code: public JSONArray names() {
          JSONArray ja = new JSONArray();
          for (String key : m_map.keySet()) {
              ja.put(key);
          }
          return ja.length() == 0 ? null : ja;
      }
docs: Produce a JSONArray containing the names of the elements of this
  JSONObject.
  @return A JSONArray containing the key strings, or null if the JSONObject
  is empty.