sort and group a java collection

Question

I have an object which has a name and a score. I would like to sort a collection of such objects so that they are grouped by name and sorted by maximum score in each group (and

Answer 1

Foreach over the collection, and put the objects into a Map>, keyed by name, where the SortedSet is a TreeSet with a custom comparator that compares by score.

Then foreach over the map's values() collection, and put the groups into a SortedSet>, with a second custom comparator that compares SortedSets according to their largest element. Actually, instead of foreaching, you can simply use addAll().

Here's the code:

public class SortThings {

    static class Thing {
        public final String name;
        public final int score;
        public Thing(String name, int score) {
            this.name = name;
            this.score = score;
        }
        @Override
        public String toString() {
            return "(" + name + ", " + score + ")";
        }
    }

    public static void main(String[] args) {
        Collection things = Arrays.asList(
            new Thing("a", 3),
            new Thing("a", 9),
            new Thing("b", 7),
            new Thing("b", 10),
            new Thing("c", 8),
            new Thing("c", 3)
        );

        SortedSet> sortedGroups = sortThings(things);

        System.out.println(sortedGroups);
    }

    private static SortedSet> sortThings(Collection things) {
        final Comparator compareThings = new Comparator() {
            public int compare(Thing a, Thing b) {
                Integer aScore = a.score;
                Integer bScore = b.score;
                return aScore.compareTo(bScore);
            }
        };

        // first pass
        Map> groups = new HashMap>();
        for (Thing obj: things) {
            SortedSet group = groups.get(obj.name);
            if (group == null) {
                group = new TreeSet(compareThings);
                groups.put(obj.name, group);
            }
            group.add(obj);
        }

        // second pass
        SortedSet> sortedGroups = new TreeSet>(new Comparator>() {
            public int compare(SortedSet a, SortedSet b) {
                return compareThings.compare(a.last(), b.last());
            }
        });
        sortedGroups.addAll(groups.values());
        return sortedGroups;
    }

}

Note that the output is in smallest-to-largest order. That's the natural order with Java's collections; it would be trivial to modify this to sort the other way if that's what you need.