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"
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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).
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如何实现呢?
案例结构图
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案例类图
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组合模式总览
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"
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模式的理解
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类图
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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
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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
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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
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案例代码部分
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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(); } }
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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()); } }
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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(); } }
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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
