迷糊了，为什么说找不到符号呢？？？

[waiterNet]

import java.util.*;
class Test{
public static void main(String[] args){
ArrayList<Object> al=new ArrayList<Object>();
al.add(new A(32,41));
al.add(new A(44,60));
al.add(new A(44,41));
al.add(new A(30,54));
al.add(new A(39,24));
al.add(new A(43,44));
//为什么说下句话是找不到符号呢
Collections.sort(al);

System.out.println(al);


}
}
class A implements Comparable{
int x,y;
A(int x,int y){
this.x=x;
this.y=y;
}
public String toString(){
return x+" "+y;
}
public int compareTo(Object o){
A a=(A)o;
return x>a.x?1:(x==a.x?0:-1);
}

}