first part of r22260

1 files changed, 8 insertions, 7 deletions

diff --git a/src/library/scala/collection/TraversableLike.scala b/src/library/scala/collection/TraversableLike.scala
index 04d9002eae..2169dcdd02 100644
--- a/src/library/scala/collection/TraversableLike.scala
+++ b/src/library/scala/collection/TraversableLike.scala
@@ -330,17 +330,18 @@ trait TraversableLike[+A, +Repr] extends HasNewBuilder[A, Repr]
    *               for which `f(x)` equals `k`.
    *
    */
-  def groupBy[K](f: A => K): Map[K, Repr] = {
-    var m = Map[K, Builder[A, Repr]]()
+  def groupBy[K](f: A => K): immutable.Map[K, Repr] = {
+    val m = mutable.Map.empty[K, Builder[A, Repr]]
     for (elem <- this) {
       val key = f(elem)
-      val bldr = m get key match {
-        case None => val b = newBuilder; m = m updated (key, b); b
-        case Some(b) => b
-      }
+      val bldr = m.getOrElseUpdate(key, newBuilder)
       bldr += elem
     }
-    m map { case (k, b) => (k, b.result) }
+    val b = immutable.Map.newBuilder[K, Repr]
+    for ((k, v) <- m)
+      b += ((k, v.result))
+
+    b.result
   }
 
   /** Tests whether a predicate holds for all elements of this $coll.