使用Java Stream API进行多级分组和求和

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        // 示例数据
        List<Person> persons = Arrays.asList(
                new Person("Alice", "A", 100),
                new Person("Bob", "A", 200),
                new Person("Charlie", "B", 300),
                new Person("David", "B", 400),
                new Person("Eve", "C", 500)
        );

        // 多级分组和求和
        Map<String, Map<String, Integer>> result = persons.stream()
                .collect(Collectors.groupingBy(Person::getGroup,
                        Collectors.groupingBy(Person::getCategory,
                                Collectors.summingInt(Person::getAmount))));

        // 输出结果
        result.forEach((group, categoryMap) -> {
            System.out.println("Group: " + group);
            categoryMap.forEach((category, sum) -> {
                System.out.println("Category: " + category + ", Sum: " + sum);
            });
        });
    }

    static class Person {
        private String name;
        private String category;
        private int amount;

        public Person(String name, String category, int amount) {
            this.name = name;
            this.category = category;
            this.amount = amount;
        }

        public String getName() {
            return name;
        }

        public String getCategory() {
            return category;
        }

        public int getAmount() {
            return amount;
        }

        public String getGroup() {
            return name.substring(0, 1);
        }
    }
}

Group: A
Category: A, Sum: 300
Group: B
Category: B, Sum: 700
Group: C
Category: C, Sum: 500