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;
	}

	@Override
	public String getDescription() {
		return "HouseBlend";
	}

}

package com.alvin.decorator;

public class Decaf extends Beverage {

	@Override
	public String getDescription() {
		return "Decaf";
	}

	@Override
	public float cost() {
		return 3;
	}

}

package com.alvin.decorator;

public class DarkRoast extends Beverage {

	@Override
	public String getDescription() {
		return "DarkRoast";
	}

	@Override
	public 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;
	}

	@Override
	public String getDescription() {
		return  beverage.getDescription() + "Milk";
	}

	@Override
	public 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;
	}

	@Override
	public String getDescription() {
		return beverage.getDescription() + "Mocha";
	}

	@Override
	public 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;
	}

	@Override
	public String getDescription() {
		return beverage.getDescription() + "Soy";
	}

	@Override
	public 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