有許多方法可以把對(duì)象堆起來成為一個(gè)集合（Collection），比如放入數(shù)組、堆?；蛏⒘斜碇?。若用戶直接從這些數(shù)據(jù)結(jié)構(gòu)中取出對(duì)象，則需要知道具體是存在什么數(shù)據(jù)結(jié)構(gòu)中（如棧就用peek，數(shù)組[]）。迭代器能夠讓客戶遍歷你的對(duì)象而又無法窺視你存儲(chǔ)對(duì)象的方式。

對(duì)象村餐廳和煎餅屋合并了，它們有著不同的菜單列表，但菜單項(xiàng)基礎(chǔ)都是一樣的。

class MenuItem
{
private:string name;string description;bool vegetarian;double price;public:MenuItem(string name, string description, bool vegetarian, double price){this->name = name;this->description = description;this->vegetarian = vegetarian;this->price = price;}string getName(){return name;}string getDescription(){return description;}bool isVegetarian(){return vegetarian;}double getpPrice(){return price;}
};

下面就寫Java代碼了，改成C++一時(shí)半會(huì)還是做不過來。

public class PancakeHouseMenu
{ArrayList menuItems;public PancakeHouseMenu(){menuItems = new ArrayList();addItem("K&B's Pancake Breakfast", "Pancakes with scrambled eggs, and toast", true, 2.99);}public void addItem(String name, String description, boolean vegetarian, double price){MenuItem menuItem = new MenuItem(name, description, vegetarian, price);menuItems.add(menuItem);}public ArrayList getMenuItems(){return menuItems;}
};/ ********************************************************/
public class DinerMenu
{static final int MAX_ITEMS = 6;int numberOfItems = 0;MenuItem[] menuItems;public DinerMenu(){menuItems = new MenuItem[MAX_ITEMS];addItem("Vegetarian BLT", "Fakin Bacon", true, 2.99);}public void addItem(String name, String description, boolean vegetarian, double price){MenuItem menuItem = new MenuItem(name, description, vegetarian, price);if (numberOfItems >= MAX_ITEMS){System.err.println("Sorry, menu is full! Can't add item to menu");}else{menuItems[numberOfItems++] = menuItem;}}public MenuItem[] getMenuItems(){return menuItems;}
};

這兩種不同的菜單表現(xiàn)方式，會(huì)使得女招待需要知道菜單的實(shí)現(xiàn)細(xì)節(jié)，才能對(duì)菜單進(jìn)行遍歷。

PancakeHouseMenu pancakeHouseMenu = new PancakeHouseMenu();
ArrayList breakfastItems = pancakeHouseMenu.getMenuItems();for breakfastItems.size()
MenuItem menuItem = (MenuItem)breakfastItems.get(i);/ ******************************************************************* /
DinerMenu dinerMenu = new DinerMenu();
MenuItem[] lunchItems = DinerMenu.getMenuItems();for lunchItems.size()
MenuItem menuItem = lunchItems[i];

如果還有第三家餐廳以不同的實(shí)現(xiàn)出現(xiàn)，我們就需要有三個(gè)循環(huán)。

因此，我們需要?jiǎng)?chuàng)建一個(gè)對(duì)象（迭代器），封裝“遍歷集合內(nèi)的每個(gè)對(duì)象的過程”。

Iterator iter = breakfastItems.createIterator();while (iter.hasNext())
{MenuItem menuItem = (MenuItem)iter.next();
}

?當(dāng)我們擁有迭代器接口后，我們就可以為各種對(duì)象集合實(shí)現(xiàn)迭代器

public interface Iterator
{boolean hasNext();Object next();
};public class DinerMenuIterator implements Iterator
{MenuItem[] items;int position = 0;public DinerMenuIterator(MenuItem[] items){this.items = items;}public Object next(){MenuItem menuItem = items[position++];return menuItem;}public boolean hasNext(){if (position >= items.length || items[position] == null) return false;else return true;}
};

有了DinerMenuIterator后就可以改造DinerMenu和PancakeHouseMenu。

public class DinerMenu
{static final int MAX_ITEMS = 6;int numberOfItems = 0;MenuItem[] menuItems;// public DinerMenu()// addItem()// 刪除getMenuItems()public Iterator createIterator(){// 返回迭代器接口?？蛻舨恍枰啦蛷d菜單如何維護(hù)菜單項(xiàng)return new DinerMenuIterator(menuItems);}
};

public class Waitress {PancakeHouseMenu pancakeHouseMenu;DinerMenu dinerMenu;public Waitress(PancakeHouseMenu pancakeHouseMenu, DinerMenu dinerMenu) {this.pancakeHouseMenu = pancakeHouseMenu;this.dinerMenu = dinerMenu;}public void printMenu() {Iterator pancakeIterator = pancakeHouseMenu.createIterator();Iterator dinerIterator = dinerMenu.createIterator();System.out.println("MENU\n----\nBREAKFAST");printMenu(pancakeIterator);System.out.println("\nLUNCH");printMenu(dinerIterator);}private void printMenu(Iterator iterator) {while (iterator.hasNext()) {MenuItem menuItem = iterator.next();System.out.print(menuItem.getName() + ", ");System.out.print(menuItem.getPrice() + " -- ");System.out.println(menuItem.getDescription());}}
}

現(xiàn)在可以進(jìn)一步對(duì)waitress進(jìn)行優(yōu)化，因?yàn)樗€捆綁與兩個(gè)具體的菜單類。但在優(yōu)化之前，我們先看下目前的設(shè)計(jì)。

?除了使用自己構(gòu)建的迭代器接口外，還可以直接使用java.util的迭代器接口，同時(shí)ArrayList也有一個(gè)返回迭代器的方法。

?

// 煎餅屋的代碼public Iterator createIterator()
{return menuItems.iterator();
}// 餐廳的代碼public class DinerMenuIterator implements Iterator {MenuItem[] list;int position = 0;public DinerMenuIterator(MenuItem[] list) {this.list = list;}public MenuItem next() {MenuItem menuItem = list[position];position = position + 1;return menuItem;}public boolean hasNext() {if (position >= list.length || list[position] == null) {return false;} else {return true;}}public void remove() {if (position <= 0) {throw new IllegalStateException("You can't remove an item until you've done at least one next()");}if (list[position-1] != null) {for (int i = position-1; i < (list.length-1); i++) {list[i] = list[i+1];}list[list.length-1] = null;}}}

最后我們?cè)俳o菜單一個(gè)共同的接口，然后修改下女招待。

public interface Menu
{public Iterator createIterator();
}public class Waitress {Menu pancakeHouseMenu;Menu dinerMenu;public Waitress(Menu pancakeHouseMenu, Menu dinerMenu) {this.pancakeHouseMenu = pancakeHouseMenu;this.dinerMenu = dinerMenu;}public void printMenu() {Iterator<MenuItem> pancakeIterator = pancakeHouseMenu.createIterator();Iterator<MenuItem> dinerIterator = dinerMenu.createIterator();System.out.println("MENU\n----\nBREAKFAST");printMenu(pancakeIterator);System.out.println("\nLUNCH");printMenu(dinerIterator);}private void printMenu(Iterator iterator) {while (iterator.hasNext()) {MenuItem menuItem = (MenuItem)iterator.next();System.out.print(menuItem.getName() + ", ");System.out.print(menuItem.getPrice() + " -- ");System.out.println(menuItem.getDescription());}}}

?