Question

Scala’s Ordering trait has a method reverse which seems to be the “official” way to get a TreeMap which is sorted “the wrong” way.

The snippet of the trait looks like this:

trait Ordering[T] extends Comparator[T] with PartialOrdering[T] with Serializable {
  outer =>

  override def reverse: Ordering[T] = new Ordering[T]{
    override def reverse = outer
    def compare(x: T, y: T) = outer.compare(y, x)
  }
  /*snip*/
}

I thought it would work comparable to Java’s Collections.reverseOrder, but Ordering.reverse doesn’t work of course.

How can I use the reverse Ordering with a TreeMap, e. g.:

new TreeMap[Foo, Bar](/*???*/)

Answer 1

new TreeMap[Foo, Bar]()(implicitly[Ordering[Foo]].reverse)

Assuming, of course, that Ordering[Foo] is implicitly available (such as Ordering[Int] or Ordering[String]). If you have it defined as object X, just pass X.reverse instead.

Note that the first set of parenthesis here must be empty — it is only the second set of parenthesis that receive the implicit parameter in this particular case.