code: public void setUserAgent(String userAgent) {
          if (userAgent == null || userAgent.isEmpty()) {
              throw new IllegalArgumentException("User agent can't be null or empty.");
          }
  
          this.userAgent = userAgent;
      }
docs: Set the User agent that will be used when making a tile request.
  
  Some tile server usage policies requires application to identify itself,
  so please make sure that it is set properly.
  
  @param userAgent User agent to be used.