目的
讓外界以一致的方式處理個別物件和整體物件（多個遞迴組合的物件）。
Treat individual objects & multiple, recursively composed objects uniformly.
適用
- 物件必須是遞迴組合（部分-全體關係）
- 個別物件和複合物件之間沒有差異或無需考慮其差異
- 能以一致的方式處理複合結構裡的物件
結構及成員
Collaborations: Client透過Component類別介面，與複合結構裡的Object互動。如果對象是個Leaf，就直接處理，如果是Composite，就會將訊息傳遞給子節點去處理，在傳遞之前或之後也可能會做些額外的事情。
影響結果
好處
壞處
實作
設置父節點的Reference
較易上溯結構、刪除節點，也利於製作Chain of Responsibility Pattern。
共用Component
如果一個組件可以有好幾個父節點，將父節點記錄在子節點中。
極大化Component介面
Component類別應盡可能涵蓋Composite和Leaf可能會有的操作。若遇到Leaf不會有的操作（如：存取子節點），可以將其改為不會傳回任何子節點（或是Exception）。
宣告管理子節點的操作（如：add()和remove()）
安全性（Safety）和通透性（Transparency）的取捨：
- 若定義在Component：通透性較好，安全性較差。無法避免別人寫出「可增、刪Leaf子節點」的程式。
- 若定義在Composite：安全性較好，但通透性較差。因為Leaf和Composite的介面不同。
做法差異、顧及安全性的改寫方法：
在此模式中，較強調通透性。因為如果型別資訊遺失，就必須手動將Component轉型成Composite。
該把「一群Component」的功能做進Component類別裡面嗎？（Component裡的Component物件存放的變數）
如果在Component裡宣告子節點的Component的變數，則Leaf也會背負這些空間。唯有整個結構中只有少數幾個子節點時才值得這麼做。
子節點之間的相對順序
若有必要維持子節點的相對順序，就須設計存取管理子節點的介面，可以使用Iterator Pattern。
以快取機制增進效率
若常常需要巡訪或搜尋composite object，可以使用快取處理（如快取Picture所佔的長方形區域）。但需要連帶做「向父節點申報作廢快取資訊」的介面。
誰該負責刪除Component？
若程式語言沒有垃圾回收機制（Garbage Collection），應讓Composite負責刪除動作。但共用的Leaf不能刪除。
儲存Component的最佳資料結構為何？
Linked List、Tree、Array、Hash Table等等都行，但有時一個變數對一個子節點就夠用。
Example: Menu
//Class: MenuComponent
package 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();
}
}//Class: Menu
package 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();
}
}
}//Class: MenuItem
package 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());
}
}//Class: Waitress
package composite.menu;
import java.util.Iterator;
public class Waitress {
MenuComponent allMenus;
public Waitress(MenuComponent allMenus) {
this.allMenus = allMenus;
}
public void printMenu() {
allMenus.print();
}
}//Class: MenuTestDrive
package 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();
}
}Result:
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 fried eggs, sausage
Blueberry Pancakes(v), 3.49
-- Pancakes made with fresh blueberries, and blueberry syrup
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
BLT, 2.99
-- Bacon with lettuce & tomato on whole wheat
Soup of the day, 3.29
-- A bowl of the soup of the day, with a side of potato salad
Hotdog, 3.05
-- A hot dog, with saurkraut, relish, onions, topped with cheese
Steamed Veggies and Brown Rice(v), 3.99
-- Steamed vegetables over brown rice
Pasta(v), 3.89
-- Spaghetti with Marinara Sauce, and a slice of sourdough bread
DESSERT MENU, Dessert of course!
---------------------
Apple Pie(v), 1.59
-- Apple pie with a flakey crust, topped with vanilla icecream
Cheesecake(v), 1.99
-- Creamy New York cheesecake, with a chocolate graham crust
Sorbet(v), 1.89
-- A scoop of raspberry and a scoop of lime
CAFE MENU, Dinner
---------------------
Veggie Burger and Air Fries(v), 3.99
-- Veggie burger on a whole wheat bun, lettuce, tomato, and fries
Soup of the day, 3.69
-- A cup of the soup of the day, with a side salad
Burrito(v), 4.29
-- A large burrito, with whole pinto beans, salsa, guacamole
COFFEE MENU, Stuff to go with your afternoon coffee
---------------------
Coffee Cake(v), 1.59
-- Crumbly cake topped with cinnamon and walnuts
Bagel, 0.69
-- Flavors include sesame, poppyseed, cinnamon raisin, pumpkin
Biscotti(v), 0.89
-- Three almond or hazelnut biscotti cookies
