code: public void setButton(String type, String title, String url, String payload)
      {
          HashMap<String, String> button = new HashMap<String, String>();
          button.put("type", type);
          button.put("title", title);
          button.put("url", url);
          button.put("payload", payload);
          this.buttons.add(button);
      }
docs: Set Button
  
  @param type the button type
  @param title the button title
  @param url the button url
  @param payload the button payload