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