I've implemented a Minecraft-like game for assignm...

# codereview

[Ellen Spertus]

I've implemented a Minecraft-like game for assignments in a class I am teaching. Classes include Mob (abstract), Zombie, and Spider. I would like to create a class Spawner<T: Mob> to create, for example, Spider Spawners (blocks that periodically construct and place new instances of Spider). I'm trying to figure out the best way to instantiate a new instance of T from within Spawner<T>. I tried the following, but it doesn't work because of type erasure:

class Spawner<T: Mob> {
    fun spawn() {
        val constructor = T::class.constructors.find { it.parameters.isEmpty() } // error: Cannot use 'T' as reified type parameter
        val newMob = constructor?.call()
    }
}

I'd rather not require the students to create factory classes. (This is an introductory programming class.) Instead I was thinking of faking a factory with a copy() method:

class Spawner<T: Mob>(val mob: T) {
    fun spawn() {
        val newMob = mob.copy()
        // Place newMob on game board.
    }
}

abstract class Mob {
    abstract fun copy(): Mob
}

class Pigman: Mob() {
    override fun copy() = Pigman()
}

fun main() {
    val pigmanSpawner = Spawner<Pigman>(Pigman())
    pigmanSpawner.spawn()
}

Something I don't like about this is the type parameter isn't used and could be removed (if I changed the type of the mob property from T to Mob).

[ephemient]

class Spawner<out T: Mob>(val newMob: () -> T) {
    fun spawn() {
        val newMob = newMob()

val pigmanSpawner = Spaner(::Pigman)

with some better naming but that should be the idea

[Ellen Spertus]

Any ideas on how I can incorporate non-hacky generic types into the assignment? I could create a `Hunter<T>`but I have the same problem because of type erasure. I can't filterIsInstance<T>

[Matteo Mirk]

Alternatively you could make the spawner an inline function so you can use the generic type

inline fun <reified T : Mob> spawn() : T {
    val constructor = T::class.constructors.first { it.parameters.isEmpty() }
    return constructor.call()
}

fun main() {
    val z = spawn<Zombie>()
}

I replaced ‘find’ with ‘first’ that produces a non-nullable type or throws if the element is not found.

[ephemient]

why would you do that when you can just T::class.createInstance()

[ephemient]

but I'd avoid reflection for this use case, it doesn't fit that well

[Matteo Mirk]

Wow I wasn’t aware of that function, totally missed it! Thanks. Anyway yes, I didn’t realize your solution didn’t use reflection, cool!

[Ellen Spertus]

FYI, after I applied @ephemient’s advice to create Spawner<T> and tested it out with Spawner<Zombie>, I realized I could create Spawner<Spawner<Zombie>> and did so. 🧟