code: public JSONObject optJSONObject( String key ) {
        verifyIsNull();
        Object o = opt( key );
        return o instanceof JSONObject ? (JSONObject) o : null;
     }
docs: Get an optional JSONObject associated with a key. It returns null if there
  is no such key, or if its value is not a JSONObject.
  
  @param key A key string.
  @return A JSONObject which is the value.