ArrayList<class>中，class有int学号属性和String班级属性，怎么样按学号升序输出？

你可以使用`Comparator`接口来定义按学号升序排序的规则，并使用`Collections.sort()`方法来排序。以下是示例代码：

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class Student {
    private int id;
    private String className;

    public Student(int id, String className) {
        this.id = id;
        this.className = className;
    }

    public int getId() {
        return id;
    }

    public String getClassName() {
        return className;
    }
}

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

        students.add(new Student(3, "A"));
        students.add(new Student(1, "B"));
        students.add(new Student(2, "C"));

        Comparator<Student> comparator = Comparator.comparing(Student::getId);

        Collections.sort(students, comparator);

        for (Student student : students) {
            System.out.println("id: " + student.getId() + ", className: " + student.getClassName());
        }
    }
}

输出结果为：

id: 1, className: B
id: 2, className: C
id: 3, className: A