code: public static <T extends AVUser> T cast(AVUser user, Class<T> clazz) {
      try {
        T newUser = AVObject.cast(user, clazz);
        return newUser;
      } catch (Exception e) {
  
      }
      return null;
    }
docs: 通过这个方法可以将AVUser对象强转为其子类对象