java list流

/ 获取id的集合

List idList = stockList.stream().map(Stock::getId).collect(Collectors.toList());

// 获取skuid集合并去重

List skuIdList = stockList.stream().map(Stock::getSkuId).distinct().collect(Collectors.toList());

// 获取supplierId集合(supplierId的类型为int，返回List,使用boxed方法装箱)

Set supplierIdSet = stockList.stream().mapToInt(Stock::getSupplierId).boxed().collect(Collectors.toSet());

6.2 分组与分片

// 按skuid分组

Map<Long, List> skuIdStockMap = stockList.stream().collect(Collectors.groupingBy(Stock::getSkuId));

// 过滤supplierId=1然后按skuId分组

Map<Long, List> filterSkuIdStockMap = stockList.stream().filter(s -> s.getSupplierId() == 1).collect(Collectors.groupingBy(Stock::getSkuId));

// 按状态分为不可用和其他两个分片

Map<Boolean, List> partitionStockMap = stockList.stream().collect(Collectors.partitioningBy(s -> s.getStatus() == 0));

6.3 计数与求和

// 统计skuId=1的记录数

long skuIdRecordNum = stockList.stream().filter(s -> s.getSkuId() == 1).count();

// 统计skuId=1的总库存量

BigDecimal skuIdAmountSum = stockList.stream().filter(s -> s.getSkuId() == 1).map(Stock::getAmount).reduce(BigDecimal.ZERO, BigDecimal::add);

6.4 特定用法

// 多重分组并排序，先按supplierId分组，再按skuId分组，排序规则，先supplierId后skuId

Map<Integer, Map<Long, List>> supplierSkuStockMap = stockList.stream().collect(Collectors.groupingBy(Stock::getSupplierId, TreeMap::new,

Collectors.groupingBy(Stock::getSkuId, TreeMap::new, Collectors.toList())));

// 多条件排序，先按supplierId正序排，再按skuId倒序排

// (非stream方法，而是集合的sort方法，直接改变原集合元素，使用Function参数)

stockList.sort(Comparator.comparing(Stock::getSupplierId)

.thenComparing(Stock::getSkuId, Comparator.reverseOrder()));