根据list中的对象某个属性 进行分组

1，跟据某个属性分组OfficeId

 Map<String, List<IncomeSumPojo>> collect = list.stream().collect(Collectors.groupingBy(IncomeSumPojo::getOfficeId));

2，根据某个属性分组OfficeId，汇总某个属性Money

Map<String, Double> collect = list.stream().
collect(Collectors.groupingBy(IncomeSumPojo::getOfficeId,Collectors.summingDouble(IncomeSumPojo::getMoney)));

3，根据某个属性添加条件过滤数据，

list = list.stream().filter(u -> !u.getAmount().equals("0.00")).collect(Collectors.toList());

4，判断一组对象里面有没有属性值是某个值

List<Menu> menuList = UserUtils.getMenuList();
boolean add = menuList.stream().anyMatch(m -> "plan:ctPlan:add".equals(m.getPermission()));

5，取出一组对象的某个属性组成一个新集合

List<String> tableNames=list.stream()
.map(User::getMessage)
.collect(Collectors.toList());

6.List 去重

List<String> list = useridList.stream()
      .distinct()
      .collect(Collectors.toList());
List<String> itemCodes = itemList.stream().map(OrderRebateMateGroupItemResponseDTO::getItemCode)
                .distinct().collect(Collectors.toList());

7.java stream中的findFirst

返回出现的第一个结果

List<String> lst1 = Arrays.asList("Jhonny", "David", "Jack", "Duke", "Jill","Dany","Julia","Jenish","Divya");


 
Optional<String> findFirst = lst1.parallelStream().filter(s -> s.startsWith("D")).findFirst();

 
System.out.println(findFirst.get()); //总是打印出David

8.stream自定义key，返回map

Map<String, AttStatisticsDailyResponseDTO> dailyRespMap = Optional.ofNullable(dailyRespList)
        .orElse(Collections.emptyList())
        .stream()
        .collect(Collectors.toMap(dailyResp -> {
    
    
            String keyPrefix = dailyResp.getOrgId() + "@" + dailyResp.getLoginId() + "@" + dailyResp.getStatisticsDate();
            String workType = dailyResp.getWorkType() == null ? "NO" : dailyResp.getWorkType();
            String key = keyPrefix + "@" + workType;
            log.info("打卡数据放到缓存的key={}", key);
            return key;

            //相同key，覆盖，取最后的值
        }, a->a,(V1,V2)->V1));

9.通过某个字段返回map

userList.stream().collect(Collectors.toMap(User::getId, User::getName));

10.多个字段进行分组并统计数量

List<PefsAdvanceOrderHeadReqDTO> list = new ArrayList<>();
PefsAdvanceOrderHeadReqDTO req = new PefsAdvanceOrderHeadReqDTO();
req.setSalesman("A");
req.setSalesmanCode("A1");
req.setTotal(1);
list.add(req);
req = new PefsAdvanceOrderHeadReqDTO();
req.setSalesman("A");
req.setSalesmanCode("A1");
req.setTotal(2);
list.add(req);
req = new PefsAdvanceOrderHeadReqDTO();
req.setSalesman("B");
req.setSalesmanCode("B1");
req.setTotal(1);
list.add(req);

Map<String, Double> collect = list.stream().collect(Collectors.groupingBy(o -> o.getSalesmanCode() + "@" + o.getSalesman(), Collectors.summingDouble(PefsAdvanceOrderHeadReqDTO::getTotal)));
System.out.println(JSON.toJSONString(collect));

11.JDK8的迭代方式

//JDK8的迭代方式

map.forEach((key, value) -> {
    
    

System.out.println(key + "：" + value);

});

12. 通过某个字段 生成List

List<String> itemCodeList = list.stream().map(OrderCustDiscountItemResponseDTO::getItemCode).collect(Collectors.toList());

13 金额统计

BigDecimal adjustAmount = Optional.ofNullable(reqList).orElse(Collections.emptyList()).stream()
.map(OrderFeeAdjustImportReqDTO::getAdjustAmount)
.reduce(BigDecimal.ZERO, BigDecimal::add);

14 根据某个属性去重

public List<RptDayMonthTargetVO> queryTarget() {
    
    
        List<RptDayMonthTarget> list = targetService.selectMonthTarget();
        //数据去重-根据Bean对象的SupplierId属性去重
        List<RptDayMonthTarget> dbList = list.stream().filter(distinctByKey(RptDayMonthTarget::getSupplierId)).collect(Collectors.toList());return voList;
    }
　　
    private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
    
    
        Map<Object,Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }