Google colleciton Library の使用は、とにかく待望のMultimapであろう。
Multimap<Integer,String> map = ArrayListMultimap.create();
map.put(1,"1A");
map.put(1,"1B");
map.put(2,"2A");
map.put(2,"2C");
map.put(2,"2B");
map.put(3,"3A");
map.put(4,"4A");
map.put(4,"4B");
List<String> list = (List<String>)map.get(2);
Multimap<String,String> mm = HashMultimap.create();
mm.put("a","A1");
mm.put("a","A2");
mm.put("b","B2");
mm.put("c","C1");
mm.put("c","C2");
Collection<String> mc = mm.get("c");
SortedSetMultimap<Integer,String> map = TreeMultimap.create();
map.put(1,"1A");
map.put(1,"1B");
map.put(2,"2A");
map.put(2,"2C");
map.put(2,"2B");
map.put(3,"3A");
map.put(4,"4A");
map.put(4,"4B");
Set<String> list = map.get(2);