用java写一个Student类实现comparable接口，使得一个类型为Student的ArrayList能够使用sort方法排序，按照年龄从小到大，如果年龄相同则按照成绩升序排序。通过标准输入读入8个学生的年龄和成绩，并输出排序后的结果。

好的，这是一个非常好的问题。下面是一个示例Student类的实现，实现了Comparable接口，并按照要求进行了排序：

import java.util.*;

class Student implements Comparable<Student>{
    int age;
    double grade;

    public Student(int age, double grade) {
        this.age = age;
        this.grade = grade;
    }

    public int compareTo(Student s) {
        if (this.age != s.age) {
            return this.age - s.age;
        } else {
            return (int) (this.grade - s.grade);
        }
    }

    public String toString() {
        return "Age: " + this.age + ", Grade: " + this.grade;
    }
}

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        ArrayList<Student> students = new ArrayList<>();

        for (int i = 1; i <= 8; i++) {
            System.out.println("Enter the age of student " + i + ":");
            int age = sc.nextInt();
            System.out.println("Enter the grade of student " + i + ":");
            double grade = sc.nextDouble();
            students.add(new Student(age, grade));
        }

        Collections.sort(students);

        System.out.println("Sorting result:");
        for (int i = 0; i < students.size(); i++) {
            System.out.println("Student " + (i+1) + ": " + students.get(i));
        }
    }
}

输出结果：

Enter the age of student 1:
20
Enter the grade of student 1:
90
Enter the age of student 2:
22
Enter the grade of student 2:
80
Enter the age of student 3:
20
Enter the grade of student 3:
85
Enter the age of student 4:
23
Enter the grade of student 4:
70
Enter the age of student 5:
21
Enter the grade of student 5:
75
Enter the age of student 6:
23
Enter the grade of student 6:
85
Enter the age of student 7:
21
Enter the grade of student 7:
80
Enter the age of student 8:
22
Enter the grade of student 8:
90
Sorting result:
Student 1: Age: 20, Grade: 85.0
Student 2: Age: 20, Grade: 90.0
Student 3: Age: 21, Grade: 75.0
Student 4: Age: 21, Grade: 80.0
Student 5: Age: 22, Grade: 80.0
Student 6: Age: 22, Grade: 90.0
Student 7: Age: 23, Grade: 70.0
Student 8: Age: 23, Grade: 85.0