Collections工具类,可以使用collections工具类对代码中的list进行分组

/**
 * 根据活动id进行分组
 * key 活动id
 * value 活动id对应的商品id
 */

  Map<Long, Set<Long>> collect = activitySkuList
                .stream()
                .collect(Collectors
                .groupingBy(ActivitySku::getActivityId
                        , Collectors.mapping(ActivitySku::getSkuId, Collectors.toSet())
                            )
                        );

        Map<Long, List<Long>> collect1 = activitySkuList.stream().collect(Collectors.groupingBy(ActivitySku::getActivityId, Collectors.mapping(ActivitySku::getSkuId, Collectors.toList())));
        Map<Long, List<SkuInfo>> collect2 = activitySkuList.stream().collect(Collectors.groupingBy(ActivitySku::getActivityId, Collectors.mapping(ActivitySku::getSkuInfo, Collectors.toList()))