Groovy vs. Google Collections, Round 1

In response to my comment, Matt Taylor challenged me to a "code-off" between Groovy and Google Collections.

Here is my response to his first challenge.

1. Create a set of boys and girls.

Set<String> boys = Sets.newHashSet("Paco", "Sven", "Roger", "Emelio");
Set<String> girls = Sets.newHashSet("Julia", "Prudence", "Lucy");

2. Discover every combination of them.

Multimap<String, String> pairs = combinations(boys, girls);

static Multimap<String, String> combinations(
 Set<String> set1, Set<String> set2) {
 Multimap<String, String> ret = Multimaps.newHashMultimap();
 for (String s1 : set1) {
     for (String s2 : set2) {
         ret.put(s1, s2);
     }
 }
 return ret;
}

3. Group the combinations by boy and girl.

for (String boy : boys) {
 for (String partner : pairs.get(boy)) {
     ...
 }
}

Multimap<String,String> pairsInverse = Multimaps.inverseHashMultimap(pairs);
for (String girl : girls) {
 for (String partner : pairsInverse.get(girl)) {
     ...
 }
}

Here is the full source code of the Google Collections version, courtesy Pastie.

Google Collections doesn't provide a combinations function like Groovy does. It would be nice to have the methods combinations(Set<T> ...) and permutations(Set<T> ...) in the Sets utility class.