使用Java Stream API按字段分组计算sum

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

public class Main {
    public static void main(String[] args) {
        // 假设有一个包含对象的List集合
        List<Person> persons = Arrays.asList(
                new Person("Alice", "A", 10),
                new Person("Bob", "A", 20),
                new Person("Charlie", "B", 30),
                new Person("David", "B", 40),
                new Person("Eve", "C", 50)
        );

        // 使用Stream API按字段分组并计算sum
        Map<String, Integer> sumByGroup = persons.stream()
                .collect(Collectors.groupingBy(Person::getGroup, Collectors.summingInt(Person::getValue)));

        // 输出结果
        sumByGroup.forEach((group, sum) -> System.out.println("Group: " + group + ", Sum: " + sum));
    }
}

class Person {
    private String name;
    private String group;
    private int value;

    public Person(String name, String group, int value) {
        this.name = name;
        this.group = group;
        this.value = value;
    }

    public String getName() {
        return name;
    }

    public String getGroup() {
        return group;
    }

    public int getValue() {
        return value;
    }
}