【Head First设计模式】-Decorator模式

 

【Head First设计模式】-Decorator模式

 

一、要完成的任务

星巴兹（Starbuzz）是以扩张速度最快而闻名的咖啡连锁店。如果你在街角看到它的店，在对面街上肯定还会看到另一家。因为扩张速度实在太快了，他们准备更新订单系统，以合乎他们的饮料供应要求。他们原先的类设计是这样的……

 

购买咖啡时，也可以要求在其中加入各种调料，例如：蒸奶（Steamed Milk）、豆浆（Soy）、摩卡（Mocha，也就是巧克力风味）或覆盖奶泡。星巴兹会根据所加入的调料收取不同的费用。所以订单系统必须考虑到这些调料部分。

 

二、Decorator模式

 

1、一个原则

 

类应该对扩展开放，对修改关闭

 

2、定义装饰者模式

装饰者模式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。

   

3.分析任务

4.设计任务

 

 

三、代码实现

 

1．定义抽象类

 

（1）饮料抽象类Beverage

Beverage.java

 

package  com.sterning.ch3_decorator;

/**/ /*
 * Beverage是一个抽象类,有两个方法
 */
public   abstract   class  Beverage  {
        public String description="Unknown Beverage";
        
        /**//*
         * getDescription()已经在此实现了,但是cost()必须在子类中实现
         */
        public String getDescription() {
            return description;
        }
        
        public abstract double cost();
}

（2）调料抽象类CondimentDecorator

CondimentDecorator.java

package  com.sterning.ch3_decorator;

/**/ /*
 * 首先,必须让CondimentDecorator能够取代Beverage,所以将CondimentDecorator扩展自Beverage类
 */
public   abstract   class  CondimentDecorator  extends  Beverage  {

    //所有的调料装饰者都必须重新实现getDescription()方法.
    public abstract String getDescription();
}

 

2．饮料实现

 

（1）Espresso

Espresso.java

package  com.sterning.ch3_decorator;

/**/ /*
 * 首先,必须让CondimentDecorator能够取代Beverage,所以将CondimentDecorator扩展自Beverage类
 */
public   abstract   class  CondimentDecorator  extends  Beverage  {

    //所有的调料装饰者都必须重新实现getDescription()方法.
    public abstract String getDescription();
}

 

（2）HouseBlend

HouseBlend.java

package  com.sterning.ch3_decorator.drink;

import  com.sterning.ch3_decorator.Beverage;

public   class  HouseBlend  extends  Beverage  {
    
    
    public HouseBlend() {
        description="House Blend Coffee";
    }

    @Override
    public double cost() {
        return 0.89;
    }

}

 

（3）DarkRoast

DarkRoast.java

package  com.sterning.ch3_decorator.drink;

import  com.sterning.ch3_decorator.Beverage;

public   class  DarkRoast  extends  Beverage  {
    
    
    public DarkRoast() {
        description="Dark Roast Coffee";
    }

    @Override
    public double cost() {
        return 0.99;
    }
}

 

（4）Decaf

Decaf.java

package  com.sterning.ch3_decorator.drink;

import  com.sterning.ch3_decorator.Beverage;

public   class  Decaf  extends  Beverage  {
    
    
    public Decaf() {
        description="Decaf Coffee";
    }

    @Override
    public double cost() {
        return 1.05;
    }
}

 

3．调料实现

（1）Mocha

Mocha.java

package  com.sterning.ch3_decorator.condiment;

import  com.sterning.ch3_decorator.Beverage;
import  com.sterning.ch3_decorator.CondimentDecorator;

public   class  Mocha  extends  CondimentDecorator  {
    /**//*
     * 要让Mocha能够引用一个Beverage,做法如下:一是用一个实例变量记录饮料,也就是被装饰者.
     * 二是想办法让装饰者(饮料)记录到实例变量中,即把饮料当作构造器的参数,再由构造器将此饮料记录在实例变量中
     */
    Beverage beverage;

    public Mocha(Beverage beverage) {
        this.beverage = beverage;
    }

    @Override
    public String getDescription() {
        /**//*
         * 我们希望叙述不只是描述饮料,而是完整的连调料都描述出来
         */
        return beverage.getDescription()+",Mocha";
    }
            
    @Override
    public double cost() {
        /**//*
         * 要计算带Mocha饮料的价钱,首先把调用委托给装饰对象,以计算价钱,然后再加上Mocha的价钱,得到最后结果
         */
        return 0.20+beverage.cost();
    }
}

 

（2）Soy

Soy.java

package  com.sterning.ch3_decorator.condiment;

import  com.sterning.ch3_decorator.Beverage;
import  com.sterning.ch3_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();
    }
}

 

（3）Whip

Whip.java

package  com.sterning.ch3_decorator.condiment;

import  com.sterning.ch3_decorator.Beverage;
import  com.sterning.ch3_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();
    }
}

 

4．测试类StarbuzzCoffee

StarbuzzCoffee.java

package  com.sterning.ch3_decorator;

import  com.sterning.ch3_decorator.condiment.Mocha;
import  com.sterning.ch3_decorator.condiment.Soy;
import  com.sterning.ch3_decorator.condiment.Whip;
import  com.sterning.ch3_decorator.drink.DarkRoast;
import  com.sterning.ch3_decorator.drink.Espresso;
import  com.sterning.ch3_decorator.drink.HouseBlend;

public   class  StarbuzzCoffee  {
    public static void main(String args[]){
        /**//*
         * 订一杯Espresso,不需要调料,打印出它的描述和价钱.
         */
        Beverage beverage=new Espresso();
        System.out.println(beverage.getDescription()+" $"+beverage.cost());
        
        /**//*
         * 制造一个DarkRoast对象,用Mocha,Whip装饰它
         */
        Beverage beverage2=new DarkRoast();
        beverage2=new Mocha(beverage2);
        beverage2=new Mocha(beverage2);
        beverage2=new Whip(beverage2);
        System.out.println(beverage2.getDescription()+" $"+beverage2.cost());    
        
        /**//*
         * 最后,再来一杯调料为豆浆,摩卡\奶泡的HouseBlend咖啡
         */
        Beverage beverage3=new HouseBlend();
        beverage3=new Soy(beverage3);
        beverage3=new Mocha(beverage3);
        beverage3=new Whip(beverage3);
        System.out.println(beverage3.getDescription()+" $"+beverage3.cost());    
    }
}

源代码下载:ch3_decorator.rar