结构型模式---装饰模式(Decorator)

一：定义:

Decorator:Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

二：引入

假设现在有一家咖啡店,经营咖啡,茶等饮料,我们来为它设计一个系统。

 

问题:

• 饮料加糖或牛奶等调料时是要另收费的(每包1元),顾客可以要也可以不要,所以这个固定价格的cost方法不符合要求. 不符合OCP(open for extension,close for modification)

重新设计:

很显然,这种设计也不合适,会导致

• 类数量爆炸性增长(class exploson).
• 也不符合要求,比如有的顾客要两包糖等情况没有考虑到.

引入Decorator设计模式:

public abstract class Beverage ...{
    private String discription;

    
    public String getDiscription() ...{
        return discription;
    }

    public void setDiscription(String discription) ...{
        this.discription = discription;
    }

    public abstract double cost() ;
}


public class Coffee extends Beverage...{
    public Coffee()
    ...{
        setDiscription("coffee");
    }
    public  double cost() 
    ...{
        return 10;
    }
}


public abstract class CondimentDecorator extends Beverage...{
    public abstract String getDiscription() ;
}

public class CondimentMilk extends CondimentDecorator...{
    private Beverage beverage;
    public CondimentMilk(Beverage beverage)
    ...{
        this.beverage=beverage;
    }
    public  double cost() 
    ...{
        return beverage.cost()+1;
    }
    public  String getDiscription() 
    ...{
        return beverage.getDiscription()+",milk";
    }
}


public class Client ...{
    public static void main(String args[])
    ...{
        Beverage coffee=new Coffee();
        System.out.println(coffee.getDiscription()+":"+coffee.cost());
        System.out.println(new CondimentMilk(coffee).getDiscription()+":"+new CondimentMilk(coffee).cost());
        System.out.println(new CondimentSugar(new CondimentMilk(coffee)).getDiscription()+":"+new CondimentSugar( new CondimentMilk(coffee)).cost());
        System.out.println(new CondimentSugar(new CondimentSugar(new CondimentMilk(coffee))).getDiscription()+":"+new CondimentSugar(new CondimentSugar( new CondimentMilk(coffee))).cost());
    }
}

 

三：结构

    

四：实际应用

1. JAVA I/O  系统是decorator模式的典型应用

 

我们自己来写一个InputStream decorator,用来使所有的输出字符变成小写.

public class LowerCaseInputStream extends FilterInputStream ...{

    protected LowerCaseInputStream(InputStream in) ...{
        super(in);
    }
    
    public  int read() throws IOException
    ...{
        int c=super.read();
        return (c==-1?c:Character.toLowerCase(c));
    }
}

public class IoTest ...{
    public static void main(String args[]) ...{
        int c;
        String filePath = "f:/temp/designPatten.txt";
        try ...{
            InputStream in = new LowerCaseInputStream(new BufferedInputStream(new FileInputStream(
                    filePath)));
            while(( c=in.read())!=-1)
            ...{
                System.out.print((char)c);
            }
            in.close();

        } catch (FileNotFoundException e) ...{

            e.printStackTrace();
        } catch (IOException e) ...{

            e.printStackTrace();
        }
    }
}

五：适用情形

Use Decorator

• to add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects.  
• for responsibilities that can be withdrawn.
• when extension by subclassing is impractical. Sometimes a large number of independent extensions are possible and would produce an explosion of subclasses to support every combination. Or a class definition may be hidden or otherwise unavailable for subclassing.

参考文献：
1:阎宏，《Java与模式》，电子工业出版社
2:Eric Freeman & Elisabeth Freeman，《Head First Design Pattern》，O'REILLY

     

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结构型模式-适配器模式(Adapter) | 行为型模式--策略模式(Strategy)

• 2007-06-30 21:55
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