Java 8 中的 Streams API Demo

Java8 Streams demo

一、获取List集合中的某个字段的List集合
例如：
List<Long> userIdList = payBillVOList.stream().mapToLong(PayBillVO::getUserId).boxed().collect(Collectors.toList());before java8 ：
List<Long> userIdList = new ArrayList<>();
for (PayBillVO payBillVO : payBillVOList) {
userIdList.add(payBillVO.getUserId());
}

List集合中的某个字段转为String
String appIds = Joiner.on(",").join(deviceUserDOList.stream().map(TDeviceUserDO::getAppId).distinct().collect(Collectors.toList()));

二、List对象转Map
例如：
单个对象：
Map<Long, UserMain> userMainMap = userMainList.stream().collect(Collectors.toMap(UserMain::getUserId, c -> c));

List对象：
Map<String, List<CollectionVO>> collectionVOListMap = collectionVOList.stream().collect(Collectors.groupingBy(CollectionVO::getCreateDate));
before java8 ：
自己写很复杂。
三、String按特定的规则转List
例如：
基础版本：withdrawInfoIds=110,120
List<Long> withdrawInfoIdList = Splitter.on(",").splitToList(withdrawInfoIds).stream().mapToLong(Long::valueOf).boxed().collect(Collectors.toList());
兼容版本：relationMobileIds=110,,120,
List<Long> relationIdList = Splitter.on(",").trimResults().omitEmptyStrings().splitToList(relationMobileIds).stream().mapToLong(Long::valueOf).boxed().collect(Collectors.toList());

before java8 ：
List<Long> withdrawInfoIdList = new ArrayList<>();
String[] withdrawInfoIdArr = withdrawInfoIds.split(",");
for (String id : withdrawInfoIdArr) {
withdrawInfoIdList.add(Long.valueOf(id));
}

四、获取list集合中某个字段出现的总数（例如统计用户ID出现的次数）
例如：
Map<Long, Long> withdrawInfoListMap = withDrawInfoDOList.stream().collect(Collectors.groupingBy(TWithDrawInfoDO::getUserId, Collectors.counting()));

五、List集合中的对象转为另一个对象例如：
List<EntryVO> entryAPIList = entryDOList.getContent().stream().map(EntryVO::toVoFromDo).collect(Collectors.toList());
其中：
public static EntryVO toVoFromDo(EntryDO entryDO){
if (entryDO == null){
return null;
}
return new EntryVO(entryDO.getAutoId(), entryDO.getUserId());
}
六、从list集合中筛选出符合条件的一条记录
例如：
Long entryId = 8;
EntryAPI entryAPI = entryAPIList.stream().filter(p -> p.getAutoId().equals(entryId)).findFirst().orElse(null);
七、排序
例如：先排序，再格式化输出

List<Date> openTimeList;
List<String> openTimeStringList = openTimeList.stream().sorted(Comparator.comparing(date -> date)).map((date)-> DateUtils.formatHourMinutes(date)).collect(Collectors.toList());

例如：把list集合中的数据转成String输出 空格隔开
List<String> openTimeStringList
String openTimeString = openTimeStringList.stream().collect(Collectors.joining(" "));八、求和Integer point = billPointsDOList.stream().mapToInt(TBillPointsDO::getTotalPointsAvaliable).sum();

BigDecimal paidAmount = billPaymentDOList.stream().map(TBillPaymentDO::getPayment).reduce(BigDecimal.ZERO, BigDecimal::add);