java stream 删除元素

    List<Long> userIds = userList.stream().filter(item -> item.getUserId() != null).map(User::getUserId).collect(Collectors.toList());
    List<String> list = new ArrayList<>();
        list.add("AA");
        list.add("DD");

        List<String> dd = list.stream().filter(s -> !s.equals("DD")).collect(Collectors.toList());