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.

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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.

Answer 2

MultimapBuilder was introduced in Guava 16:

<K extends Comparable<? super K>, V> ListMultimap<K, V> multimap() {     return MultimapBuilder.treeKeys().linkedListValues().build(); } 

That keeps your keys sorted by their natural order (MultimapBuilder::treeKeys is also overloaded to accept a custom comparator), and the values associated with each key are maintained in a LinkedList (ArrayList and HashSet are among the other options).