import java.util.*;//15天13听的不是很懂class GenericDemo {    public static void main(String[] args)     {        ArrayList<Person> a = new ArrayList<Person>();        a.add(new Person("a"));        a.add(new Person("b"));        a.add(new Person("c"));               Iterator it = al.iterator();        while(it.hasNext())        {            String s = it.next(); //注意这里缺少了向下转型            System.out.println(s);        }        ArrayList<Stu> al1 = new ArrayList<Stu>();        al1.add(new Stu("A"));

        al1.add(new Stu("B"));        al1.add(new Stu("C"));        printCln(al1);    }
}