设计模式——装饰者模式

装饰者模式：
 
首先定义抽象的组件类，子类继承该类后持有抽象类的引用，在抽象类的基础上可以在调用父类方法前后分别执行自己的行为。从而实现装饰添加新的功能特性。再次强调了使用组合的好处。
强调对扩展开放，对修改关闭。
在Java I/O中应用了装饰者模式。
 

装饰者模式类图：

以下程序来自Head First设计模式中的实例代码，模拟了不同种类的饮料不同的售价计算方法。
 
1.定义饮料接口

package net.dp.decorator; public abstract class Beverage {    protected String description = "Unknown Beverage";     public String getDescription() {        return description;    }     public abstract double cost();}

2.实现三种不同种类的饮料

package net.dp.decorator.coffee; import net.dp.decorator.Beverage; public class Espresso extends Beverage {     public Espresso() {        description = "Espresso";    }     public double cost() {        return 1.99;    }}

package net.dp.decorator; public abstract class Beverage {    protected String description = "Unknown Beverage";     public String getDescription() {        return description;    }     public abstract double cost();}

package net.dp.decorator.coffee; import net.dp.decorator.Beverage; public class HouseBlend extends Beverage {     public HouseBlend() {        description = "House Blend Coffee";    }     public double cost() {        return .89;    }}

3.定义添加了调料的饮料抽象类

package net.dp.decorator; public abstract class CondimentDecorator extends Beverage {    public abstract String getDescription();}

4.实现不同调料的饮料

package net.dp.decorator.flovoring; import net.dp.decorator.Beverage;import net.dp.decorator.CondimentDecorator; public class Mocha extends CondimentDecorator {    Beverage beverage;     public Mocha(Beverage beverage) {        this.beverage = beverage;    }     public String getDescription() {        return beverage.getDescription() + ", Mocha";    }     public double cost() {        return .20 + beverage.cost();    }}

package net.dp.decorator.flovoring; import net.dp.decorator.Beverage;import net.dp.decorator.CondimentDecorator; public class Soy extends CondimentDecorator {    Beverage beverage;     public Soy(Beverage beverage) {        this.beverage = beverage;    }     public String getDescription() {        return beverage.getDescription() + ", Soy";    }     public double cost() {        return .15 + beverage.cost();    }}

package net.dp.decorator.flovoring; import net.dp.decorator.Beverage;import net.dp.decorator.CondimentDecorator; public class Whip extends CondimentDecorator {    Beverage beverage;     public Whip(Beverage beverage) {        this.beverage = beverage;    }     public String getDescription() {        return beverage.getDescription() + ", Whip";    }     public double cost() {        return .10 + beverage.cost();    }}

5.终于大功告成！跑一把！

package net.dp.decorator; import net.dp.decorator.coffee.DarkRoast;import net.dp.decorator.coffee.Espresso;import net.dp.decorator.coffee.HouseBlend;import net.dp.decorator.flovoring.Mocha;import net.dp.decorator.flovoring.Soy;import net.dp.decorator.flovoring.Whip; public class StarbuzzCoffee {     public static void main(String[] args) {        Beverage beverage = new Espresso();        System.out.println(beverage.getDescription() + " $" + beverage.cost());         Beverage beverage2 = new DarkRoast();        beverage2 = new Mocha(beverage2);        beverage2 = new Mocha(beverage2);        beverage2 = new Whip(beverage2);        System.out                .println(beverage2.getDescription() + " $" + beverage2.cost());         Beverage beverage3 = new HouseBlend();        beverage3 = new Soy(beverage2);        beverage3 = new Mocha(beverage2);        beverage3 = new Whip(beverage2);        System.out                .println(beverage3.getDescription() + " $" + beverage3.cost());    }}

可以看到不同种类的饮料得出了不同的价格。