高手帮忙看一下这个JAVA程序

哪位帮我看一下,编译显示"使用了未经检查或不安全的操作"我想可能泛型的问题.但我不知道哪里应该改一下.
import java.util.*;
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;
int result=num>s.num?1:(num==s.num?0:-1);
if(result==0)
result=name.compareTo(s.name);
return result;
}
public String toString()
{
return num+":name="+name;
}
}
class CollectionTest
{
public static void main(String[] args)
{
ArrayList<Student> al=new ArrayList<Student>();
al.add(new Student(2,"zhangsan"));
al.add(new Student(3,"whangwu"));
al.add(new Student(3,"lisi"));
Collections.sort(al);
for(int i=0;i<al.size();i++)
{
System.out.println(al.get(i));