设计模式之迭代器与组合模式（四）

由于这系列篇幅较长，因此在这里也不进行任何铺垫，直奔主题去啦。

利用组合设计菜单

咱们要如何在菜单上应用组合模式呢？一开始，咱们须要建立一个组件接口来做为菜单和菜单项的共同接口，让咱们可以用统一的作法来处理菜单和菜单项。换句话说，咱们能够针对菜单或菜单项调用相同的方法。

让咱们从头来看看如何让菜单可以符合组合模式的结构：

实现菜单组件

好了，咱们开始编写菜单组件的抽象类；请记住，菜单组件的角色是为叶节点和组合节点提供一个共同的接口。

public abstract class MenuComponent {
   
	public void add(MenuComponent menuComponent) {
		throw new UnsupportedOperationException();
	}
	public void remove(MenuComponent menuComponent) {
		throw new UnsupportedOperationException();
	}
	public MenuComponent getChild(int i) {
		throw new UnsupportedOperationException();
	}
  
	public String getName() {
		throw new UnsupportedOperationException();
	}
	public String getDescription() {
		throw new UnsupportedOperationException();
	}
	public double getPrice() {
		throw new UnsupportedOperationException();
	}
	public boolean isVegetarian() {
		throw new UnsupportedOperationException();
	}

	public abstract Iterator<MenuComponent> createIterator();
 
	public void print() {
		throw new UnsupportedOperationException();
	}
}
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让咱们来看菜单类。别忘了，这是组合类图里的叶类，它实现组合内元素的行为。

public class MenuItem extends MenuComponent {
 
	String name;
	String description;
	boolean vegetarian;
	double price;
    
	public MenuItem(String name, String description, boolean vegetarian, double price) { 
		this.name = name;
		this.description = description;
		this.vegetarian = vegetarian;
		this.price = price;
	}
  
	public String getName() {
		return name;
	}
  
	public String getDescription() {
		return description;
	}
  
	public double getPrice() {
		return price;
	}
  
	public boolean isVegetarian() {
		return vegetarian;
	}

	public Iterator<MenuComponent> createIterator() {
		return new NullIterator();
	}
 
	public void print() {
		System.out.print(" " + getName());
		if (isVegetarian()) {
			System.out.print("(v)");
		}
		System.out.println(", " + getPrice());
		System.out.println(" -- " + getDescription());
	}

}
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咱们已经有了菜单项，还须要组合类，这就是咱们叫作菜单的。别忘了，此组合类能够持有菜单项或其余菜单。

public class Menu extends MenuComponent {
	Iterator<MenuComponent> iterator = null;
	ArrayList<MenuComponent> menuComponents = new ArrayList<MenuComponent>();
	String name;
	String description;
  
	public Menu(String name, String description) {
		this.name = name;
		this.description = description;
	}
 
	public void add(MenuComponent menuComponent) {
		menuComponents.add(menuComponent);
	}
 
	public void remove(MenuComponent menuComponent) {
		menuComponents.remove(menuComponent);
	}
 
	public MenuComponent getChild(int i) {
		return menuComponents.get(i);
	}
 
	public String getName() {
		return name;
	}
 
	public String getDescription() {
		return description;
	}

  
	public Iterator<MenuComponent> createIterator() {
		if (iterator == null) {
			iterator = new CompositeIterator(menuComponents.iterator());
		}
		return iterator;
	}
 
 
	public void print() {
		System.out.print("\n" + getName());
		System.out.println(", " + getDescription());
		System.out.println("---------------------");
	}
}
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由于菜单是一个组合，包含了菜单项和其余的菜单，因此它的print()应该打印出它所包含的一切。若是它不这么作，咱们就必须遍历整个组合的每一个节点，而后将每一项打印出来。这么一来，也就失去了使用组合结构的意义。

因此，print还得进行优化，以下:

public void print() {
	System.out.print("\n" + getName());
	System.out.println(", " + getDescription());
	System.out.println("---------------------");
 
