How to obtain all subclasses of a given sealed class?

In Kotlin 1.3+ you can use sealedSubclasses.

In prior versions, if you nest the subclasses in your base class then you can use nestedClasses:

Base::class.nestedClasses

If you nest other classes within your base class then you'll need to add filtering. e.g.:

Base::class.nestedClasses.filter { it.isFinal && it.isSubclassOf(Base::class) }

Note that this gives you the subclasses and not the instances of those subclasses (unlike Enum.values()).

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With your particular example, if all of your nested classes in State are your object states then you can use the following to get all of the instances (like Enum.values()):

State::class.nestedClasses.map { it.objectInstance as State }

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And if you want to get really fancy you can even extend Enum<E: Enum<E>> and create your own class hierarchy from it to your concrete objects using reflection. e.g.:

sealed class State(name: String, ordinal: Int) : Enum<State>(name, ordinal) {
    companion object {
        @JvmStatic private val map = State::class.nestedClasses
                .filter { klass -> klass.isSubclassOf(State::class) }
                .map { klass -> klass.objectInstance }
                .filterIsInstance<State>()
                .associateBy { value -> value.name }

        @JvmStatic fun valueOf(value: String) = requireNotNull(map[value]) {
            "No enum constant ${State::class.java.name}.$value"
        }

        @JvmStatic fun values() = map.values.toTypedArray()
    }

    abstract class VanillaState(name: String, ordinal: Int) : State(name, ordinal)
    abstract class ChocolateState(name: String, ordinal: Int) : State(name, ordinal)

    object StateA : VanillaState("StateA", 0)
    object StateB : VanillaState("StateB", 1)
    object StateC : ChocolateState("StateC", 2)
}

This makes it so that you can call the following just like with any other Enum:

State.valueOf("StateB")
State.values()
enumValueOf<State>("StateC")
enumValues<State>()

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UPDATE

Extending Enum directly is no longer supported in Kotlin. See Disallow to explicitly extend Enum class : KT-7773.

Full example:

sealed class State{
    companion object {
        fun find(state: State) =
            State::class.sealedSubclasses
                    .map { it.objectInstance as State}
                    .firstOrNull { it == state }
                    .let {
                        when (it) {
                            null -> UNKNOWN
                            else -> it
                        }
                    }
    }
    object StateA: State()
    object StateB: State()
    object StateC: State()
    object UNKNOWN: State()

}

With Kotlin 1.3+ you can use reflection to list all sealed sub-classes without having to use nested classes: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.reflect/-k-class/sealed-subclasses.html

I asked for some feature to achieve the same without reflection: https://discuss.kotlinlang.org/t/list-of-sealed-class-objects/10087