JAVA设计模式——组合模式+迭代器模式

         组合模式

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

我们以菜单为例思考这一切：这个模式能够创建一个树形结构，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，我们创建了一个“整体/部分”层次结构，即由菜单和菜单项组成的对象树。但是可以将它视为一个整体，像是一个丰富的大菜单。一旦有了丰富的大菜单，我们就可以使用这个模式来“统一处理个别对象和组合对象”。这意味着，如果我们有了一个树形结构的菜单、子菜单和可能还带有菜单项的子菜单，那么任何一个菜单都是一种“组合”。因为它既可以包含其他菜单，也可以包含菜单项。个别对象只是菜单项——并未持有其他对象。

        具体例子

组件类：MenuComponent.java

package com.designpattern.composite_iterator;import java.util.Iterator;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();}public Iterator<?> createIterator(){throw new UnsupportedOperationException();}}

菜单项：MenuItem.java

package com.designpattern.composite_iterator;import java.util.Iterator;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;}//菜单项无需迭代，直接打印即可；public void print() {System.out.print("2 " + getName());if(isVegetarian()){System.out.print("(v)");}System.out.println(", " + getPrice());System.out.println("  -- " + getDescription());}//外部迭代器；public Iterator<?> createIterator() {return new NullIterator();}}

菜单：Menu.java

package com.designpattern.composite_iterator;import java.util.ArrayList;import java.util.Iterator;public class Menu extends MenuComponent{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 (MenuComponent)menuComponents.get(i);}public String getName() {return name;}public String getDescription() {return description;}public void print() {System.out.print("1\n" + getName());System.out.println(", " + getDescription());System.out.println("-------------------");/** * 内部迭代器 *  * 用它遍历所有菜单组件，可能是菜单项，也有可能是菜单； * 注意：如果遇到另一个菜单对象，它的print（）方法会开始另一个遍历，以此类推； *  * */Iterator<?> iterator = menuComponents.iterator();while (iterator.hasNext()) {MenuComponent menuComponent = (MenuComponent)iterator.next();menuComponent.print();}}//外部迭代器public Iterator<?> createIterator() {return new CompositeIterator(menuComponents.iterator());}}

菜单项迭代器：NullIterator.java

package com.designpattern.composite_iterator;import java.util.Iterator;public class NullIterator implements Iterator<Object>{@Overridepublic boolean hasNext() {// TODO Auto-generated method stubreturn false;}@Overridepublic Object next() {// TODO Auto-generated method stubreturn null;}@Overridepublic void remove() {// TODO Auto-generated method stubthrow new UnsupportedOperationException();}}

菜单迭代器：CompositeIterator.java

package com.designpattern.composite_iterator;import java.util.Iterator;import java.util.Stack;public class CompositeIterator implements Iterator<Object>{Stack<Object> stack = new Stack<Object>();public CompositeIterator(Iterator<?> iterator){stack.push(iterator);}public boolean hasNext() {if(stack.empty()){return false;}else{Iterator<?> iterator = (Iterator<?>) stack.peek();if(!iterator.hasNext()){stack.pop();return hasNext();} else{return true;}}}public Object next() {if(hasNext()){Iterator<?> iterator = (Iterator<?>) stack.peek();MenuComponent component = (MenuComponent) iterator.next();if(component instanceof Menu){stack.push(component.createIterator());}return component;}else{return null;}}public void remove() {throw new UnsupportedOperationException();}}

操作者：Waitress.java

package com.designpattern.composite_iterator;import java.util.Iterator;/** * 操作菜单的类  * */public class Waitress {MenuComponent allMenus;public Waitress(MenuComponent allMenus){this.allMenus = allMenus;}public void printMenu(){allMenus.print();}//使用外部迭代器public void printVegetarianMenu(){Iterator<?> iterator = allMenus.createIterator();System.out.println("\nVEGETARIAN MENU\n----");while(iterator.hasNext()){MenuComponent menuComponent = (MenuComponent)iterator.next();try {if(menuComponent.isVegetarian()){menuComponent.print();}} catch (UnsupportedOperationException e) {}}}}

测试类：Test.java

package com.designpattern.composite_iterator;public class Test {public static void main(String[] args) {MenuComponent pancakeHouseMenu = new Menu("PANCAKE HOUSE MENU", "Breakfast");MenuComponent dinerMenu = new Menu("DINER MENU", "Lunch");dinerMenu.add(new MenuItem("Pasta", "Spaghetti with Marinara Sauce, and a slice of sourdough bread", true, 3.89));MenuComponent dessertMenu = new Menu("DESSERT MENU", "Dessert of course!");dessertMenu.add(new MenuItem("Apple Pie", "Apple pie with a flakey crust, topped with vanilla ice cream", true, 1.59));dinerMenu.add(dessertMenu);MenuComponent cafeMenu = new Menu("CAFE MENU", "Dinner");MenuComponent allMenus = new Menu("ALL MENUS", "All menus combined");allMenus.add(pancakeHouseMenu);allMenus.add(dinerMenu);allMenus.add(cafeMenu);Waitress waitress = new Waitress(allMenus);waitress.printMenu();waitress.printVegetarianMenu();}}

         总结

在写MenuComponent类的print()方法时，我们利用了一个迭代器来遍历组件内的每个项。如果遇到的是菜单（而不是菜单项），我们就会递归地调用print()方法处理它。换句话说，MenuComponent是在“内部”自行处理遍历。

但在Waitress类中，我们实现的是一个“外部”迭代器，所以有许多需要追踪的事情。外部迭代器必须维护它在遍历中的位置，以便外部客户可以通过调用hasNext()和next()来驱动遍历。在这个例子中，我们的代码也必须维护组合递归结构的位置。这也就是为什么当我们在组合层次结构中上上下下时，使用堆栈来维护我们的位置。

但是存在一个问题：使用外部迭代器时有的数据会打印多遍，取决于层级结构，这个貌似是迭代器本身的问题，想只打印一遍数据，需另作处理，或者直接用“内部”迭代器的方式。