code: public Boolean challenge()
      {
          if( (this.hub_mode.equals("subscribe")) && (this.hub_verify_token.equals(this.configs.get("verify_token", ""))) ){
              Logger.info("Verify token validated successfully.");
              return true;
          }
  
          Logger.error("Error validating verify token.");
          return false;
      }
docs: Verify Challenge Data
  
  @return boolean whether challenge passed or not