学习笔记之装饰者模式

装饰者（Decorator）模式又名包装（Wrapper）模式，它动态地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。

package org.kangsg219.abst;

//星巴兹咖啡抽象类
public abstract class Beverage {
	
	public String description="Unknown Beverage!";
	
	public String getDescription(){
		return description;
	}
	
	public abstract double cost();

}

package org.kangsg219.abst;

//调料(Condiment)抽象类
public abstract class CondimentDecorator extends Beverage {
	public abstract String getDescription();

}

package org.kangsg219.impl;

import org.kangsg219.abst.Beverage;

//浓缩咖啡(Espresso)具体类
public class Espresso extends Beverage {

	public Espresso(){
		description="Espresso";
	}
	
	@Override
	public double cost() {
		
		return 1.99;
	}

}

package org.kangsg219.impl;

import org.kangsg219.abst.Beverage;
//HouseBlend咖啡具体类
public class HouseBlend extends Beverage {

	public HouseBlend(){
		description="HouseBlend";
	}
	
	@Override
	public double cost() {
		
		return 0.89;
	}

}

package org.kangsg219.impl;

import org.kangsg219.abst.Beverage;
import org.kangsg219.abst.CondimentDecorator;

public class Mocha extends CondimentDecorator {

	public Beverage beverage;
	
	public Mocha(Beverage beverage){
		this.beverage=beverage;
	}
	
	@Override
	public String getDescription() {
		
		return beverage.getDescription()+",mocha";
	}

	@Override
	public double cost() {
		
		return beverage.cost()+0.20;
	}

}

package org.kangsg219.impl;

import org.kangsg219.abst.Beverage;
import org.kangsg219.abst.CondimentDecorator;

//奶泡具体装饰类
public class Whip extends CondimentDecorator {

	public Beverage beverage;
	
	public Whip(Beverage beverage){
		this.beverage=beverage;
	}
	
	@Override
	public String getDescription() {
		
		return beverage.getDescription()+",Whip";
	}

	@Override
	public double cost() {
		
		return beverage.cost()+ 0.10;
	}

}

package org.kangsg219.test;

import org.kangsg219.abst.Beverage;
import org.kangsg219.impl.Espresso;
import org.kangsg219.impl.HouseBlend;
import org.kangsg219.impl.Mocha;
import org.kangsg219.impl.Whip;

public class StarbuzzCoffee {

	
	public static void main(String[] args) {
		//仅要一杯浓缩咖啡，不要调料。
		Beverage beverage=new Espresso();
		System.out.println(beverage.getDescription()+" $"+beverage.cost());
		
		//双倍摩卡的HouseBlend咖啡
		Beverage beverage2=new HouseBlend();
		beverage2=new Mocha(beverage2);
		beverage2=new Mocha(beverage2);
		beverage2=new Whip(beverage2);
		System.out.println(beverage2.getDescription()+" $"+beverage2.cost());		
	}

}

运行结果：
Espresso $1.99
HouseBlend,mocha,mocha,Whip $1.39

评论

2 楼 kangsg219 2007-10-20  

网友happyboy2007对奇怪问题的解答：
因为double是双精度浮点型，它本身是并不精确的。而int等整型才是真正精确的数据类型。比如你给a 赋值为 0.89，其实0.89被a接收到后，真正的值并不是精确的0.89，后面还有若干个零，就比如你所看到的运行结果。
正因为浮点数据类型的不精确性，所以它们不能应用于switch的测试表达式。

1 楼 kangsg219 2007-10-19  

在调这个程序的时候发现了一个奇怪的问题：

package org.kangsg219.test;

public class Test {

	
	public static void main(String[] args) {
		double n1=0.89,n2=0.20,n3=0.10;
		System.out.println(n1+n2+n3);
		System.out.println(n1+n3+n2);
	}

}

运行的结果是：
1.1900000000000002
1.19
为什么不相等呢？仅仅是相加顺序不同而已，怎么会这样？不理解，知道的网友请赐教！