JDK8集合部分相关语法

1.List转map

1.1 构造参数

ArrayList<ProductReturnDetailResult> productReturnDetailResults = new ArrayList<>();//退货结果ProductReturnDetailResult returnDetailResult = new ProductReturnDetailResult();returnDetailResult.setMsg("该订单状态更新成功!");returnDetailResult.setProductNo(productSerialNos);returnDetailResult.setStatus(true);productReturnDetailResults.add(returnDetailResult);

1.2 使用

Map<Boolean, ProductReturnDetailResult> productResult = productReturnDetailResults.stream().collect(Collectors.toMap(ProductReturnDetailResult::isStatus,detailResult -> detailResult, (oldDetail, newDetail) -> newDetail));

1.3 使用说明

该段代码的含义是:以isStatus(boolean)的值作为key,value使用ProductReturnDetailResult，如果key相同 那么使用新的覆盖原来的value.

2.排序

List<Integer> list1 = Arrays.asList(1, 3, 2, 5, 4);Collections.sort(list1,(o1,o2)->o1-o2);//升序Collections.sort(list1,(o1,o2)->o2-o1);//降序

3.for循环

        ArrayList<SupplierOrder> supplierOrders = new ArrayList<>();SupplierOrder supplierOrder = new SupplierOrder();supplierOrder.setSupplyOrderNo("9999");supplierOrder.setComment("test");supplierOrders.add(supplierOrder);supplierOrders.forEach(order -> {System.out.println(order.getSupplyOrderNo());System.out.println(order.getComment());});

4.filter(保留)

ArrayList<SupplierOrder> supplierOrders = new ArrayList<>();SupplierOrder supplierOrder = new SupplierOrder();supplierOrder.setSupplyOrderNo("9999");supplierOrder.setComment("test");supplierOrders.add(supplierOrder);SupplierOrder supplierOrder1 = new SupplierOrder();supplierOrder1.setSupplyOrderNo("9998");supplierOrder1.setComment("test");supplierOrders.add(supplierOrder1);List<SupplierOrder> newOrder = supplierOrders.stream().filter(order -> order.getSupplyOrderNo().contains("9998")).collect(Collectors.toList());System.out.println(JSON.toJSONString(newOrder));

PS: filter理解成保留的意思不要理解成过滤，该结果输出为:[{"comment":"test","supplyOrderNo":"9998"}]

5.取出List中对象中的某一个字段作为一个集合

        List<MerchantInvitation> merchantInvitations =  new ArrayList<>();List<Integer> merchantIds = merchantInvitations.stream().map(MerchantInvitation::getMerchantId).collect(Collectors.toList());

6.计算数值返回整形

List<InvitationStatisticsResult> list = new ArrayList<>();
int invitationCountSum = list.stream().mapToInt(InvitationStatisticsResult::getInvitationCount).sum();

或者

List<InvitationStatistics> temp = new ArrayList<>();InvitationStatistics t = new InvitationStatistics();t.setInvitationCount(2);InvitationStatistics t1 = new InvitationStatistics();t1.setInvitationCount(2);temp.add(t);temp.add(t1);Integer collect = temp.stream().collect(Collectors.summingInt(InvitationStatistics::getInvitationCount));

7.计算BigDecimal

List<InvitationStatisticsResult> list = new ArrayList<>();
BigDecimal transactionAmountSum = list.stream().map(InvitationStatisticsResult::getTransactionAmount).reduce(BigDecimal.ZERO, BigDecimal::add);

8.连接字符串

List<InvitationStatistics> temp = new ArrayList<>();InvitationStatistics t = new InvitationStatistics();t.setMerchantName("test1");InvitationStatistics t1 = new InvitationStatistics();t1.setMerchantName("test2");temp.add(t);temp.add(t1);String collect = temp.stream().map(InvitationStatistics::getMerchantName).collect(Collectors.joining("-"));System.out.println(collect);

9.获取最大值

List<InvitationStatistics> temp = new ArrayList<>();InvitationStatistics t = new InvitationStatistics();t.setInvitationCount(2);InvitationStatistics t1 = new InvitationStatistics();t1.setInvitationCount(3);temp.add(t);temp.add(t1);Optional<InvitationStatistics> collect = temp.stream().collect(Collectors.maxBy(Comparator.comparingInt(InvitationStatistics::getInvitationCount)));System.out.println(JSON.toJSONString(collect));

10.获取最小值

List<InvitationStatistics> temp = new ArrayList<>();InvitationStatistics t = new InvitationStatistics();t.setInvitationCount(2);InvitationStatistics t1 = new InvitationStatistics();t1.setInvitationCount(3);temp.add(t);temp.add(t1);Optional<InvitationStatistics> collect = temp.stream().collect(Collectors.minBy(Comparator.comparingInt(InvitationStatistics::getInvitationCount)));System.out.println(JSON.toJSONString(collect));

11.分组

Map<Integer, List<SupplierOrder>> supplierOrderGroupBy = supplierOrders.stream().collect(Collectors.groupingBy(SupplierOrder::getMerchantId));//根据卖家id进行分组

12.排序

List<InvitationRecordModel> result2 = invitationRecordModels.stream().filter(record -> record.getIsCreateOrder().equals(Const.MERCHANT_IS_CREATE_ORDER_FALSE)).collect(Collectors.toList());Collections.sort(result2,Comparator.comparing(InvitationRecordModel::getCreateDt).reversed());