E7%A4%8E%E4%BD%BF%E7%94%A8-de660dbb63d2

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