设计模式之九：迭代器与组合模式

有许多方法可以把对象堆起来成为一个集合（Collection），比如放入数组、堆栈或散列表中。若用户直接从这些数据结构中取出对象，则需要知道具体是存在什么数据结构中（如栈就用peek，数组[]）。迭代器能够让客户遍历你的对象而又无法窥视你存储对象的方式。

对象村餐厅和煎饼屋合并了，它们有着不同的菜单列表，但菜单项基础都是一样的。

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;}
};

下面就写Java代码了，改成C++一时半会还是做不过来。

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;}
};

这两种不同的菜单表现方式，会使得女招待需要知道菜单的实现细节，才能对菜单进行遍历。

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];

如果还有第三家餐厅以不同的实现出现，我们就需要有三个循环。

因此，我们需要创建一个对象（迭代器），封装“遍历集合内的每个对象的过程”。

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

当我们拥有迭代器接口后，我们就可以为各种对象集合实现迭代器

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()// 删除getMenuItems()public Iterator createIterator(){// 返回迭代器接口。客户不需要知道餐厅菜单如何维护菜单项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());}}
}

现在可以进一步对waitress进行优化，因为她还捆绑与两个具体的菜单类。但在优化之前，我们先看下目前的设计。

除了使用自己构建的迭代器接口外，还可以直接使用java.util的迭代器接口，同时ArrayList也有一个返回迭代器的方法。

// 煎饼屋的代码public Iterator createIterator()
{return menuItems.iterator();
}// 餐厅的代码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;}}}

最后我们再给菜单一个共同的接口，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());}}}