code: public Object get(Object key,Object subKey){
          HashMap a = (HashMap)super.get(key);
          if(a!=null){
              Object b=a.get(subKey);
              return b;
          }
          return null;
      }
docs: Returns the value to which this map maps the specified key and subKey. Returns null if the map contains no mapping
  for this key and subKey. A return value of null does not necessarily indicate that the map contains no mapping
  for the key and subKey; it's also possible that the map explicitly maps the key to null.
  The containsKey operation may be used to distinguish these two cases.
  @param key  whose associated value is to be returned.
  @param subKey whose associated value is to be returned
  @return the value to which this map maps the specified key.