设计模式学习专栏十--------组合模式

设计模式学习专栏十--------组合模式

场景

---

回顾迭代器模式的案例

我们希望能够加上一份 餐后甜点的 "子菜单"

我们需要什么?

• 我们需要某种树形结构 , 可以容纳菜单 , 子菜单和菜单项
• 我们需要确定能够在每个菜单的各个项之间游走 . 而且至少要像现在用迭代器一样方便
• 我们也需要能够更有弹性地在菜单项之间游走 . 比如说 , 可以只需要遍历甜品菜单, 或者可以遍历餐厅的整个菜单(包括甜点菜单在内).

如何实现呢?

案例结构图

案例类图

组合模式总览

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定义:允许你将对象组成树形结构来表现"整体 / 部分"的层次结构.组合能让客户以一致的方式处理个别对象和对象组合(忽略整体/部分的区别)
我们可以使用这个模式来 "统一处理个别对象 和 组合对象"

• 模式的理解

  • 类图

    

  • 角色

    • 叶子结点Leaf
    • 组合结点 Composite
    • 叶子结点与组合结点共同接口 Component

  • 细节

    • 当我们有数个对象的集合, 它们之间有"整体/部分"的关系,并且我们想用一致的方法对待这些对象时,就可以使用组合模式
    • 组合模式提供了一个结构,可同时包容个别对象和组合对象
    • 组合结构内的任意对象为组件 , 组件可以是组合,也可以是叶子结点
    • 如果有些对象具备一些没有意义的调用(比如叶子结点的getChild), 我们可以返回null值或者false

      我们可以把没意义的调用放到父类中去默认实现 , 但这样会失去一些"安全性" (调用无效操作),

      或者把责任区分别放到不同的接口中 (比如把add , getChild放到叶子结点) , 但那样子对客户就失去了"透明性"(需要使用instanceof判断类型并强转) , 因此这需要我们折衷

案例代码部分

• 抽象组件MenuComponent

  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();
      }
  }

• 菜单项MenuItem

  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());
      }
  
  }

• 菜单Menu

  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;
      }
       //菜单Menu重写 add , remove ,getChild方法
      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("---------------------");
    
          Iterator<MenuComponent> iterator = menuComponents.iterator();
          while (iterator.hasNext()) {
              MenuComponent menuComponent = iterator.next();
              menuComponent.print();
          }
      }
  }

• 客户端

  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 allMenus = new Menu("ALL MENUS", "All menus combined");
    
          allMenus.add(pancakeHouseMenu);
          allMenus.add(dinerMenu);
    
          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));
  
          Waitress waitress = new Waitress(allMenus);
          waitress.printMenu();
      }
  }

• 输出结果

  ALL MENUS, All menus combined
  ---------------------
  
  PANCAKE HOUSE MENU, Breakfast
  ---------------------
    K&B's Pancake Breakfast(v), 2.99
       -- Pancakes with scrambled eggs, and toast
    Regular Pancake Breakfast, 2.99
       -- Pancakes with fried eggs, sausage
    Blueberry Pancakes(v), 3.49
       -- Pancakes made with fresh blueberries, and blueberry syrup
    Waffles(v), 3.59
       -- Waffles, with your choice of blueberries or strawberries
  
  DINER MENU, Lunch
  ---------------------
    Vegetarian BLT(v), 2.99
       -- (Fakin') Bacon with lettuce & tomato on whole wheat
    BLT, 2.99
       -- Bacon with lettuce & tomato on whole wheat
    Soup of the day, 3.29
       -- A bowl of the soup of the day, with a side of potato salad
    Hotdog, 3.05
       -- A hot dog, with saurkraut, relish, onions, topped with cheese
    Steamed Veggies and Brown Rice(v), 3.99
       -- A medly of steamed vegetables over brown rice
    Pasta(v), 3.89
       -- Spaghetti with Marinara Sauce, and a slice of sourdough bread

参考

​ 书籍: HeadFirst设计模式

​ 代码参考地址: 我就是那个地址