code: public List getAll(Object key) {
          Object value = mMap.get(key);
          if (value instanceof List) {
              return ((List)value);
          }
          else {
              List list = new ArrayList();
              if (value != null || mMap.containsKey(key)) {
                  list.add(value);
              }
              mMap.put(key, list);
              return list;
          }
      }
docs: Returns all the values associated with the given key. Changes to the
  returned list will be reflected in this map.