设计模式之组合模式

组合模式允许你将对象组合成树形结构来表现“整体/部分”层次结构。组合能让客户以一致的方式处理个别对象以及对象组合。

组合模式能够创建一个树型结构,在同一个结构中处理嵌套菜单和菜单项组。通过将菜单和项放在相同的结构中,我们创建一个“整体/部分”层次结构,即由菜单和菜单项组成的对象树。其实 WordPress的菜单项也很有可能是这种模式,因为菜单中不仅仅有菜单项还可以有菜单,这个要等我的源码分析到那里的时候在讲了。


这是组合模式的示意图,Component为组合中的所有对象定义一个接口,不管是组合还是叶节点,它可以为 add()、remove()、setChild()和它的操作实现一些默认的行为。不管是叶节点还是枝节点都继承自 Component。

下面还是那个服务员和菜单的例子:

MenuComponent类,菜单和菜单项类都会继承这个类:

package headfirst.composite.menu;

import java.util.*;

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 void print() {
		throw new UnsupportedOperationException();
	}
}

菜单项类:

package headfirst.composite.menu;

import java.util.Iterator;
import java.util.ArrayList;

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 void print() {
		System.out.print("  " + getName());
		if (isVegetarian()) {
			System.out.print("(v)");
		}
		System.out.println(", " + getPrice());
		System.out.println("     -- " + getDescription());
	}
}

菜单类:

package headfirst.composite.menu;

import java.util.Iterator;
import java.util.ArrayList;

public class Menu extends MenuComponent {
	ArrayList menuComponents = new ArrayList();
	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 (MenuComponent)menuComponents.get(i);
	}
 
	public String getName() {
		return name;
	}
 
	public String getDescription() {
		return description;
	}
 
	public void print() {
		System.out.print("\n" + getName());
		System.out.println(", " + getDescription());
		System.out.println("---------------------");
  
		Iterator iterator = menuComponents.iterator();
		while (iterator.hasNext()) {
			MenuComponent menuComponent = 
				(MenuComponent)iterator.next();
			menuComponent.print();
		}
	}
}

Client类也就是服务员类:

package headfirst.composite.menu;

import java.util.Iterator;
  
public class Waitress {
	MenuComponent allMenus;
 
	public Waitress(MenuComponent allMenus) {
		this.allMenus = allMenus;
	}
 
	public void printMenu() {
		allMenus.print();
	}
}

入口类:

package headfirst.composite.menu;

import java.util.*;

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 coffeeMenu = new Menu("COFFEE MENU", "Stuff to go with your afternoon coffee");
  
		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",
			"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));

		cafeMenu.add(coffeeMenu);

		coffeeMenu.add(new MenuItem(
			"Coffee Cake",
			"Crumbly cake topped with cinnamon and walnuts",
			true,
			1.59));
		coffeeMenu.add(new MenuItem(
			"Bagel",
			"Flavors include sesame, poppyseed, cinnamon raisin, pumpkin",
			false,
			0.69));
		coffeeMenu.add(new MenuItem(
			"Biscotti",
			"Three almond or hazelnut biscotti cookies",
			true,
			0.89));
 
		Waitress waitress = new Waitress(allMenus);
   
		waitress.printMenu();
	}
}