1. 定義，來自wiki（http://en.wikipedia.org/wiki/Decorator_pattern）

The decorator pattern can be used to extend (decorate) the functionality of a certain object statically, or in some cases at?run-time, independently of other instances of the same?class, provided some groundwork is done at design time. This is achieved by designing a new?decoratorclass that?wraps?the original class. This wrapping could be achieved by the following sequence of steps:

Subclass the original "Component" class into a "Decorator" class (see UML diagram);

In the Decorator class, add a Component pointer as a field;

Pass a Component to the Decorator constructor to initialize the Component pointer;

In the Decorator class, redirect all "Component" methods to the "Component" pointer; and

In the ConcreteDecorator class, override any Component method(s) whose behavior needs to be modified.

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2. 實例，來自wiki（http://en.wikipedia.org/wiki/Decorator_pattern）

// The abstract Coffee class defines the functionality of Coffee implemented by decorator public abstract class Coffee {public abstract double getCost(); // Returns the cost of the coffeepublic abstract String getIngredients(); // Returns the ingredients of the coffee }// Extension of a simple coffee without any extra ingredients public class SimpleCoffee extends Coffee {public double getCost() {return 1;}public String getIngredients() {return "Coffee";} } // Abstract decorator class - note that it extends Coffee abstract class public abstract class CoffeeDecorator extends Coffee {protected final Coffee decoratedCoffee;protected String ingredientSeparator = ", ";public CoffeeDecorator (Coffee decoratedCoffee) {this.decoratedCoffee = decoratedCoffee;}public double getCost() { // Implementing methods of the abstract classreturn decoratedCoffee.getCost();}public String getIngredients() {return decoratedCoffee.getIngredients();} } // Decorator Milk that mixes milk with coffee. // Note it extends CoffeeDecorator. class Milk extends CoffeeDecorator {public Milk (Coffee decoratedCoffee) {super(decoratedCoffee);}public double getCost() { // Overriding methods defined in the abstract superclassreturn super.getCost() + 0.5;}public String getIngredients() {return super.getIngredients() + ingredientSeparator + "Milk";} }// Decorator Whip that mixes whip with coffee. // Note it extends CoffeeDecorator. class Whip extends CoffeeDecorator {public Whip (Coffee decoratedCoffee) {super(decoratedCoffee);}public double getCost() {return super.getCost() + 0.7;}public String getIngredients() {return super.getIngredients() + ingredientSeparator + "Whip";} }// Decorator Sprinkles that mixes sprinkles with coffee. // Note it extends CoffeeDecorator. class Sprinkles extends CoffeeDecorator {public Sprinkles (Coffee decoratedCoffee) {super(decoratedCoffee);}public double getCost() {return super.getCost() + 0.2;}public String getIngredients() {return super.getIngredients() + ingredientSeparator + "Sprinkles";} }

測試類

public class Main {public static final void main(String[] args) {Coffee c = new SimpleCoffee();System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());c = new Milk(c);System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());c = new Sprinkles(c);System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());c = new Whip(c);System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());// Note that you can also stack more than one decorator of the same typec = new Sprinkles(c);System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());}}

輸出結果

Cost: 1.0; Ingredients: Coffee Cost: 1.5; Ingredients: Coffee, Milk Cost: 1.7; Ingredients: Coffee, Milk, Sprinkles Cost: 2.4; Ingredients: Coffee, Milk, Sprinkles, Whip Cost: 2.6; Ingredients: Coffee, Milk, Sprinkles, Whip, Sprinkles

3. 優缺點（http://tianli.blog.51cto.com/190322/35287/）

Decorator模式有以下的優缺點： 1.??????比靜態繼承更靈活?與對象的靜態繼承相比，Decorator模式提供了更加靈活的向對象添加職責的方式，可以使用添加和分離的方法，用裝飾在運行時刻增加和刪除職責。使用繼承機制增加職責需要創建一個新的子 ? ? ? ? ? ?類，如果需要為原來所有的子類都添加功能的話，每個子類都需要重寫，增加系統的復雜度，此外可以為一個特定的Component類提供多個Decorator，這種混合匹配是適用繼承很難做到的。 2.??????避免在層次結構高層的類有太多的特征，Decorator模式提供了一種“即用即付”的方法來添加職責，他并不試圖在一個復雜的可訂制的類中支持所有可預見的特征，相反可以定義一個簡單的類，并且用Decorator ? ? ? ? ? ?類給他逐漸的添加功能，可以從簡單的部件組合出復雜的功能。 3.??????Decorator?與它的Component不一樣?Decorator是一個透明的包裝，如果我們從對象標識的觀點出發，一個被裝飾了的組件與這個組件是有差別的，因此使用裝飾時不應該以來對象標識。 4.??????產生許多小對象，采用Decorator模式進行系統設計往往會產生許多看上去類似的小對象，這些對象僅僅在他們相互連接的方式上有所不同。

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轉載于:https://www.cnblogs.com/davidwang456/p/3844770.html

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