java 8 lambda表达式list操作分组、过滤、求和、最值、排序、去重_gsls200808的专栏-CSDN博客_lambda 分组 - 亲测可用 很实用
java8的lambda表达式提供了一些方便list操作的方法，主要涵盖分组、过滤、求和、最值、排序、去重。跟之前的传统写法对比，能少写不少代码。
新建实体类
package com.vvvtimes.vo; import java.math.BigDecimal;import java.util.Date; public class User { private Long id; //姓名 private String name; //年龄 private int age; //工号 private String jobNumber; //性别 private String sex; //入职日期 private Date entryDate; //家庭成员数量 private BigDecimal familyMemberQuantity; public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getJobNumber() { return jobNumber; } public void setJobNumber(String jobNumber) { this.jobNumber = jobNumber; } public String getSex() { return sex; } public void setSex(String sex) { this.sex = sex; } public Date getEntryDate() { return entryDate; } public void setEntryDate(Date entryDate) { this.entryDate = entryDate; } public BigDecimal getFamilyMemberQuantity() { return familyMemberQuantity; } public void setFamilyMemberQuantity(BigDecimal familyMemberQuantity) { this.familyMemberQuantity = familyMemberQuantity; }}1.分组
通过groupingBy可以分组指定字段
//分组 Map<String, List<User>> groupBySex = userList.stream().collect(Collectors.groupingBy(User::getSex)); //遍历分组 for (Map.Entry<String, List<User>> entryUser : groupBySex.entrySet()) { String key = entryUser.getKey(); List<User> entryUserList = entryUser.getValue(); }多字段分组
Function<WarehouseReceiptLineBatch, List<Object>> compositeKey = wlb -> Arrays.<Object>asList(wlb.getWarehouseReceiptLineId(), wlb.getWarehouseAreaId(), wlb.getWarehouseLocationId()); Map<Object, List<WarehouseReceiptLineBatch>> map = warehouseReceiptLineBatchList.stream().collect(Collectors.groupingBy(compositeKey, Collectors.toList())); //遍历分组 for (Map.Entry<Object, List<WarehouseReceiptLineBatch>> entryUser : map.entrySet()) { List<Object> key = (List<Object>) entryUser.getKey(); List<WarehouseReceiptLineBatch> entryUserList = entryUser.getValue(); Long warehouseReceiptLineId = (Long) key.get(0); Long warehouseAreaId = (Long) key.get(0); Long warehouseLocationId = (Long) key.get(0); }2.过滤
通过filter方法可以过滤某些条件
//过滤 //排除掉工号为201901的用户 List<User> userCommonList = userList.stream().filter(a -> !a.getJobNumber().equals("201901")).collect(Collectors.toList());3.求和
分基本类型和大数类型求和，基本类型先mapToInt，然后调用sum方法，大数类型使用reduce调用BigDecimal::add方法
//求和 //基本类型 int sumAge = userList.stream().mapToInt(User::getAge).sum(); //BigDecimal求和 BigDecimal totalQuantity = userList.stream().map(User::getFamilyMemberQuantity).reduce(BigDecimal.ZERO, BigDecimal::add);上面的求和不能过滤bigDecimal对象为null的情况，可能会报空指针，这种情况，我们可以用filter方法过滤，或者重写求和方法
重写求和方法
package com.vvvtimes.util; import java.math.BigDecimal; public class BigDecimalUtils { public static BigDecimal ifNullSet0(BigDecimal in) { if (in != null) { return in; } return BigDecimal.ZERO; } public static BigDecimal sum(BigDecimal ...in){ BigDecimal result = BigDecimal.ZERO; for (int i = 0; i < in.length; i++){ result = result.add(ifNullSet0(in[i])); } return result; }}使用重写的方法
BigDecimal totalQuantity2 = userList.stream().map(User::getFamilyMemberQuantity).reduce(BigDecimal.ZERO, BigDecimalUtils::sum);
判断对象空
stream.filter(x -> x!=null)stream.filter(Objects::nonNull)判断字段空
stream.filter(x -> x.getDateTime()!=null)4.最值
求最小与最大，使用min max方法
//最小 Date minEntryDate = userList.stream().map(User::getEntryDate).min(Date::compareTo).get(); //最大 Date maxEntryDate = userList.stream().map(User::getEntryDate).max(Date::compareTo).get();有时候我们需要知道最大最小对应的这个对象，我们可以通过如下方法获取
Comparator<LeasingBusinessContract> comparator = Comparator.comparing(LeasingBusinessContract::getLeaseEndDate);LeasingBusinessContract maxObject = leasingBusinessContractList.stream().max(comparator).get();5.List 转map
/** * List -> Map * 需要注意的是： * toMap 如果集合对象有重复的key，会报错Duplicate key .... * user1,user2的id都为1。 * 可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2 */ Map<Long, User> userMap = userList.stream().collect(Collectors.toMap(User::getId, a -> a,(k1,k2)->k1));list转map的时候有时候会将date类型作为key，实际情况中使用string的多，我们可以将某个字段转成string
Map<String, WorkCenterLoadVo> workCenterMap = list.stream().collect(Collectors.toMap(key->DateFormatUtils.format(key.getDate(), "yyyy-MM-dd"), a -> a,(k1,k2)->k1));6.排序
可通过Sort对单字段多字段排序
//排序 //单字段排序，根据id排序 userList.sort(Comparator.comparing(User::getId)); //多字段排序，根据id，年龄排序 userList.sort(Comparator.comparing(User::getId).thenComparing(User::getAge));7.去重
可通过distinct方法进行去重
//去重 List<Long> idList = new ArrayList<Long>(); idList.add(1L); idList.add(1L); idList.add(2L); List<Long> distinctIdList = idList.stream().distinct().collect(Collectors.toList());针对属性去重
List<AddOutboundNoticeDetailsBatchVo> entryDetailsBatchDistinctBatchIdList = entryDetailsBatchList.stream().filter(distinctByKey(b -> b.getMaterialBatchNumberId())).collect(Collectors.toList()); //distinctByKey自己定义 public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) { Map<Object, Boolean> seen = new ConcurrentHashMap<>(); return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null; }8.获取list某个字段组装新list
//获取list对象的某个字段组装成新list List<Long> userIdList = userList.stream().map(a -> a.getId()).collect(Collectors.toList());9.批量设置list列表字段为同一个值
addList.stream().forEach(a -> a.setDelFlag("0"));10.不同实体的list拷贝
List<TimePeriodDate> timePeriodDateList1 = calendarModelVoList.stream().map(p->{TimePeriodDate e = new TimePeriodDate(); e.setStartDate(p.getBegin());e.setEndDate(p.getEnd()); return e;}).collect(Collectors.toList());
Original url: Access
Created at: 2020-06-23 17:17:30
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