Converting mutable to immutable map

The immutable hierarchy doesn't contain a MultiMap, so you won't be able to use the converted structure with the same convenient syntax. But if you're happy to deal with key/valueset pairs, then:

If you just want a mutable HashMap, you can just use x.toMap in 2.8 or collection.immutable.Map(x.toList: _*) in 2.7.

But if you want the whole structure to be immutable--including the underlying set!--then you have to do more: you need to convert the sets along the way. In 2.8:

x.map(kv => (kv._1,kv._2.toSet)).toMap

In 2.7:

collection.immutable.Map(
  x.map(kv => (kv._1,collection.immutable.Set(kv._2.toList: _*))).toList: _*
)

scala> val mutableMap = new HashMap[Int, String]
mutableMap: scala.collection.mutable.HashMap[Int,String] = Map()

scala> mutableMap += 1 -> "a"
res5: mutableMap.type = Map((1,a))

scala> mutableMap
res6: scala.collection.mutable.HashMap[Int,String] = Map((1,a))

scala> val immutableMap = mutableMap.toMap
immutableMap: scala.collection.immutable.Map[Int,String] = Map((1,a))

scala> immutableMap += 2 -> "b"
<console>:11: error: reassignment to val
       immutableMap += 2 -> "b"
                ^