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();
}
上门的分组存在key空值安全问题，需要做过滤或封装处理
用Optional封装使用中会报No value present
//分组添加累计
Map<Optional<Long>, List<SalesOrderMonthlyStatementVo>> groupMap = list.stream().collect(Collectors.groupingBy(x -> Optional.ofNullable(x.getReviewerGroupId())));
//遍历分组
for (Map.Entry<Optional<Long>, List<SalesOrderMonthlyStatementVo>> entryGroup : groupMap.entrySet()) {
Optional<Long> key = entryGroup.getKey();
Long groupId = key.get();
List<SalesOrderMonthlyStatementVo> entryGroupList = entryGroup.getValue();
}
正确使用方法如下，自定义groupingBy_WithNullKeys
/** Like Collectors.groupingBy, but accepts null keys. */
public static <T, A> Collector<T, ?, Map<A, List<T>>>
groupingBy_WithNullKeys(Function<super T, extends A> classifier) {
return Collectors.toMap(
classifier,
Collections::singletonList,
(List<T> oldList, List<T> newEl) -> {
List<T> newList = new ArrayList<>(oldList.size() + 1);
newList.addAll(oldList);
newList.addAll(newEl);
return newList;
});
}
然后调用
Map<Long, List<SalesOrderMonthlyStatementVo>> groupMap = list.stream().collect(groupingBy_WithNullKeys(x -> x.getReviewerGroupId()));
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(1);
Long warehouseLocationId = (Long) key.get(2);
}
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));
list转map有时候会用到多字段key的map结构，除了常用的key用字符串连接，也可以使用apache commons的多字段key形式的map结构MultiKeyMap
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));
实际上这个写法存在空值安全问题，建议改成
userList.sort(Comparator.comparing(User::getId,Comparator.nullsLast(Comparator.naturalOrder())).thenComparing(User::getAge,Comparator.nullsLast(Comparator.naturalOrder())));
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());