关于ArrayList自动排序的问题

[syj247]

请各位大虾帮忙看看 。，。
这样用arraylist自动排序

package day2;

public class Student implements Comparable<Student> {
private String name;
private double scorll;

public String getName() {
return name;
}

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

public double getScorll() {
return scorll;
}

public void setScorll(double scorll) {
this.scorll = scorll;
}

public Student(String name, double scorll) {
this.setName(name);
this.setScorll(scorll);
}

public Student() {
}

@Override//重写toString
public String toString() {
return name + ":" + scorll;
}

@Override//实现compareTo方法
public int compareTo(Student o) {
// TODO Auto-generated method stub
return (int) (o.getScorll() - this.scorll);
}

}
_______________________________________分割线————————————————————

package day2;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

public class TestStudent2 {
public static void main(String[] args) {
// TODO Auto-generated method stub
Student s1 = new Student("syj1", 90);
Student s2 = new Student("syj2", 60);
Student s3 = new Student("syj7", 30);
Student s4 = new Student("syj4", 50);
Student s5 = new Student("syj5", 40);
Student s6 = new Student("syj5", 100);
Student s7 = new Student("syj3", 70);
List<Student> al = new ArrayList<Student>();
al.add(s1);
al.add(s2);
al.add(s3);
al.add(s4);
al.add(s5);
al.add(s6);
al.add(s7);
// 这里怎么写？？？？
Iterator it = al.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}
}

}

[weberwong]

Collections.sort(al);

[syj247]

感谢万分@@@@@！！！！！！！！！！！！！！！！！！！！！！