code: public String optString(String key, String defaultValue) {
          Object object = this.opt(key);
          return NULL.equals(object) ? defaultValue : object.toString();
      }
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.