java集合类问题

import java.util.*;
class Test
{
public static void diedai(Collection c)
{
Iterator i=c.iterator();
while(i.hasNext())
{
System.out.println(i.next());
}
}
public static void main(String[] args)
{
Student s1=new Student(2,"zhangsan");
Student s2=new Student(3,"lisi");
Student s3=new Student(1,"wangwu");
ArrayList<Object> a=new ArrayList<Object>();
a.add(s1);
a.add(s2);
a.add(s3);
Collections.sort(a);
diedai(a);

}
}

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 s=(Student)o;
return num>s.num ? 1 : (num==s.num ? 0 : -1);
}
public String toString()
{
return "

import java.util.*;
class Test
{
public static void diedai(Collection c)
{
Iterator i=c.iterator();
while(i.hasNext())
{
System.out.println(i.next());
}
}
public static void main(String[] args)
{
Student s1=new Student(2,"zhangsan");
Student s2=new Student(3,"lisi");
Student s3=new Student(1,"wangwu");
ArrayList<Student> a=new ArrayList<Student>();
a.add(s1);
a.add(s2);
a.add(s3);
Collections.sort(a);
diedai(a);

}
}

class Student implements Comparable <Student>
{
int num;
String name;
Student(int num,String name)
{
this.num=num;
this.name=name;
}

public int compareTo(Student o)
{
Student s=o;
return num>s.num ? 1 : (num==s.num ? 0 : -1);
}
public String toString()
{