Java Collection - 如何按标记排序学生

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

class Student implements Comparable<Student> {
  public Student(String name, double grade) {
    this.name = name;
    this.grade = grade;
  }

  String name;
  double grade;

  @Override
  public int compareTo(Student o) {
    if (o == null) {
      return -1;
    }
    int c = Double.valueOf(grade).compareTo(o.grade);
    if (c != 0) {
      return c;
    }
    return name.compareTo(o.name);
  }

  @Override
  public String toString() {
    return String.format("%s has grade %.2f", name, grade);
  }
}

public class Main {
  public static void main(String[] args) {
    List<Student> al = new ArrayList<>();
    al.add(new Student("Z", 34.34));
    al.add(new Student("M", 123.22));
    al.add(new Student("A", 1378.00));
    al.add(new Student("D", 99.22));
    al.add(new Student("Q", -19.08));
    Collections.sort(al);
    System.out.println(al);
  }

}