Kotlin Tutorial · Object-Oriented Kotlin · Lesson 16 of 31

Kotlin Enums and Sealed Classes

Model fixed sets of values with enum classes and closed hierarchies with sealed classes and exhaustive when.

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enum class

Enums can have properties and functions. entries lists the values and valueOf parses a name.

Enum classes

enum class Plan(val monthlyPrice: Double) {
    FREE(0.0), PRO(10.11), TEAM(25.0);

    fun yearly() = monthlyPrice * 12
}

fun main() {
    for (plan in Plan.entries) println("${plan.name} ${plan.ordinal} ${"%.2f".format(plan.yearly())}")
    println(Plan.valueOf("PRO").monthlyPrice)
}

Output

FREE 0 0.00
PRO 1 121.32
TEAM 2 300.00
10.11

Run this on your computer — see “Get Started” for setup.

Sealed classes and interfaces

A sealed hierarchy has a fixed set of subtypes known at compile time, which can carry different data. A when over a sealed type is exhaustive without else: add a subtype and the compiler shows every when you need to update. This is the standard way to model results and UI state.

Sealed hierarchies and exhaustive when

sealed interface LoadState {
    data object Loading : LoadState
    data class Success(val items: List<String>) : LoadState
    data class Error(val message: String) : LoadState
}

fun render(state: LoadState): String = when (state) {     // exhaustive: no else needed
    LoadState.Loading -> "Spinner"
    is LoadState.Success -> "List of ${state.items.size} items"
    is LoadState.Error -> "Error: ${state.message}"
}

fun main() {
    listOf(LoadState.Loading, LoadState.Success(listOf("a", "b")), LoadState.Error("offline"))
        .forEach { println(render(it)) }
}

Output

Spinner
List of 2 items
Error: offline

Run this on your computer — see “Get Started” for setup.

Test yourself

Why doesn't a when over a sealed type need an else branch?

else is optional everywhere
The compiler knows every subtype, so it can check the when is exhaustive
Sealed classes can't be used in when
It returns null by default

Show answer

Answer: The compiler knows every subtype, so it can check the when is exhaustive

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