//List里面的对象元素，以某个属性来分组，例如，以id分组，将id相同的放在一起
        //List 以ID分组 Map<Long,List<Sku>>
        Map<Long, List<Sku>> mapList = skuList.stream().collect(Collectors.groupingBy(Sku::getProductSpecificationId));
        System.out.println(mapList);
        //List转Map
        Map<Long, Sku> appleMap = skuList.stream().collect(Collectors.toMap(Sku::getProductSpecificationId, item -> item));
        System.out.println(appleMap);