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java 8 lambda表达式list操作分组、过滤、求和、最值、排序、去重

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());


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