[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());
（3）处填（）。