code: public WebhookClient build()
      {
          OkHttpClient client = this.client;
          if (client == null)
          {
              if (builder == null)
                  builder = DEFAULT_HTTP_BUILDER;
              client = builder.build();
          }
          if (pool == null)
          {
              if (threadFactory == null)
                  threadFactory = new DefaultWebhookThreadFactory();
              pool = Executors.newSingleThreadScheduledExecutor(threadFactory);
          }
          return new WebhookClient(id, token, client, pool);
      }
docs: Builds a new {@link net.dv8tion.jda.webhook.WebhookClient WebhookClient} instance
  with the current state of this builder.
  
  <p><b><u>Remember to close the WebhookClient once you don't need it anymore to free resources!</u></b>
  
  @return The new WebhookClient instance