关于TreeSet的add（Integer data）的出错

import java.util.Collection;
import java.util.TreeSet;
public class ContainerT{

public static void main(String[] args){
Collection c =new TreeSet();
c.add("csupper used son`s object");
c.add(new String("as"));
c.add(new String("b"));
c.add(new Integer(11).toString());

//c.add(new Integer(11))出错？？
System.out.println(c.size());
System.out.println(c);
}
}

 

 

 

import java.util.Collection;
import java.util.HashSet;
public class ContainerT{

public static void main(String[] args){
Collection c =new HashSet();
c.add("csupper used son`s object");
c.add(new String("as"));
c.add(new String("b"));
c.add(new Integer(11));

//c.add(new Integer(11).toString());也可以

System.out.println(c.size());
System.out.println(c);
}
}