設計模式之迭代器與組合模式（四）

因為這系列篇幅較長，所以在這裡也不進行任何鋪墊，直奔主題去啦。

利用組合設計菜單

我們要如何在菜單上應用組合模式呢？一開始，我們需要創建一個組件介面來作為菜單和菜單項的共同介面，讓我們能夠用統一的做法來處理菜單和菜單項。換句話說，我們可以針對菜單或菜單項調用相同的方法。

讓我們從頭來看看如何讓菜單能夠符合組合模式的結構：

實現菜單組件

好了，我們開始編寫菜單組件的抽象類；請記住，菜單組件的角色是為葉節點和組合節點提供一個共同的介面。

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();
    }
}

讓我們來看菜單類。別忘了，這是組合類圖裡的葉類，它實現組合內元素的行為。

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());
    }

}

我們已經有了菜單項，還需要組合類，這就是我們叫做菜單的。別忘了，此組合類可以持有菜單項或其他菜單。

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("---------------------");
    }
}

因為菜單是一個組合，包含了菜單項和其他的菜單，所以它的print()應該列印出它所包含的一切。如果它不這麼做，我們就必須遍歷整個組合的每個節點，然後將每一項列印出來。這麼一來，也就失去了使用組合結構的意義。

所以，print還得進行優化，如下:

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();
    }
}

看到上面了沒，我們用了迭代器。用它遍歷所有菜單組件，遍歷過程中，可能遇到其他菜單，或者是遇到菜單項。由於菜單和菜單項都實現了print，那我們只要調用print即可。

開始測試數據之前，我們瞭解一下，在運行時菜單組合是什麼樣的：

開始運行我們的測試程式啦:

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();
 
    }
}

結果這裡就不附上了，請大家自行去跑代碼實現吧。相信你們又對組合模式也已經有了一個大概了吧。下一篇，還有更犀利的，組合迭代器等著我們。小編馬上回去搞起來，安排上。