如何按Arraylist多维分组

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class ArrayListGrouping {
    public static void main(String[] args) {
        // 创建ArrayList对象，存储待分组的数据
        List<Student> students = new ArrayList<>();
        students.add(new Student("Alice", "Math"));
        students.add(new Student("Bob", "English"));
        students.add(new Student("Charlie", "Math"));
        students.add(new Student("David", "Science"));
        students.add(new Student("Eve", "English"));
        students.add(new Student("Frank", "Science"));

        // 定义HashMap对象，用于存储分组结果
        Map<String, List<Student>> groups = new HashMap<>();

        // 遍历ArrayList中的每个元素，进行分组
        for (Student student : students) {
            String subject = student.getSubject();

            // 判断HashMap中是否存在以该属性值为键的分组
            if (!groups.containsKey(subject)) {
                // 如果不存在，则创建一个新的ArrayList作为该属性值的分组，并将该分组添加到HashMap中
                groups.put(subject, new ArrayList<>());
            }

            // 将当前元素添加到对应的分组中
            groups.get(subject).add(student);
        }

        // 输出分组结果
        for (Map.Entry<String, List<Student>> entry : groups.entrySet()) {
            String subject = entry.getKey();
            List<Student> group = entry.getValue();

            System.out.println("Subject: " + subject);
            for (Student student : group) {
                System.out.println("  " + student.getName());
            }
        }
    }
}

class Student {
    private String name;
    private String subject;

    public Student(String name, String subject) {
        this.name = name;
        this.subject = subject;
    }

    public String getName() {
        return name;
    }

    public String getSubject() {
        return subject;
    }
}

Subject: Math
  Alice
  Charlie
Subject: English
  Bob
  Eve
Subject: Science
  David
  Frank