組合模式（Composite Pattern）

組合模式：允許你將對象組合成樹形結構來表現“整體/部分”層次結構。組合能讓客戶以一致的方式處理個別對象以及對象組合

本質上就是以樹形的數據結構來完成數據的存儲， 而對其的遍歷就是對樹形結構的遍歷。

案例：帶有子菜單的菜單

爲了可以讓客戶以一致的方式處理，我們自然而然的就需要菜單和菜單項多繼承同一個抽象基類

代碼：

import java.util.Iterator;

public abstract class MenuComponent {

	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 add(MenuComponent component) {
		throw new UnsupportedOperationException();
	}
	
	public void remove(MenuComponent component) {
		throw new UnsupportedOperationException();
	}
	
	public MenuComponent getChild(int i) {
		throw new UnsupportedOperationException();
	}
	
	public void print() {
		throw new UnsupportedOperationException();
	}
	
	public Iterator createIterator() {
		throw new UnsupportedOperationException();
	}
}

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

public class Menu extends MenuComponent {

	private String name;
	private String description;
	
	private ArrayList<MenuComponent> list;
	
	public Menu(String name, String description) {
		this.name = name;
		this.description = description;
		list = new ArrayList<MenuComponent>();
	}
	
	@Override
	public String getName() {
		return name;
	}

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

	@Override
	public void add(MenuComponent component) {
		list.add(component);
	}

	@Override
	public void remove(MenuComponent component) {
		list.remove(component);
	}

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

	@Override
	public void print() {
		System.out.println("Menu: name = " + name + ", subItems below: ");
		Iterator i = list.iterator();
		while (i.hasNext()) {
			MenuComponent menuComponent = (MenuComponent)i.next();
			menuComponent.print();
		}
	}

	@Override
	public Iterator createIterator() {
		return new MenuComponentIterator(list.iterator());
	}
}

import java.util.Iterator;

public class MenuItem extends MenuComponent {

	private String name;
	private String description;
	private boolean vegetarian;
	private 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 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.println("Menu item: name = " + name + ", price = " + price);
	}

	@Override
	public Iterator createIterator() {
		return null;
	}
}

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

public class MenuComponentIterator implements Iterator {
	
	private Stack<Iterator> stack;
	
	public MenuComponentIterator(Iterator iterator) {
		if (stack == null) {
			stack = new Stack<Iterator>();
		}
		stack.push(iterator);
	}

	@Override
	public boolean hasNext() {
		if (stack.isEmpty()) {
			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 menuComponent = (MenuComponent)iterator.next();
			if (menuComponent instanceof Menu) {
				stack.push(menuComponent.createIterator());
			}
			return menuComponent;
		} else {
			return null;
		}
	}

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

public class PatternDemo {
	
	public static void main(String[] args) {
		Menu allMenu = new Menu("All menu", "");
		MenuItem menuItem = new MenuItem("Hot dog", "", false, 2.39);
		MenuItem subMenuItem = new MenuItem("V BLT", "", false, 2.39);
		MenuItem subMenuItem1 = new MenuItem("V1", "", true, 1.39);
		MenuItem subMenuItem2 = new MenuItem("BLT", "", true, 3.39);
		
		Menu subMenu = new Menu("Sub menu", "sub menu");
		allMenu.add(menuItem);
		subMenu.add(subMenuItem);
		subMenu.add(subMenuItem1);
		subMenu.add(subMenuItem2);
		allMenu.add(subMenu);
		
		allMenu.print();
	}
}

這裏兩個地方用到了遞歸，print()方法以及MenuComponentIterator的實現裏面。