Question

When I create an immutable map with a standard call to Map() or by concatenating the existing maps created that way, in all my tests I get that traversing its members provides them in the order of addition. That’s exactly the way I need them to be sorted, but there’s not a word in the documentation about the reliability of the ordering of the members of the map.

So I was wondering whether it is safe to expect the standard Map to return its items in the order of addition or I should look for some other implementations and which ones in that case.

Answer 1

I don’t think it’s safe, the order is not preserved starting from 5 elements (Scala 2.9.1):

scala> Map(1 -> 1, 2 -> 2, 3 -> 3, 4 -> 4, 5 -> 5)
res9: scala.collection.immutable.Map[Int,Int] = 
  Map(5 -> 5, 1 -> 1, 2 -> 2, 3 -> 3, 4 -> 4)

With bigger maps the order is completely “random”, try Map((1 to 100) zip (1 to 100): _*).

Try LinkedHashMap for ordered entries and TreeMap to achieve sorted entries.