设计模式笔记之Decorator Pattern

装饰模式: Decorator Pattern

概括: 动态地给一个对象添加一些额外的职责。单从增加功能来说，用装饰模式比简单使用子类继承的方式来的灵活。

关键字: Has-a，

重点: 每个装饰类都要实现定义好的实现功能的纯虚装饰函数。各个装饰类都has-a一个拥有基本功能的类，通过对接口的实现，给has-a关系的基础功能增加新的功能

类图:

简单例子: http://en.wikipedia.org/wiki/Decorator_pattern

待装饰的基类:

// The Coffee Interface defines the functionality of Coffee implemented by decoratorpublic interface Coffee {    public double getCost(); // returns the cost of the coffee    public String getIngredients(); // returns the ingredients of the coffee} // implementation of a simple coffee without any extra ingredientspublic class SimpleCoffee implements Coffee {    public double getCost() {        return 1;    }     public String getIngredients() {        return "Coffee";    }}

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虚装饰类:

// abstract decorator class - note that it implements Coffee interfaceabstract public class CoffeeDecorator implements Coffee {    protected final Coffee decoratedCoffee; // has-a一个待装饰的基类    protected String ingredientSeparator = ", ";     public CoffeeDecorator(Coffee decoratedCoffee) {        this.decoratedCoffee = decoratedCoffee;    }     public double getCost() { // implementing methods of the interface        return decoratedCoffee.getCost();    }     public String getIngredients() {        return decoratedCoffee.getIngredients();    }}

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实际增加功能的装饰类,即提供给用户使用的装饰类：

// Decorator Milk that mixes milk with coffee// note it extends CoffeeDecoratorpublic class Milk extends CoffeeDecorator {    public Milk(Coffee decoratedCoffee) {        super(decoratedCoffee);    }     public double getCost() { // overriding methods defined in the abstract superclass        return super.getCost() + 0.5;    }     public String getIngredients() {        return super.getIngredients() + ingredientSeparator + "Milk";    }}

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实际用户子类:

public class Main{    public static 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());                ...    }}