code: public String optString(String key, String defaultValue) {
      Object o = opt(key);
      return o != null ? o.toString() : defaultValue;
    }
docs: Get an optional string associated with a key. It returns the defaultValue if there is no such key.
  
  @param key
  A key string.
  @param defaultValue
  The default.
  @return A string which is the value.