	Iterator<MenuComponent> iterator = menuComponents.iterator();
	while (iterator.hasNext()) {
		MenuComponent menuComponent = iterator.next();
		menuComponent.print();
	}
}
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看到上面了没，咱们用了迭代器。用它遍历全部菜单组件，遍历过程当中，可能遇到其余菜单，或者是遇到菜单项。因为菜单和菜单项都实现了print，那咱们只要调用print便可。

开始测试数据以前，咱们了解一下，在运行时菜单组合是什么样的：

开始运行咱们的测试程序啦:

public class MenuTestDrive {
	public static void main(String args[]) {

		MenuComponent pancakeHouseMenu = 
			new Menu("PANCAKE HOUSE MENU", "Breakfast");
		MenuComponent dinerMenu = 
			new Menu("DINER MENU", "Lunch");
		MenuComponent cafeMenu = 
			new Menu("CAFE MENU", "Dinner");
		MenuComponent dessertMenu = 
			new Menu("DESSERT MENU", "Dessert of course!");
  
		MenuComponent allMenus = new Menu("ALL MENUS", "All menus combined");
  
		allMenus.add(pancakeHouseMenu);
		allMenus.add(dinerMenu);
		allMenus.add(cafeMenu);
  
		pancakeHouseMenu.add(new MenuItem(
			"K&B's Pancake Breakfast", 
			"Pancakes with scrambled eggs, and toast", 
			true,
			2.99));
		pancakeHouseMenu.add(new MenuItem(
			"Regular Pancake Breakfast", 
			"Pancakes with fried eggs, sausage", 
			false,
			2.99));
		pancakeHouseMenu.add(new MenuItem(
			"Blueberry Pancakes",
			"Pancakes made with fresh blueberries, and blueberry syrup",
			true,
			3.49));
		pancakeHouseMenu.add(new MenuItem(
			"Waffles",
			"Waffles, with your choice of blueberries or strawberries",
			true,
			3.59));

		dinerMenu.add(new MenuItem(
			"Vegetarian BLT",
			"(Fakin') Bacon with lettuce & tomato on whole wheat", 
			true, 
			2.99));
		dinerMenu.add(new MenuItem(
			"BLT",
			"Bacon with lettuce & tomato on whole wheat", 
			false, 
			2.99));
		dinerMenu.add(new MenuItem(
			"Soup of the day",
			"A bowl of the soup of the day, with a side of potato salad", 
			false, 
			3.29));
		dinerMenu.add(new MenuItem(
			"Hotdog",
			"A hot dog, with saurkraut, relish, onions, topped with cheese",
			false, 
			3.05));
		dinerMenu.add(new MenuItem(
			"Steamed Veggies and Brown Rice",
			"A medly of steamed vegetables over brown rice", 
			true, 
			3.99));
 
		dinerMenu.add(new MenuItem(
			"Pasta",
			"Spaghetti with Marinara Sauce, and a slice of sourdough bread",
			true, 
			3.89));
   
		dinerMenu.add(dessertMenu);
  
		dessertMenu.add(new MenuItem(
			"Apple Pie",
			"Apple pie with a flakey crust, topped with vanilla icecream",
			true,
			1.59));
		dessertMenu.add(new MenuItem(
			"Cheesecake",
			"Creamy New York cheesecake, with a chocolate graham crust",
			true,
			1.99));
		dessertMenu.add(new MenuItem(
			"Sorbet",
			"A scoop of raspberry and a scoop of lime",
			true,
			1.89));

		cafeMenu.add(new MenuItem(
			"Veggie Burger and Air Fries",
			"Veggie burger on a whole wheat bun, lettuce, tomato, and fries",
			true, 
			3.99));
		cafeMenu.add(new MenuItem(
			"Soup of the day",
			"A cup of the soup of the day, with a side salad",
			false, 
			3.69));
		cafeMenu.add(new MenuItem(
			"Burrito",
			"A large burrito, with whole pinto beans, salsa, guacamole",
			true, 
			4.29));
 
		Waitress waitress = new Waitress(allMenus);
   
		waitress.printVegetarianMenu();
 
	}
}
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结果这里就不附上了，请你们自行去跑代码实现吧。相信大家又对组合模式也已经有了一个大概了吧。下一篇，还有更犀利的，组合迭代器等着咱们。小编立刻回去搞起来，安排上。

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