java list分组聚合

废话不多说所直接上代码

1. 只对某个属性分组 (personsList为存储实体类的集合)

group by age

 Map<Integer,List<Person>> map = personsList.stream().collect(Collectors.groupingBy(Person::getAge));

结果：

2.先分组，再获取所有聚合统计的参数

Map<Integer, DoubleSummaryStatistics> collect = persons.stream().collect(
                Collectors.groupingBy(Person::getAge,
                        Collectors.summarizingDouble(Person::getSalary)));

结果：

3.先分组，然后对工资聚合

sum(salary)
group by age

Map collect1 = persons.stream().collect(
               Collectors.groupingBy(Person::getAge, Collectors.summingDouble(Person::getSalary)));

结果：

整体代码总体如下：

import java.util.*;
import java.util.stream.Collectors;

public class Test {

    public static void main(String[] args) {

        List<Person> persons = new ArrayList<>();

        persons.add(new Person("Brandon", 15, 5000));
        persons.add(new Person("Braney", 15, 15000));
        persons.add(new Person("Jack", 10, 5000));
        persons.add(new Person("Robin", 10, 500000));
        persons.add(new Person("Tony", 10, 1400000));

		// 只对 age 分组
        Map<Integer,List<Person>> map = persons.stream().collect(Collectors.groupingBy(Person::getAge));
        for (Map.Entry<Integer, List<Person>> entry : map.entrySet()) {
            System.out.println("分组" +entry);
        }
        System.out.println("=======================");
        
        // 对age分组 然后取所有的聚合结果集
        Map<Integer, DoubleSummaryStatistics> collect = persons.stream().collect(
                Collectors.groupingBy(Person::getAge,
                        Collectors.summarizingDouble(Person::getSalary)));
        for (Map.Entry<Integer, DoubleSummaryStatistics> entry : collect.entrySet()) {
            DoubleSummaryStatistics longSummaryStatistics = entry.getValue();
            System.out.println("----------------key----------------" + entry.getKey());
            System.out.println("求和:" + longSummaryStatistics.getSum());
            System.out.println("求平均" + longSummaryStatistics.getAverage());
            System.out.println("求最大:" + longSummaryStatistics.getMax());
            System.out.println("求最小:" + longSummaryStatistics.getMin());
            System.out.println("求总数:" + longSummaryStatistics.getCount());
        }

        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");

		// 对age分组 然后取求和结果集
        Map<Integer, Double> collect1 = persons.stream().collect(
                Collectors.groupingBy(Person::getAge, Collectors.summingDouble(Person::getSalary)));
        System.out.println(collect1);
        System.out.println("1");


    }
}

结果:

分组10=[Person [name=Jack, age=10, salary=5000.0], Person [name=Robin, age=10, salary=500000.0], Person [name=Tony, age=10, salary=1400000.0]]
分组15=[Person [name=Brandon, age=15, salary=5000.0], Person [name=Braney, age=15, salary=15000.0]]
=======================
----------------key----------------10
求和:1905000.0
求平均635000.0
求最大:1400000.0
求最小:5000.0
求总数:3
----------------key----------------15
求和:20000.0
求平均10000.0
求最大:15000.0
求最小:5000.0
求总数:2
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
{10=1905000.0, 15=20000.0}
1

实体类

/**
 * 要聚合的实体类
 /
class Person {
    private String name;
    private int age;
    private double salary;

    public Person(String name, int age, double salary) {
        super();
        this.name = name;
        this.age = age;
        this.salary = salary;
    }

    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 double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    public String getNameAndAge() {
        return this.getName() + "-" + this.getAge();
    }

    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + ", salary=" + salary
                + "]";
    }
}