code: public <T extends WebCrawler> void start(WebCrawlerFactory<T> crawlerFactory,
                                               int numberOfCrawlers) {
          this.start(crawlerFactory, numberOfCrawlers, true);
      }
docs: Start the crawling session and wait for it to finish.
  
  @param crawlerFactory
  factory to create crawlers on demand for each thread
  @param numberOfCrawlers
  the number of concurrent threads that will be contributing in
  this crawling session.
  @param <T> Your class extending WebCrawler