《Head First》第三節 裝飾者模式

背景介紹：星巴茲是一家很火爆的咖啡連鎖店，因爲擴張很快，需求多樣化，原系統已不能滿足。

原系統設計：

購買咖啡時，會要求加各種調料，系統要根據加的不同調料收取不同的費用，導致結構圖如下：

解決方案：

以飲料爲主體，然後運行時，以調料來“裝飾”(decorate)飲料。

比如，顧客要加摩卡和奶泡的深焙咖啡。那麼要做的是：

1.拿一個深焙咖啡(DarkRoast)對象

2.以摩卡(Mocha)對象裝飾它

3.以奶泡(Whip)對象裝飾它

4.調用cost()方法，並依賴委託(delegate)將調料的價錢加上去。

創建的圖如下：

結算時，最外圈裝飾者(Whip)的cost()就可以得到：

裝飾者模式類圖：

參照此類圖，得到星巴茲飲料類圖：

具體實現代碼：

Beverage(飲料)

public abstract class Beverage {
    public String description = "Unknown Beverage";

    public String getDescription() {
        return description;
    }

    public abstract double cost();
}

裝飾者類，Condiment(調料)

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

各種飲料

HouseBlend（綜合咖啡）

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

    public double cost() {
        return .89;
    }
}

Espresso(濃縮咖啡)

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

    public double cost() {
        return 1.99;
    }
}

Decaf（低咖啡因）

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

    public double cost() {
        return 1.05;
    }
}

DarkRoast（深焙咖啡）

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

    public double cost() {
        return .99;
    }
}

各種調料

Mocha（摩卡）

public class Mocha extends CondimentDecorator {
    Beverage beverage;

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

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

    public double cost() {
        return beverage.cost() + .20;
    }
}

Soy（豆漿）

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 beverage.cost() + .30;
    }
}

Whip（牛奶）

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 beverage.cost() + .30;
    }
}

測試：

public class StarbuzzCoffee {
    public static void main(String[] args) {
        Beverage beverage = new Espresso();
        System.out.println(beverage.getDescription() + " $" + beverage.cost());
        Beverage beverage1 = new Mocha(new Mocha(new Whip(new DarkRoast())));
        System.out.println(beverage1.getDescription() + " $" + beverage1.cost());
        Beverage beverage2 = new Soy(new Mocha(new Whip(new DarkRoast())));
        System.out.println(beverage2.getDescription() + " $" + beverage2.cost());
    }
}

運行結果：

Java I/O流中的裝飾者模式

編寫自己的I/O裝飾者，實現大小寫轉換

public class LowerCaseInputStream extends FilterInputStream {
    public LowerCaseInputStream(InputStream in) {
        super(in);
    }

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

    public int read(byte b[], int off, int len) throws IOException {
        int result = super.read(b, off, len);
        for (int i = off; i < off + result; i++) {
            b[i] = (byte) Character.toLowerCase((char)b[i]);
        }
        return result;
    }
}

測試

public class InputTest {
    public static void main(String[] args) {
        int c;
        try {
            InputStream is = new LowerCaseInputStream(new BufferedInputStream(new FileInputStream("test.txt")));
            while ((c = is.read()) >= 0) {
                System.out.print((char)c);
            }
            is.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

運行結果

總結：

裝飾者模式--動態的將責任附加到對象上，想要擴展功能，裝飾者提供有別於繼承的另一種選擇。