How to call the correct method in Scala/Java based the types of two objects without using a switch statement?

Question

I am currently developing a game in Scala where I have a number of entities (e.g. GunBattery, Squadron, EnemyShip, EnemyFighter) that all inherit from a GameEntity class. Ga

Answer 1

You can easily implement multiple dispatch in Scala, although it doesn't have first-class support. With the simple implementation below, you can encode your example as follows:

object Receive extends MultiMethod[(Entity, Message), String]("")

Receive defImpl { case (_: Fighter, _: Explosion) => "fighter handling explosion message" }
Receive defImpl { case (_: PlayerShip, _: HullBreach) => "player ship handling hull breach" }

You can use your multi-method like any other function:

Receive(fighter, explosion) // returns "fighter handling explosion message"

Note that each multi-method implementation (i.e. defImpl call) must be contained in a top-level definition (a class/object/trait body), and it's up to you to ensure that the relevant defImpl calls occur before the method is used. This implementation has lots of other limitations and shortcomings, but I'll leave those as an exercise for the reader.

Implementation:

class MultiMethod[A, R](default: => R) {
  private var handlers: List[PartialFunction[A, R]] = Nil

  def apply(args: A): R = {
    handlers find {
      _.isDefinedAt(args)
    } map {
      _.apply(args)
    } getOrElse default
  }

  def defImpl(handler: PartialFunction[A, R]) = {
    handlers +:= handler
  }
}