Java8实现List转Map

//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);

//获取 List 集合获取指定的字段数组
List<Vip> vipList = new ArrayList<Vip>();
vipList.add(new Vip().setId(1L));
vipList.add(new Vip().setId(2L));
List<Long> idList = vipList.stream().map(Vip::getId).collect(Collectors.toList());

List转换逗号分隔的字符串
StringUtils.join(productSpecificationIdList.toArray(), ",")

//Set转List
Set<String> idSet = buyerCartMap.keySet();
List<Long> productSpecificationIds = Arrays.asList(idSet.toArray(new Long[idSet.size()]));