用java的stream流库在实际项目的使用案例

由于自己用stream+lambda很不熟悉，用文章记录在一个电商项目中的用法
购物车模块：

		//获取购物车，校验（是否有商品、库存）List<Cart> cartList = cartService.listForCart(uid).stream().filter(Cart::getProductSelected).collect(Collectors.toList());

订单模块：

//获取cartList里的productIdsSet<Integer> productIdSet = cartList.stream().map(Cart::getProductId).collect(Collectors.toSet());List<Product> productList = productMapper.selectByProductIdSet(productIdSet);Map<Integer, Product> map  = productList.stream().collect(Collectors.toMap(Product::getId, product -> product));List<OrderItem> orderItemList = new ArrayList<>();Long orderNo = generateOrderNo();for (Cart cart : cartList) {//根据productId查数据库Product product = map.get(cart.getProductId());//是否有商品if (product == null) {return ResponseVo.error(ResponseEnum.PRODUCT_NOT_EXIST,"商品不存在. productId = " + cart.getProductId());}//商品上下架状态if (!ProductStatusEnum.ON_SALE.getCode().equals(product.getStatus())) {return ResponseVo.error(ResponseEnum.PRODUCT_OFF_SALE_OR_DELETE,"商品不是在售状态. " + product.getName());}}

返回订单视图

	private OrderVo buildOrderVo(Order order, List<OrderItem> orderItemList, Shipping shipping) {OrderVo orderVo = new OrderVo();BeanUtils.copyProperties(order, orderVo);List<OrderItemVo> OrderItemVoList = orderItemList.stream().map(e -> {OrderItemVo orderItemVo = new OrderItemVo();BeanUtils.copyProperties(e, orderItemVo);return orderItemVo;}).collect(Collectors.toList());orderVo.setOrderItemVoList(OrderItemVoList);if (shipping != null) {orderVo.setShippingId(shipping.getId());orderVo.setShippingVo(shipping);}return orderVo;}

创建订单

private Order buildOrder(Integer uid,Long orderNo,Integer shippingId,List<OrderItem> orderItemList) {BigDecimal payment = orderItemList.stream().map(OrderItem::getTotalPrice).reduce(BigDecimal.ZERO, BigDecimal::add);Order order = new Order();order.setOrderNo(orderNo);order.setUserId(uid);order.setShippingId(shippingId);order.setPayment(payment);order.setPaymentType(PaymentTypeEnum.PAY_ONLINE.getCode());order.setPostage(0);order.setStatus(OrderStatusEnum.NO_PAY.getCode());return order;}

获取订单列表

	@Overridepublic ResponseVo<PageInfo> list(Integer uid, Integer pageNum, Integer pageSize) {PageHelper.startPage(pageNum, pageSize);List<Order> orderList = orderMapper.selectByUid(uid);Set<Long> orderNoSet = orderList.stream().map(Order::getOrderNo).collect(Collectors.toSet());List<OrderItem> orderItemList = orderItemMapper.selectByOrderNoSet(orderNoSet);Map<Long, List<OrderItem>> orderItemMap = orderItemList.stream().collect(Collectors.groupingBy(OrderItem::getOrderNo));Set<Integer> shippingIdSet = orderList.stream().map(Order::getShippingId).collect(Collectors.toSet());List<Shipping> shippingList = shippingMapper.selectByIdSet(shippingIdSet);Map<Integer, Shipping> shippingMap = shippingList.stream().collect(Collectors.toMap(Shipping::getId, shipping -> shipping));List<OrderVo> orderVoList = new ArrayList<>();for (Order order : orderList) {OrderVo orderVo = buildOrderVo(order,orderItemMap.get(order.getOrderNo()),shippingMap.get(order.getShippingId()));orderVoList.add(orderVo);}PageInfo pageInfo = new PageInfo<>(orderList);pageInfo.setList(orderVoList);return ResponseVo.success(pageInfo);}

关于Collectors.groupingBy：根据一个或多个属性对集合中的项目进行分组，返回一个map:

Map<Integer, List<Integer>> mapGroupBy = Stream.of(1, 2, 3, 4, 5, 4, 3).collect(Collectors.groupingBy(x -> x * 10));
System.out.println(mapGroupBy); // {50=[5], 20=[2], 40=[4, 4], 10=[1], 30=[3, 3]}
for(Map.Entry entry:mapGroupBy.entrySet())
{System.out.print(entry.getKey()+":");System.out.println(String.valueOf(entry.getValue()));
}

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