Update mutable HashMap value which is a mutable co

I'm have a map that looks like this: Map[ A -> Collection[B]]. This map gets updated in a loop - the special thing is however, that updates mostly just mean adding an element B to the Collection[B] (for some key A).

I am trying to find out if I can get some speedup by changing the type of my Collection from List[ ] to ListBuffer[ ].

Up to now my code looked like this (simplified):

var incoming = new HashMap[A, List[B]() {
  override def default(a: A) = List()
}
..
for(b < someCollectionOfBs){
  ..
  incoming(b.getA) = b :: incoming(b.getA)
  ..
}

This works fine. Now, I changed the type of the map so it looks like this:

var incoming = new collection.mutable.HashMap[A, ListBuffer[B]() {
  override def default(a: A) = collection.mutable.ListBuffer()
}
..
for(b < someCollectionOfBs){
  ..
  incoming(b.getA) += b
  ..
}

Note the change in how the element B is added to the collection in the 2nd example (no more immutable List, hence we do not need to create and assign new collection...).

But. This does not work: incoming(X) += .. does not update the value of the map for X, actually it does not change anything.

What am I missing here? I thought that I should be able to update the values of a mutable HashMap... So, if my values are mutable collections, why can't I just add elements to those?

The default is returned when the key is not found, but it does not update the map with the default value. You can use getOrElseUpdate for that.

incoming.getOrElseUpdate(b.getA, ListBuffer()) += b

That should do what you want.

Additional note:

If you're concerned about performance, I don't think replacing List with ListBuffer will buy you much, because you are prepending to a List and that should be very fast. ListBuffer is handy when you want to append to a list. You should look at using java.util.HashMap and see if that helps.