设计模式学习（三）之装饰者模式

设计原则：类应该对扩展开放，对修改关闭

我们的目标是允许类容易扩展，在不修改代码的情况下，就可以搭配新的行为，如能实现这样的目标，这样的设计具有弹性，可以应对改变，可以接受新的功能来应对改变的需求

装饰者和被装饰者对象有相同的超类型

可以用一个或者多个装饰者包装一个对象

既然装饰者和被装饰者有相同的超类型，所以在任何需要原始对象的场合，可以用装饰过的对象代替它

装饰者模式：动态的将责任附加到对象上，若要扩展功能，装饰者提供了比继承更有弹性的替代方案

装饰者类图

看个具体问题：

有一个咖啡店，他们有几种饮料，比如HouseBlend, DarkRoast.Decaf,Espresso

每种饮料里面可以加调料，可以加一种或者几种，也可以不加，调料有，SteamedMilk.Soy,Mocha等

计算一杯饮料的价格，包括饮料的价格和调料的价格

看一下类图

那么问题来了

如果需要添加新的调料，需要加上新的方法

如果价格修改了，需要修改父类的代码

有些饮料并不适合某些调料，仍然加入了判断

如果需要加入双倍的某种调料呢？

看新的类图

用代码来实现 

package com.alvin.decorator;public abstract class Beverage {protected String description;public abstract String getDescription();public abstract float cost();}

package com.alvin.decorator;public class HouseBlend extends Beverage{public float cost(){return 1;}@Overridepublic String getDescription() {return "HouseBlend";}}

package com.alvin.decorator;public class Decaf extends Beverage {@Overridepublic String getDescription() {return "Decaf";}@Overridepublic float cost() {return 3;}}

package com.alvin.decorator;public class DarkRoast extends Beverage {@Overridepublic String getDescription() {return "DarkRoast";}@Overridepublic float cost() {return 2;}}

package com.alvin.decorator.impl;import com.alvin.decorator.Beverage;public abstract class CondimentDecorator extends Beverage {}

package com.alvin.decorator.impl;import com.alvin.decorator.Beverage;public class Milk extends CondimentDecorator {private Beverage beverage;public Milk(Beverage beverage) {this.beverage = beverage;}@Overridepublic String getDescription() {return  beverage.getDescription() + "Milk";}@Overridepublic float cost() {return beverage.cost() + 0.2f;}}

package com.alvin.decorator.impl;import com.alvin.decorator.Beverage;public class Mocha extends CondimentDecorator {private Beverage beverage;public Mocha(Beverage beverage) {this.beverage = beverage;}@Overridepublic String getDescription() {return beverage.getDescription() + "Mocha";}@Overridepublic float cost() {return beverage.cost() + 0.1f;}}

package com.alvin.decorator.impl;import com.alvin.decorator.Beverage;public class Soy extends CondimentDecorator {private Beverage beverage;public Soy(Beverage beverage) {this.beverage = beverage;}@Overridepublic String getDescription() {return beverage.getDescription() + "Soy";}@Overridepublic float cost() {return beverage.cost() + 0.3F;}}

package com.alvin.decorator;import com.alvin.decorator.impl.Milk;import com.alvin.decorator.impl.Mocha;import com.alvin.decorator.impl.Soy;public class Run {public static void main(String[] args) {Beverage beverage = new DarkRoast();System.out.println(beverage.getDescription() + beverage.cost());Beverage beverage1 = new HouseBlend();beverage1 = new Mocha(beverage1);beverage1 = new Milk(beverage1);beverage1 = new Soy(beverage1);System.out.println(beverage1.getDescription() + beverage1.cost());}}

运行结果

DarkRoast2.0
HouseBlendMochaMilkSoy1.6000001

在看一个JDK里面的类图来帮助理解输入输出流