java8的lambda表达式

转自：https://blog.csdn.net/gsls200808/article/details/86501905

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

1.  
   //分组
2.  
   Map<String, List<User>> groupBySex = userList.stream().collect(Collectors.groupingBy(User::getSex));
3.  
   //遍历分组
4.  
   for (Map.Entry<String, List<User>> entryUser : groupBySex.entrySet()) {
5.  
   String key = entryUser.getKey();
6.  
   List<User> entryUserList = entryUser.getValue();
7.  
   }

多字段分组

1.  
   Function<WarehouseReceiptLineBatch, List<Object>> compositeKey = wlb ->
2.  
   Arrays.<Object>asList(wlb.getWarehouseReceiptLineId(), wlb.getWarehouseAreaId(), wlb.getWarehouseLocationId());
3.  
   Map<Object, List<WarehouseReceiptLineBatch>> map =
4.  
   warehouseReceiptLineBatchList.stream().collect(Collectors.groupingBy(compositeKey, Collectors.toList()));
5.  
   //遍历分组
6.  
   for (Map.Entry<Object, List<WarehouseReceiptLineBatch>> entryUser : map.entrySet()) {
7.  
   List<Object> key = (List<Object>) entryUser.getKey();
8.  
   List<WarehouseReceiptLineBatch> entryUserList = entryUser.getValue();
9.  
   Long warehouseReceiptLineId = (Long) key.get(0);
10.  
    Long warehouseAreaId = (Long) key.get(0);
11.  
    Long warehouseLocationId = (Long) key.get(0);
12.  
     
13.  
    }

2.过滤

通过filter方法可以过滤某些条件

1.  
   //过滤
2.  
   //排除掉工号为201901的用户
3.  
   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

1.  
   //求和
2.  
   //基本类型
3.  
   int sumAge = userList.stream().mapToInt(User::getAge).sum();
4.  
   //BigDecimal求和
5.  
   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

1.  
   //最小
2.  
   Date minEntryDate = userList.stream().map(User::getEntryDate).min(Date::compareTo).get();
3.  
    
4.  
   //最大
5.  
   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

1.  
   Comparator<LeasingBusinessContract> comparator = Comparator.comparing(LeasingBusinessContract::getLeaseEndDate);
2.  
   LeasingBusinessContract maxObject = leasingBusinessContractList.stream().max(comparator).get();

5.List 转map

1.  
   /**
2.  
   * List -> Map
3.  
   * 需要注意的是：
4.  
   * toMap 如果集合对象有重复的key，会报错Duplicate key ....
5.  
   * user1,user2的id都为1。
6.  
   * 可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2
7.  
   */
8.  
   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

1.  
   //排序
2.  
   //单字段排序，根据id排序
3.  
   userList.sort(Comparator.comparing(User::getId));
4.  
   //多字段排序，根据id，年龄排序
5.  
   userList.sort(Comparator.comparing(User::getId).thenComparing(User::getAge));

7.去重

可通过distinct方法进行去重

1.  
   //去重
2.  
   List<Long> idList = new ArrayList<Long>();
3.  
   idList.add(1L);
4.  
   idList.add(1L);
5.  
   idList.add(2L);
6.  
   List<Long> distinctIdList = idList.stream().distinct().collect(Collectors.toList());

针对属性去重

1.  
   List<AddOutboundNoticeDetailsBatchVo> entryDetailsBatchDistinctBatchIdList = entryDetailsBatchList.stream().filter(distinctByKey(b -> b.getMaterialBatchNumberId())).collect(Collectors.toList());
2.  
    
3.  
   //distinctByKey自己定义
4.  
   public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
5.  
   Map<Object, Boolean> seen = new ConcurrentHashMap<>();
6.  
   return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
7.  
   }

8.获取list某个字段组装新list

1.  
   //获取list对象的某个字段组装成新list
2.  
   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());