Java8实现List转Map

这些仅仅是在开发中遇到的一些转换，总结一下！

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> mapList = skuList.stream().collect(Collectors.groupingBy(Sku::getProductSpecificationId));
System.out.println(mapList);

Map>分组

List

List转Map

Map 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 vipList = new ArrayList();
vipList.add(new Vip().setId(1L));
vipList.add(new Vip().setId(2L));
List idList = vipList.stream().map(Vip::getId).collect(Collectors.toList());

从集合中取出某个字段的集合

List<String> numbers = houseList.stream().map(p->p.get("number").toString()).collect(Collectors.toList());

去重

List<String> numbers = houseList.stream().map(p->p.get("number").toString()).distinct().collect(Collectors.toList());

List转换逗号分隔的字符串

StringUtils.join(productSpecificationIdList.toArray(), ",")

Set转List

Set idSet = buyerCartMap.keySet();
List productSpecificationIds = Arrays.asList(idSet.toArray(new Long[idSet.size()]));

排序

正序排序：list.sort(Comparator.comparing((Map m) -> (new BigDecimal(m.get("sort").toString())));
倒序排序：list.sort(Comparator.comparing((Map m) -> (new BigDecimal(m.get("sort").toString()))).reversed());