阅读以下说明和Java代码，回答问题

[说明]

对多个元素的聚合进行遍历访问时，需要依次推移元素，例如对数组通过递增下标的方式，数组下标功能抽象化、一般化的结果就称为迭代器(Iterator)。模式以下程序模拟将书籍(Book)放到书架(BookShelf)上并依次输出书名。这样就要涉及到遍历整个书架的过程。使用迭代器Iterator实现。图6-1显示了各个类间的关系。以下是JAVA语言实现，能够正确编译通过。

[图6-1]

[Java代码]

//Iterator. java文件

public interface Iterator {

public abstract boolean hasNext();

public abstract Object next();

}

//Aggregate. java文件

public interface Aggregate {

public abstract Iterator iterator();

}

//Book. java

public class Book {

//省略具体方法和属性

}

//BookshelfIterator. java文件

public class Bookshelf工terator (1) Iterator{

private BookShelf bookShelf;

private int index;

public BookshelfIterator(BookShelf bookShelf) {

this. bookShelf = bookShelf;

this. index = 0;

}

public boolean hasNext(){//判断是否还有下一个元素

if(index < bookShelf. getLength()){

return true;

}else{

return false;

}

}

public Object next()f//取得下一个元素

Book book = bookShelf. getBookAt(index);

index++;

return book;

}

}

//BookShelf. java

import java. util. Vector;

public class BookShelf {

private Vector books;

public BookShelf(int initialsize){

this. books = new Vector(initialsize);

}

public Book getBookAt(int index){

return(Book)books.get(index);

}

public int getLength(){

return books.size();

}

public Iterator iterator(){

return new BookShelfIterator( (2) );

}

}

//Main. java文件

public class Main {

public static void main(String args){

BookShelf bookShelf = new BookShelf(4);

//将书籍上架，省略代码

Iterator it = bookShelf. (3) ;

while( (4) ){//遍历书架，输出书名

Book book = (Book)it. (5) ;

System.out.printin(" "+book.getName());

}

}

}