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> groupBySex = userList.stream().collect(Collectors.groupingBy(User::getSex));
    //遍历分组
    for (Map.Entry> entryUser : groupBySex.entrySet()) {
        String key = entryUser.getKey();
        List 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, List> groupMap = list.stream().collect(Collectors.groupingBy(x -> Optional.ofNullable(x.getReviewerGroupId())));
//遍历分组
for (Map.Entry, List> entryGroup : groupMap.entrySet()) {
Optional key = entryGroup.getKey();
Long groupId = key.get();
List 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 Collector>>
groupingBy_WithNullKeys(Function classifier) {
return Collectors.toMap(
classifier,
Collections::singletonList,
(List oldList, List newEl) -> {
List newList = new ArrayList<>(oldList.size() + 1);
newList.addAll(oldList);
newList.addAll(newEl);
return newList;
});
}
然后调用

Map> groupMap = list.stream().collect(groupingBy_WithNullKeys(x -> x.getReviewerGroupId()));
多字段分组

    Function> compositeKey = wlb ->
            Arrays.