如何实现对ArrayList排序 sort()

package com.collection;

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

public class Test {
     public static void main(String[] args) {

Student zlj = new Student("丁晓宇", 21);
          Student dxy = new Student("赵四", 22);
          Student cjc = new Student("张三", 11);
          Student lgc = new Student("刘武", 19);

List<Student> studentList = new ArrayList<Student>();
          studentList.add(zlj);
          studentList.add(dxy);
          studentList.add(cjc);
          studentList.add(lgc);

System.out.println("按年龄升序：");
          Collections.sort(studentList, new SortByAge());
          for (Student student : studentList) {
           System.out.println(student.getName() + " / " + student.getAge());
          }
          System.out.println();
          System.out.println("按姓名排序：");
          Collections.sort(studentList, new SortByName());
          for (Student student : studentList) {
           System.out.println(student.getName() + " / " + student.getAge());
          }
         }
        }

class SortByAge implements Comparator {
         public int compare(Object o1, Object o2) {
          Student s1 = (Student) o1;
          Student s2 = (Student) o2;
         if (s1.getAge() > s2.getAge())
           return 1;
          return -1;
         }
        }

class SortByName implements Comparator {
         public int compare(Object o1, Object o2) {
          Student s1 = (Student) o1;
          Student s2 = (Student) o2;
          return s1.getName().compareTo(s2.getName());
         }
}

输出结果：

按年龄升序：
张三 / 11
刘武 / 19
丁晓宇 / 21
赵四 / 22

按姓名排序：
丁晓宇 / 21
刘武 / 19
张三 / 11
赵四 / 22