创建一个学生类，存储到ArrayList集合中完成指定排序操作。

import java.util.Collections;

public class Student implements Comparable<Student> {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

/*    @Override
    public int compareTo(Object o) {
       // return 0;
        return this.age-o
    }*/

    @Override
    public int compareTo(Student o) {
//        return 0;
        return this.age-o.age;
    }
}

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

public class Demo02 {
    public static void main(String[] args) {
        ArrayList<Student> arrayList = new ArrayList<>();
        arrayList.add(new Student("aaa",19));
        arrayList.add(new Student("bbb",16));
        arrayList.add(new Student("ccc",20));
        arrayList.add(new Student("ddd",16));
        System.out.println(arrayList);
        Collections.sort(arrayList, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                int result = o2.getAge()-o1.getAge();//降序
                if (result == 0) {
                    result = o1.getName().charAt(0)-o2.getName().charAt(0);//升序

                }
                return result;
            }
        });
        System.out.println(arrayList);
    }
}