implement multimap in scala

LasMultiMap.scala

package pis.chap22.multimap
import scala.collection.immutable.HashMap

class LasMultiMap[A, B] {
    var mmap = new HashMap[A, List[B]]
	def put(key:A, value:B) = {
	  if (mmap.contains(key)) {
	    mmap = mmap + ( (key, mmap.get(key).toList(0) ::: List(value) ))
	  } else {
	    mmap = mmap + ( (key, List(value)) )
	  }
	}
    
    def apply(key:A):List[B] = {
      if (mmap.get(key).isEmpty) {
        Nil
      } else {
        mmap.get(key).toList(0)
      }
    }
    
    def update(key:A, value:B) = {
      put(key, value)
    }
    
    def keySet() = {
      mmap.keySet
    }
}