Having a Multimap sorted on keys only in Java

Question

I would like to have a c.g.c.c.Multimap that is sorted based on keys only. The values shouldn't be sorted. I've tried to build something with guava's TreeMultimap, but I can't use it because the value type doesn't implement Comparable.

public class MyObject /* doesn't implement Comparable */ {
  private String name;
  private int score;
  // Getters/setters are implemented
  public static Function<MyObject,Integer> myObjectToScore {
    @Override public Integer apply (MyObject o) { return o.score; }
  }
  public static Multimap<Integer,MyObject> indexOnScore(Iterable<MyObject> i) {
    Multimap<Integer,MyObject> m = Multimaps.index(i, myObjectToScore());
    // Do the sort of the keys.
    return m;
  }
}

I've thought about getting a SortedSet of the keys, then iterating over each of these keys in the sorted set to fetch the various values, but I was hoping using an existing (yet undiscovered) feature in Guava rather than using this kind of hack.

Note: I won't make MyObject implement Comparable because it makes no sense with my actual object.

---

Example of input/output:

Set<MyObject> s = Sets.newHashSet(
  new MyObject("a", 2),
  new MyObject("b", 3),
  new MyObject("c", 1),
  new MyObject("d", 3),
  new MyObject("e", 1)
); // Assuming constructor MyObject(String name, int score)

for (Map.Entry<Integer, MyObject> e: MyObject.indexedOnScore(s).entries()) {
  System.out.printf("%d -> %s%n", e.getKey(), e.getValue().getName());
}

Prints:

1 -> c // or switched with line below
1 -> e
2 -> a
3 -> b // or switched with line below
3 -> d

Answer 1

Multimaps.index returns an ImmutableListMultimap, so you wouldn't be able to sort it after creating it. You could, however, first create a sorted copy of your Iterable<MyObject> and feed that to Multimap.index... ImmutableListMultimap keeps things in the same order it was given them.

public static ImmutableMultimap<Integer, MyObject> indexOnScore(Iterable<MyObject> i) {
  List<MyObject> sorted = Ordering.natural().onResultOf(myObjectToScore())
      .sortedCopy(i);
  return Multimaps.index(sorted, myObjectToScore());
}

Another option might be to create a TreeMultimap and use Ordering.arbitrary() as the Comparator for the values.