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。

示例—合并两种菜单并访问甜品菜单和素食菜单

有两种餐厅菜单，分别用数组和ArrayList实现。现在需要合并两种菜单，且合并后可以让一个女招待的类去访问，不需要重复代码就可以同时访问两种菜单。 将甜品菜单添加到午餐菜单中，做到既可以访问餐厅的所有的菜单也可以单独访问甜品菜单和素食菜单。

UML图表示

组合模式-遍历树形菜单

代码演示

组件基类

package TreeMenu;

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();
    }
}

叶子菜单项

package TreeMenu;

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;
    }

    @Override
    public String toString() {
        return name + ", " + price + " -- " + description;
    }

    @Override
    public String getName() {
        return name;
    }

    @Override
    public String getDescription() {
        return description;
    }

    @Override
    public boolean isVegetarian() {
        return vegetarian;
    }

    @Override
    public double getPrice() {
        return price;
    }

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

    @Override
    public Iterator createIterator() {
        return new NullIterator();
    }
}

菜单

package TreeMenu;

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

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;
    }

    @Override
    public void add(MenuComponent menuComponent){
        menuComponents.add(menuComponent);
    }

    @Override
    public void remove(MenuComponent menuComponent){
        menuComponents.remove(menuComponent);
    }

    @Override
    public MenuComponent getChild(int i) {
        return (MenuComponent)menuComponents.get(i);
    }

    @Override
    public String getName() {
        return name;
    }

    @Override
    public String getDescription() {
        return description;
    }

    @Override
    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();
        }
    }

    @Override
    public Iterator createIterator() {
        return new CompositeIterator(menuComponents.iterator());
    }

}

组合迭代器

package TreeMenu;

import java.util.Iterator;
import java.util.Stack;

public class CompositeIterator implements Iterator {

    Stack stack = new Stack();

    public CompositeIterator(Iterator iterator){
        stack.push(iterator);
    }

    @Override
    public boolean hasNext() {
        if (stack.empty()){
            return false;
        }
        else {
            Iterator iterator = (Iterator) stack.peek();
            if (!iterator.hasNext()) {
                stack.pop();
                return hasNext();
            }
            else{
                return true;
            }
        }
    }

    @Override
    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;
        }
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }
}

空迭代器

package TreeMenu;

import java.util.Iterator;

public class NullIterator implements Iterator {
    @Override
    public boolean hasNext() {
        return false;
    }

    @Override
    public Object next() {
        return null;
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }
}

服务员

package TreeMenu;

import java.util.Iterator;

public class Waitress {
    MenuComponent allMenus;

    public Waitress(MenuComponent allMenus){
        this.allMenus = allMenus;
    }

    public void printMenu(){
        allMenus.print();
    }

    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) {}
        }
    }
}

测试代码

package TreeMenu;

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 dessertMenu = new Menu("DESSERT MENU" , "Dessert of course!");

        MenuComponent allMenus = new Menu("All MENUS", "All menus combined");

        allMenus.add(pancakeHouseMenu);
        allMenus.add(dinerMenu);

        MenuItem d1 = new MenuItem("Vegetarian BLT",
                "(Fakin')Bacon with lettuce & tomato on whole wheat", true, 2.99);

        MenuItem d2 = new MenuItem("Vegetarian BLT",
                "(Fakin')Bacon with lettuce & tomato on whole wheat", true, 2.99);
        MenuItem d3 = new MenuItem("BLT","Bacon with lettuce & tomato on whole wheat", false,2.99);
        MenuItem d4 = new MenuItem("Soup of the day"
                ,"Soup of the day, with a side of potato salad", false,3.29);
        MenuItem d5 = new MenuItem("Hotdog","A hot dog, with saurkraut, relish, onions, topped with cheese",
                false,3.05);

        dinerMenu.add(d1);
        dinerMenu.add(d2);
        dinerMenu.add(d3);
        dinerMenu.add(d4);
        dinerMenu.add(d5);


        MenuItem p1 = new MenuItem("K&B's Pancake Breakfast",
                "Pancakes with scrambled eggs, and toast",
                true,2.99);
        MenuItem p2 = new MenuItem("Regular Pancake Breakfast",
                "Pancakes with fired eggs, sausage",
                false,2.99);
        MenuItem p3 = new MenuItem("Blueberry Pancakes",
                "Pancakes made with fresh blueberries",
                true,3.49);
        MenuItem p4 = new MenuItem("Waffles",
                "Waffles, with your choice of blueberries or strawberries",
                true,3.59);

        pancakeHouseMenu.add(p1);
        pancakeHouseMenu.add(p2);
        pancakeHouseMenu.add(p3);
        pancakeHouseMenu.add(p4);


        MenuItem ds1 = new MenuItem("Apple Pie",
                "Apple Pie with a flaky crust",
                true,1.59);
        MenuItem ds2 = new MenuItem("Banner Pie",
                "Banner Pie topped with vanilla ice cream",
                true,1.59);
        dessertMenu.add(ds1);
        dessertMenu.add(ds2);
        dinerMenu.add(dessertMenu);

        Waitress waitress = new Waitress(allMenus);
        waitress.printMenu();

        waitress.printVegetarianMenu();
    }
}

测试结果

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 fired eggs, sausage
 Blueberry Pancakes(v), 3.49
    --Pancakes made with fresh blueberries
 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
 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
    --Soup of the day, with a side of potato salad
 Hotdog, 3.05
    --A hot dog, with saurkraut, relish, onions, topped with cheese

DESSERT MENU,Dessert of course!
-------------------------
 Apple Pie(v), 1.59
    --Apple Pie with a flaky crust
 Banner Pie(v), 1.59
    --Banner Pie topped with vanilla ice cream

VEGETARIAN MENU
-----
 K&B's Pancake Breakfast(v), 2.99
    --Pancakes with scrambled eggs, and toast
 Blueberry Pancakes(v), 3.49
    --Pancakes made with fresh blueberries
 Waffles(v), 3.59
    --Waffles, with your choice of blueberries or strawberries
 Vegetarian BLT(v), 2.99
    --(Fakin')Bacon with lettuce & tomato on whole wheat
 Vegetarian BLT(v), 2.99
    --(Fakin')Bacon with lettuce & tomato on whole wheat
 Apple Pie(v), 1.59
    --Apple Pie with a flaky crust
 Banner Pie(v), 1.59
    --Banner Pie topped with vanilla ice cream
 Apple Pie(v), 1.59
    --Apple Pie with a flaky crust
 Banner Pie(v), 1.59
    --Banner Pie topped with vanilla ice cream