public static <T, S extends Comparable<? super S>> List<T> valueSortedKeys(final Map<T, S> map)
{
  List<T> list = new ArrayList<T>(map.keySet());
 
  Collections.sort(list, new Comparator<T>()
  {
    public int compare(T t1, T t2)
    {
      return map.get(t1).compareTo(map.get(t2));
    }
  });
 
  return list;
}