java8 Stream流、Optional的用法总结

java8 Stream流、Optional的用法总结

1.分组

 Map> collectMap = list.stream().collect(Collectors.groupingBy(p -> p.getProductNo(), Collectors.toList()));

2.list转map

 Map proDTOMap = list.stream().collect(Collectors.toMap(PurchaseDetailDTO::getProductNo, p->p,(p1,p2)->p1));

3.计算集合某元素之和

 BigDecimal reduce = typeList.stream().map(m -> m.getCategoryDTO().getTaxAmount()).reduce(BigDecimal.ZERO, BigDecimal::add);

4.分组计算某元素之和

  Map numMap = list.stream().collect(Collectors.groupingBy(p -> p.getGroupId(), Collectors.reducing(
                BigDecimal.ZERO,
                QuoteStatics::getNum,
                ArithUtil::add2)));

5.list转嵌套map

  Map> borrowMap = list.stream().collect(Collectors.groupingBy(CrmBorrowSummary::getUserId, Collectors.toMap(CrmBorrowSummary::getProductId, v -> v)));

6.list循环拼接字符串

  String productStr = detailDTOList.stream().map(v -> v.getProductName() + "，" + v.getNum() + v.getProductUnit() + "，" +
                    v.getSupplyName() + "，" + v.getPrice() + "元/" + v.getProductUnit() + "，" +
                    "协议" + v.getSupplyPrice() + "元/" + v.getProductUnit())
                    .collect(Collectors.joining("n"));

Optional的用法
1.结合filter、ifPresent、orElseThrow等用法

Optional.ofNullable(deliverPlanDTO).filter(v -> StringUtils.isEmpty(v.getContractNo())).ifPresent(v ->
                message.setStatus(CommonStatus.WAIT_APPROVE));				
Optional.ofNullable(groupId).filter(StringUtils::isNotBlank).orElseThrow(
				()-> new CommonException(ReturnCode.FAILED, "common.error", new String[]{"参数不正确！"}));
Optional.ofNullable(message.getToUsers()).filter(CollectionUtils::isNotEmpty).ifPresent(users -> {
            users.stream().filter(v -> PersonType.TO.equals(v.getType()))
                    .filter(v -> CommonStatus.CLOSE.equals(v.getStatus()))
                    .forEach(v -> v.setStatus(CommonStatus.NORMAL));
        });

2.取代if/else的判断

CrmQuoteFee fee=beanMapper.toCrmQuoteFee(reqdto);
Optional.ofNullable(reqdto.getId())
	.map(u->fee.setModifyBy(reqdto.getCreateBy()))
	.orElseGet(()->fee.setCreateBy(reqdto.getCreateBy()));
Optional.ofNullable(reqdto.getId())
	.map(u->fee.setModifyBy(reqdto.getCreateBy()))
	.orElseGet(()->{
		fee.setCanDel(Boolean.TRUE);
		fee.setType("1");
		return fee;
	}
  );

3.链式调用,中间任意环节为null则返回

String managerStr = Optional.ofNullable(project).map(Project::getAgentId).map(crmCustomerDao::getCrmCustomerById).map(CrmCustomer::getCustomerManagerId).map(userService::getUserInfoByUserId)
.map(m -> "(" + m.getName() + "," + m.getMobile() + ")").orElse(null);