Class to Instance Multimap in scala

Question

What's the idiomatic way to have associate Class[T]s to List[T]s? Essentially, I want the equivalent of Guava's ClassToInstanceMap, but in a Multimap form.

I am uncertain about how Scala's mutable.Multimap would fit in here.

Answer 1

There's nothing in the standard library, but it's quite trivial to implement (using ClassTags instead of Class to simplify the API a bit):

class ClassToInstanceMultiMap private (private val delegate: Map[ClassTag[_], List[_]]) {
  def addInstance[T](x: T)(implicit ct: ClassTag[T]) = 
    new ClassToInstanceMultiMap(delegate + (ct -> x))

  def getInstances[T](implicit ct: ClassTag[T]) = delegate.getOrElse(ct, List.empty).asInstanceOf[List[T]]

  // whatever other methods you want
}

object ClassToInstanceMultiMap {
  val empty = new ClassToInstanceMultiMap()
}

// usage
val cimm = ClassToInstanceMultiMap.empty.addInstance(1).addInstance(2).addInstance("a").getInstances[Int]
// returns List(1, 2)

Use TypeTags instead of ClassTags if you want to store instances with the same class and different generic type arguments (e.g. Option[Int] and Option[String]) separately.