code: public List<V> getAll(Object key) {
          List<V> res = map.get(key);
          if (res == null) {
              res = Collections.emptyList();
          }
          return res;
      }
docs: Get all values for the given key. Returns the empty set if there are none.
  <p>
  Changes to the returned set will update the underlying MultiMap if the return set is not empty.
  </p>
  
  @param key
  @return all values for the given key.