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

短命女 2022-03-26 06:44 472阅读 0赞

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());