# 【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 |
```JavaScript
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) {
val machineState = MachineState.SLEEP
val state = machineStateChecker(machineState)
println("current state = $state")
//current state = Machine is sleeping
}
```
```JavaScript
sealed class WorkingState {
// data class
data class Finished(val result: List): 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) {
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](https://louis383.medium.com/kotlin-sealed-classes-%E7%9A%84%E5%9F%BA%E7%A4%8E%E4%BD%BF%E7%94%A8-de660dbb63d2)