import java.util.*;

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

              Student std1=new Student(2,"xiaoxin");
                Student std2=new Student(3,"leiyin");
                Student std3=new Student(1,"wuhen");
                ArrayList<Object> al=new ArrayList<Object>();
                al.add(std1);
                al.add(std2);
                al.add(std3);
                Collections.sort(al);
             for(int i=0;i<al.size();i++)
             {
                    System.out.println(al.get(i)); 
             }
     }
}
class Student implements Comparable
{
        int num;
        String name;
        Student(int num, String name)
        {
                  this.num=num;
                  this.name=name; 
        }
          public int compareTo(Object o)
          {
                  Student stu=(Student)o;
                  return ( num>stu.num ? 1:(num==stu.num ? 0:-1));
                   
          }
          public String toString()
          {
                  return num+":"+name; 
          }
}

說找不到Collections.sort(al);

這是為什麼呢？