stream 流  集合 根据某个字段去重  + 提出多个字段生成新集合

List<WebCouponIdAndNameDTO> webCouponIdAndNameDTOList = new ArrayList<>();
if (CollectionUtil.isEmpty(couponDOByCodeList)) {
    return webCouponIdAndNameDTOList;
}
//根据 code去重
webCouponIdAndNameDTOList = couponDOByCodeList.stream()
        .collect(Collectors.collectingAndThen(
                Collectors.toCollection(
                        () -> new TreeSet<>
                                (Comparator.comparing(CouponDO::getCpnCode))), ArrayList::new))
//取出三个字段 组成新集合
        .stream().map(e -> new WebCouponIdAndNameDTO(e.getCpnName(), e.getCpnCode(), e.getCpnCode()))
        .collect(Collectors.toList());