設計模式 - Decorator

介紹一下 Decorator (裝飾者模式)

• 在不改變一個物件本身的功能的基礎上，動態地給一個物件增加額外的職責(是物件不是整個類別)，替代繼承的方式之一。其行為模式如下圖所例：

• 實際應用　Java I/O

            FileInputStream fis = new FileInputStream("/objects.gz");

　       BufferedInputStream bis = new BufferedInputStream(fis);

     　   GzipInputStream gis = new GzipInputStream(bis);

     　   ObjectInputStream ois = new ObjectInputStream(gis);

• 類別圖

• 範例程式

//飲料 Beverage  抽像類別
public abstract class Beverage {
    private String description;
 
    public abstract String getDescription();
 
    public abstract double cost();
}
//飲料 咖啡 繼承 Beverage
public class Coffee extends Beverage {

    public Coffee () {
        // TODO Auto-generated constructor stub
    }

    @Override
    public String getDescription() {
        return "This is Coffee";
    }

    @Override
    public double cost() {
        return 20;
    }

}
// 調味品 為一個裝飾者的抽象類別
public abstract class CondimentDecorator extends Beverage {

    protected Beverage beverage;
 
    public CondimentDecorator(Beverage beverage) {
        this.beverage = beverage;
    }
}
// 牛奶 繼承 CondimentDecorator 
public class Milk extends CondimentDecorator {

    public Milk(Beverage beverage) {
        super(beverage);
    }

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

    /* (non-Javadoc)
     * @see com.sample.Beverage#cost()
     */
    @Override
    public double cost() {
     return super.beverage.cost() + 5;
    }

}
// 摩卡 繼承 CondimentDecorator 
public class Mocha extends CondimentDecorator {

    public Mocha(Beverage beverage) {
        super(beverage);
    }

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

    @Override
    public double cost() {
        return super.beverage.cost() + 15;
    }

}

public class Main {
    public static void main(String[] args) {
       Beverage beverage = new Coffee();
       beverage = new Milk(beverage);
       beverage = new Mocha(beverage);
       System.out.println("價格 : "+beverage.cost());
       System.out.println("描述 : "+beverage.getDescription());
    }
}