Java8的stream用法整理

• map

1 直接获取对象的值
this.categoryMapper.selectByIdList(ids).stream().map(Category::getName).collect(Collectors.toList());

2 游标循环处理
pageInfo.getResult().stream().map(spu -> {
            // 2、把spu变为 spuBo
            SpuBo spuBo = new SpuBo();
            // 属性拷贝
            BeanUtils.copyProperties(spu, spuBo);

            // 3、查询spu的商品分类名称,要查三级分类
            List<String> names = this.categoryService.queryNameByIds(Arrays.asList(spu.getCid1(), spu.getCid2(), spu.getCid3()));
            // 将分类名称拼接后存入
            spuBo.setCname(StringUtils.join(names, "/"));

            // 4、查询spu的品牌名称
            Brand brand = this.brandMapper.selectByPrimaryKey(spu.getBrandId());
            spuBo.setBname(brand.getName());
            return spuBo;
        }).collect(Collectors.toList());

• collect

/**
 * @author index
 * @date 2020/10/27
 **/
public class TestcollectingAndThen {
    @Test
    public void test(){
        final int NUM = 14;
        List<People> peopleList = new ArrayList<>(NUM);
        String[] names = {"小张", "小龙", "小牛", "小猪", "小黑", "小红", "小白"};
        for (int i = 0; i < 5; i++) {
            //添加5个19岁的随机性别和名字的小朋友
            peopleList.add(new People(19, (int) (Math.random() * 2), names[(int) (Math.random() * names.length)]));
        }
        for (int i = 5; i < 8; i++) {
            //添加3个31岁的随机性别和名字的小朋友
            peopleList.add(new People(31, (int) (Math.random() * 2), names[(int) (Math.random() * names.length)]));
        }
        for (int i = 8; i < NUM; i++) {
            //添加6个22岁的随机性别和名字的小朋友
            peopleList.add(new People(22, (int) (Math.random() * 2), names[(int) (Math.random() * names.length)]));
        }

        //collectingAndThen先对stream里的元素进行collecting，之后再对结果进行操作，
        // 下面的结果是一个map,对map计算元素数目
        System.out.println("分组数目：");
        Integer groupCount = peopleList.stream().collect(
                Collectors.collectingAndThen(Collectors.groupingBy(People::getName), Map::size));
        System.out.println(groupCount);
        System.out.println("-------------------------------------");


        //按照名字分组
        System.out.println("按照名字分组");
        System.out.println(
                peopleList.stream().collect(Collectors.groupingBy(People::getName))
        );
        System.out.println("-------------------------------------");

        //按照名字分组(分组的结果是一个map)，并统计每一个分组(map中的每一个value)中的元素数目
        System.out.println("统计每一个分组(map中的每一个value)中的元素数目");
        System.out.println(
                peopleList.stream().collect(Collectors.groupingBy(People::getName, Collectors.counting()))
        );
        System.out.println("-------------------------------------");

        //按照名字分组(分组的结果是一个map)，并取出每一组的最大值
        System.out.println("取出每一组的最大值");
        System.out.println(
                peopleList.stream().collect(Collectors.groupingBy(People::getName, Collectors.maxBy(new Comparator<People>() {
                    @Override
                    public int compare(People o1, People o2) {
                        return o1.getAge() - o2.getAge();
                    }
                })))
        );


    }
}

List<Integer> collectList = Stream.of(1, 2, 3, 4)
        .collect(Collectors.toList());
System.out.println("collectList: " + collectList);
// 打印结果
// collectList: [1, 2, 3, 4]

Set<Integer> collectSet = Stream.of(2, 3, 3, 3)
        .collect(Collectors.toSet());
System.out.println("collectSet: " + collectSet);
// 打印结果
// collectSet: [2,3]

    private void loadStockInSku(List<Long> ids, List<Sku> skus) {
        List<Stock> stockList = stockMapper.selectByIdList(ids);
        if (CollectionUtils.isEmpty(stockList)) {
            throw new LyException(ExceptionEnum.GOODS_STOCK_NOT_FOUND);
        }
        Map<Long, Integer> stockMap = stockList.stream().collect(Collectors.toMap(Stock::getSkuId, Stock::getStock));
        skus.forEach(s -> s.setStock(stockMap.get(s.getId())));
    }