模式定義

組合模式允許你將物件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物件組合。

下圖是該模式的類圖：

可以實現下圖的需求：

一個生動的例子

元件抽象類：
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();
	}
}

葉子節點類：
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) throws HeadlessException {
		this.name = name;
		this.description = description;
		this.vegetarian = vegetarian;
		this.price = price;
	}

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

	public void setName(String name) {
		this.name = name;
	}

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

	public void setDescription(String description) {
		this.description = description;
	}

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

	public void setVegetarian(boolean vegetarian) {
		this.vegetarian = vegetarian;
	}

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

	public void setPrice(double price) {
		this.price = price;
	}

	@Override
	public void print() {
		System.out.println(toString());
	}

	@Override
	public String toString() {
		return "MenuItem [name=" + name + ", description=" + description + ", vegetarian=" + vegetarian + ", price="
				+ price + "]";
	}
}

組合節點類：
public class Menu extends MenuComponent {
	ArrayList<MenuComponent> menuComponents = new ArrayList<MenuComponent>();
	private String name;
	private String description;

	public Menu(String name, String description) {
		this.name = name;
		this.description = description;
	}

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

	public void setName(String name) {
		this.name = name;
	}

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

	public void setDescription(String description) {
		this.description = description;
	}

	@Override
	public String toString() {
		return "Menu [menuComponents=" + menuComponents + ", name=" + name + ", description=" + description + "]";
	}

	@Override
	public void print() {
		System.out.println("\nMenu [" + "name=" + name + ", description=" + description + "]");

		Iterator<MenuComponent> iterator = menuComponents.iterator();
		while (iterator.hasNext()) {
			MenuComponent menuComponent = iterator.next();
			menuComponent.print();
		}
	}

	@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 menuComponents.get(i);
	}
}

客戶端類：
public class Waitress {
	MenuComponent allMenus;

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

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

參考

1. Head First 設計模式[書籍]