Scala BiMap - a bidirectional map for Scala

BiMap.scala

import scala.collection.mutable.Map
 
class BiMap[A,B] extends BiMapHelper(Map[A,B](), Map[B,A]()) {
 
}
 
object BiMap {
 
   def apply[A,B](elems:(A,B)*) = new BiMap[A,B] ++= elems
 
}
 
/**
 * This allows a bi-directional map to be created from any two maps.
 * These maps must be the inverse of each other to work.
 */
class BiMapHelper[A,B](forward:Map[A,B], backward:Map[B,A]) extends Map[A,B] {
 
  def inverse = new BiMapHelper(backward, forward)
 
  def get(key:A) = forward get key
 
  def iterator = forward.iterator
 
  def +=(kv:(A,B)) = {
    forward += kv
    backward += kv.swap
    this
  }
 
  def -=(key:A) = {
    backward --= (forward get key)
    forward -= key
    this
  }
 
  override def empty = {
    forward.empty
    backward.empty
    this
  }
 
  override def size = forward.size
}
 
object BiMapHelper {
 
  def apply[A,B](forward:Map[A,B], backward:Map[B,A]) = new BiMapHelper(forward, backward)
 
}