List stream的9种常用功能

1、List 转List

List llla = Arrays.asList("1","2").stream().map(Long::parseLong).collect(Collectors.toList());

2、List转类型List、List、List

//（1）、List中的String属性转List

List userNameList = list.stream().map(UserInfo::getUserName).collect(Collectors.toList());

//过滤null

List userNameList1 = list.stream().map(UserInfo::getUserName).filter(Objects::nonNull).collect(Collectors.toList());

//（2）、List中的Integer属性转List

List userIds = list.stream().map(UserInfo::getAmount).map(String::valueOf).collect(Collectors.toList());

//过滤null

List userIds1 = list.stream().map(UserInfo::getAmount).filter(Objects::nonNull).map(String::valueOf).collect(Collectors.toList());

//List属性转List ,如果为 null就为0

List aountList1 = list.stream().map(i -> i.getAmount() == null ? 0 : i.getAmount()).collect(Collectors.toList());

//（3）、List属性求和

// long sum = list.stream().mapToInt(UserInfo::getAmount).sum();

//如果为 null就为0

long sum1 = list.stream().mapToInt(i -> i.getAmount() == null ? 0 : i.getAmount()).sum();

3、List转map

//（1）、List转Map<String,Bean> key重复会报错
// Map<Long, UserInfo> map1 = list.stream().collect(Collectors.toMap(UserInfo::getUserId, i -> i));
//（2）、list bean 中某两个属性 转 map (k1,k2)->k2) 表示如果key相同以后面那个覆盖前面的 。必须对value的null进行处理，因为value为null会报错
Map<Long, String> map = list.stream().collect(Collectors.toMap(UserInfo::getUserId, i -> i.getUserName() == null ? "null" : i.getUserName(), (k1, k2) -> k2));

4、分组

//List分组

// Map<Long, List> collect = list.stream().collect(Collectors.groupingBy(UserInfo::getUserId));

//List分组,null为0

Map<Long, List> collect = list.stream().collect(Collectors.groupingBy(i -> i.getUserId() == null ? 0 : i.getUserId()));

//List分组统计，相当于数据库 select name,count(*)... group by name

Map<Long, Long> collect1 = list.stream().collect(Collectors.groupingBy(i -> i.getUserId() == null ? 0 : i.getUserId(), Collectors.counting()));

log.info("=======" + (collect1));
5、limit

List collect2 = list.stream().limit(1).collect(Collectors.toList());
6、排序

List l3 = list.stream().sorted((s1, s2) -> s2.getAge().compareTo(s1.getAge())).collect(Collectors.toList());
7、判断年龄大于2

boolean check = list.stream().anyMatch(student -> student.getAge() > 2);
8、list拼接成字符串

String str = list.stream().map(UserInfo::getUserName).collect(Collectors.joining(",", "", ""));
9、获取年龄的最大值、最小值、平均值、求和等等

IntSummaryStatistics intSummaryStatistics = list.stream().mapToInt(UserInfo::getAge).summaryStatistics();

System.out.println(intSummaryStatistics.getMax());

System.out.println(intSummaryStatistics.getCount());

System.out.println(intSummaryStatistics.getSum());