裝飾者模式，可以動態地把職責附加到已有的對象上面去。又稱 Wrapper Pattern，在代碼實現的時候，確實有“包裝”的意思。
# 類圖
圖：來自《Head First Design Patterns》
可以看出，裝飾者模式里面，有4個角色：Component抽象類，ConcreteComponent具體類，Decorator抽象類，ConcreteDecorator具體類
圖：我的Java源代碼的類圖
總共10個類
一個Component抽象類
兩個ConcreteComponent具體類
一個Decorator抽象類
五個ConcreteDecorator具體類
一個Main測試類
ConcreteDecorator **繼承**了Decorator抽象類， **組合**了Component抽象類。Decorator抽象類繼承了Component抽象類，這個繼承的目的是：it’s vital that the decorators have the same type as the object they are going to decorate。簡而言之，就是為了獲得相同的類型，并不是為了獲得行為。組合是為了讓所有的concreteDecorator可以“互相包裝、裝飾”
# talk is cheap, show me the code
去食堂打菜，經濟套餐，假設總共只有5種食物可供選擇分別是：手撕包菜，茄子豆角，麻婆豆腐，青椒肉絲，紅燒里脊。
### 一個Component抽象類
~~~
package decorator;
public abstract class Meal
{
String description = "Unknown Meal";
public String getDescription()
{
return description;
}
public abstract double cost();
}
~~~
### 兩個ConcreteComponent具體類
~~~
package decorator;
public class ChineseMeal extends Meal
{
public ChineseMeal()
{
description = "Chinese Meal";
}
@Override
public double cost()
{
return 1.0; // 一次性餐具費用
}
}
~~~
~~~
package decorator;
public class WesternMeal extends Meal
{
public WesternMeal()
{
description = "Western meal";
}
@Override
public double cost()
{
return 5.0; // 不銹鋼刀、叉費用
}
}
~~~
### 一個Decorator抽象類
~~~
package decorator;
public abstract class Decorator extends Meal
{
public abstract String getDescription();
}
~~~
### 五個ConcreteDecorator具體類
~~~
package decorator;
// 茄子豆角
public class EggplantBean extends Decorator
{
Meal meal;
public EggplantBean(Meal meal)
{
this.meal = meal;
}
@Override
public String getDescription()
{
return meal.getDescription() + ", 茄子豆角";
}
@Override
public double cost()
{
return 2 + meal.cost();
}
}
~~~
~~~
package decorator;
// 青椒炒肉
public class GreenPepperPork extends Decorator
{
Meal meal;
public GreenPepperPork(Meal meal)
{
this.meal = meal;
}
@Override
public String getDescription()
{
return meal.getDescription() + ", 青椒炒肉";
}
@Override
public double cost()
{
return 3 + meal.cost();
}
}
~~~
~~~
package decorator;
// 手撕包菜
public class HandCabbage extends Decorator
{
Meal meal;
public HandCabbage(Meal meal)
{
this.meal = meal;
}
@Override
public String getDescription()
{
return meal.getDescription() + ", 手撕包菜";
}
@Override
public double cost()
{
return 2 + meal.cost();
}
}
~~~
~~~
package decorator;
// 麻婆豆腐
public class MapoTofu extends Decorator
{
Meal meal;
public MapoTofu(Meal meal)
{
this.meal = meal;
}
@Override
public String getDescription()
{
return meal.getDescription() + ", 麻婆豆腐";
}
@Override
public double cost()
{
return 4 + meal.cost();
}
}
~~~
~~~
package decorator;
// 紅燒里脊
public class BraiseTenderloin extends Decorator
{
Meal meal;
public BraiseTenderloin(Meal meal)
{
this.meal = meal;
}
@Override
public String getDescription()
{
return meal.getDescription() + ", 紅燒里脊";
}
@Override
public double cost()
{
return 5 + meal.cost();
}
}
~~~
### 一個Main測試類
~~~
package decorator;
public class Main
{
public static void main(String[] args)
{
Meal meal = new ChineseMeal();
meal = new GreenPepperPork(meal);
meal = new HandCabbage(meal);
meal = new BraiseTenderloin(meal);
System.out.println(meal.getDescription() + " ￥" + meal.cost());
Meal meal2 = new ChineseMeal();
meal2 = new EggplantBean(new MapoTofu(new BraiseTenderloin(meal2)));
System.out.println(meal2.getDescription() + " ￥" + meal2.cost());
}
}
~~~
# 運行結果
直接從eclipse復制過來
~~~
Chinese Meal, 青椒炒肉, 手撕包菜, 紅燒里脊 ￥11.0
Chinese Meal, 紅燒里脊, 麻婆豆腐, 茄子豆角 ￥12.0
~~~
# 分析討論
去食堂打菜，經濟套餐，假設總共只有5種食物可供選擇分別是：手撕包菜，茄子豆角，麻婆豆腐，青椒肉絲，紅燒里脊。如果可以隨意打菜，就可以有C15+C25+C35+C45+C55=25?1=31 種選擇。如果寫代碼把這120種選擇都分別弄成一個類，就有31個類。這樣不僅代碼重復率太高，而且如果食堂**更改**了上述五樣菜中一樣菜的**價格**，那么**31個類的代碼需要全部修改**。這不符合開閉原則，不利于代碼的維護和擴展。
裝飾者模式就可以解決上述這種“類的爆炸式增長”問題，各種各樣的排列組合太多，不能在代碼中一個類一個。
裝飾者模式的特點是“繼承（inheritance） + 組合（Composition）”：繼承是為了讓component和decorator擁有相同的類型，組合是為了讓所有的concreteDecorator可以“互相包裝、裝飾”。裝飾者又名Wrapper大概就是這樣來的。
# Java. IO
Java.IO 中應用了裝飾者模式
~~~
FileInputStream fiStream = null;
InputStreamReader iStreamReader = null;
BufferedReader bReader = null;
fiStream = new FileInputStream("C:\\xxxx");
// InputStreamReader 是字節流通向字符流的橋梁、
iStreamReader = new InputStreamReader(fiStream);
// 從字符輸入流中讀取文件中的內容,裝飾了一個InputStreamReader的對象
bReader = new BufferedReader(iStreamReader("C:\\xxxx"));
~~~
~~~
一句代碼版本，把3個new放到一起
BufferedReader bReader = new BufferedReader(new InputStreamReader(new FileInputStream("C:\\xxxx"));
~~~
