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()).

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 }

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>()

UPDATE

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