Scala best way of turning a Collection into a Map-by-key?

If I have a collection c of type T and there is a property p on T (of type P, say), what is the best way to do a map-by-extracting-key? val c: Collection[T] val m: Map[P, T] One way is the following: m = new HashMap[P, T] c foreach { t => m add (t.getP, t) } But now I need a mutable map. Is there a better way of doing this so that it's in 1 line and I end up with an immutable Map? (Obviously I cou