Iterator ——迭代器

程序示例

Aggregate 接口

// 建立一个可对应聚合的iteratorpublic interface Aggregate {    public Iterator Iterator();}

Iterator 接口

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

Book 类

public class Book {    private String name = "";    public Book(String name) {        this.name = name;    }    public String getName() {        return name;    }}

BookShelf 类

public class BookShelf implements Aggregate {    private Book[] books;    private int last = 0;    public BookShelf(int maxsize) {        this.books = new Book[maxsize];    }    public Book getBookAt(int index) {        return books[index];    }    public void appendBook(Book book) {        this.books[last] = book;        last++;    }    public int getLength() {        return last;    }    public Iterator iterator() {        return new BookShelfIterator(this);    }}

BookShelfIterator 类

public class BookShelfIterator implements Iterator {    private BookShelf bookShelf;    private int index;    public BookShelfIterator(BookShelf bookShelf) {        this.bookShelf = bookShelf;        this.index = 0;    }    public boolean hasNext() {        if (index < bookShelf.getLength()) {            return true;        } else {            return false;        }    }    public Object next() {        Book book = bookShelf.getBookAt(index);        index++;        return book;    }}

Main 类

public class Main {    public static void main(String[] args) {        BookShelf bookShelf = new BookShelf(4);        bookShelf.appendBook(new Book("Around the World in 80 Days"));        bookShelf.appendBook(new Book("Bible"));        bookShelf.appendBook(new Book("Cinderella"));        bookShelf.appendBook(new Book("Daddy-Long-Legs"));        Iterator iterator = bookShelf.iterator();        while (iterator.hasNext()) {            Book book = (Book)iterator.next();            System.out.println("" + book.getName());        }    }}

Iterator Pattern 的所有参与者

• Iterator(迭代器) 参与者
• ConcreteIterator(具体迭代器) 参与者
• Aggregate(聚合) 参与者
• ConcreteAggregate(具体聚合) 参与者

扩展自我视野的提示

无论实现结果如何，都能使用Iterator

设计Pattern的目的就是为了提高类的复用率。提高复用率则是指吧类当作一个零件来使用，只要修改某一个零件，就不需要大费周章去修改其他的零件。

抽象类、接口实在很难搞

过度依赖具体类反而会提高类与类的耦合度，增加零部件复用的困难。为了降低耦合度，让类作为零部件再利用，必须引进抽象类和接口的概念。

“下一个”容易搞错

返回现在的元素，同时进行到下一个位置。

练习题

问题1

若书籍数量超过最先设定的书架大小，就无法继续把书放上去。请利用java.util.Vector取代数组，把程序改成即使已经超过书架容量也能继续新增书籍。

答案

import java.util.Vector;public class BookShelf implements Aggregate {    private Vector books;    public BookShelf(int initialsize) {        this.books = new Vector(initialsize);    }    public Book getBookAt(int index) {    return (Book)books.get(index);    }    public void appendBook(Book book) {        books.add(book);    }    public int getLength() {        return books.length;    }    public Iterator iterator() {        return new BookShelfIterator(this);    }}