java8新特性之stream的collect實戰

1、list轉換成list

不帶return方式

 List<Long> ids=wrongTmpList.stream().map(c->c.getId()).collect(Collectors.toList());

帶return方式

// spu集合轉化成spubo集合//java8的新特性
        List<SpuBo> spuBos=spuList.stream().map(spu -> {
            SpuBo spuBo = new SpuBo();
            BeanUtils.copyProperties(spu, spuBo);
            //查詢品牌名稱
            Brand brand = this.brandMapper.selectByPrimaryKey(spu.getBrandId());
            spuBo.setBname(brand.getName());
            //查詢類別名稱
            List<String> names = this.categoryService.queryNamesByIds(Arrays.asList(spu.getCid1(), spu.getCid2(), spu.getCid3()));
            spuBo.setCname(StringUtils.join(names, "-"));
            return spuBo;
        }).collect(Collectors.toList());

2、list轉map

Map<Long, Active> activeMap = actives.stream().collect(Collectors.toMap(Active::getId, s->s));

3、分組統計計算

list轉map(根據某個屬性進行分組)

Map<Long, List<TrainPlan>> trainMaps = trainPlans.stream().collect(
        Collectors.groupingBy(TrainPlan::getModuleId));

list轉map(統計計算)

 List<StatDepartment> statDepartments = projectModuleBSDao.statProModByDepartment(params);

 Map<Long, Integer> projectNumByDep = statDepartments.stream()
                .collect(Collectors.groupingBy(StatDepartment::getDepartmentId, Collectors.summingInt(StatDepartment::getProjectNum)));