code: public static String requiredString(JSONObject o, String id) throws JSONConverterException {
          checkKeys(o, id);
          Object x = o.get(id);
          return x.toString();
      }
docs: Read an expected string.
  
  @param o  the object to parse
  @param id the key in the map that points to the string
  @return the string
  @throws JSONConverterException if the key does not point to a string