public interface Iterator
 {
 	public abstract boolean hasNext();
 	public abstract Object next();
 }

 
 public interface Collection
 {
 	public abstract Iterator iterator();
 }
 
 public interface List extends Collection
 {
 	...
 }
 
 public class ArrayList implements List
 {
 	public Iterator iterator() {
        return new Itr();
    }
    
    private class Itr implements Iterator {
        public boolean hasNext() {
            return xxx;;
        }

        public E next() {
            return xxx;
        }
    }
 }
 
 
 
 用法：
 	//多态
 	Collection c = new ArrayList();
 	//添加元素
 	c.add("hello");
 	c.add("world");
 	c.add("java);
 	
 	Iterator it = c.iterator(); //把Itr返回来了，而Itr实现了Iterator接口。所以，这是多态的体现。
 	while(it.hasNext())
 	{
 		String s = it.next();
 		System.out.println(s);
 	}