code: public <T extends WebCrawler> void start(Class<T> clazz, int numberOfCrawlers) {
          this.start(new DefaultWebCrawlerFactory<>(clazz), numberOfCrawlers, true);
      }
docs: Start the crawling session and wait for it to finish.
  This method utilizes default crawler factory that creates new crawler using Java reflection
  
  @param clazz
  the class that implements the logic for crawler threads
  @param numberOfCrawlers
  the number of concurrent threads that will be contributing in
  this crawling session.
  @param <T> Your class extending WebCrawler