is there a way to enumerate all subclasses of a given class and instantiate single instance of each?...

# announcements

[Karlo Lozovina]

is there a way to enumerate all subclasses of a given class and instantiate single instance of each? without going into too much metaprogramming etc...

[nanodeath]

well, even Kotlin doesn't really support metaprogramming. you'll need reflection though.

[nanodeath]

Class.newInstance(), IIRC, is the Java way. must be something similar for Kotlin, or you can just do that

[Russ Tennant]

I’ve done this with sealed classes

[Russ Tennant]

You can iterate over sealedSubclasses or nestedClasses. If you have abstracts in the hierarchy, check and skip.

[Russ Tennant]

Then do something like

private fun getInstance(commandClass: KClass<out ControllerCommand>): ControllerCommand {
    val objectInstance = commandClass.objectInstance
    if (objectInstance != null)
        return objectInstance
    val primaryConstructor = commandClass.primaryConstructor
    if (primaryConstructor != null)
        return primaryConstructor.call()
    throw IllegalStateException("$commandClass must be object or have primary constructor.")
}

[Karlo Lozovina]

Thanks, got it working! Combination of sealedSubclasses and objectInstance

[Karlo Lozovina]

not super familiar with Kotlin/JVM, but this is all happening at runtime, so there's a slight performance penalty?

[Karlo Lozovina]

it's not some metaprogramming happening at compile time?

[Russ Tennant]

I believe it is using reflection (runtime).

[Karlo Lozovina]

👍

[nanodeath]

Kotlin supports some amount of reflection without the kotlin-reflect library, btw. and yeah, you'll want to cache these if you can.