《Head First》第三节 装饰者模式

背景介绍：星巴兹是一家很火爆的咖啡连锁店，因为扩张很快，需求多样化，原系统已不能满足。

原系统设计：

购买咖啡时，会要求加各种调料，系统要根据加的不同调料收取不同的费用，导致结构图如下：

解决方案：

以饮料为主体，然后运行时，以调料来“装饰”(decorate)饮料。

比如，顾客要加摩卡和奶泡的深焙咖啡。那么要做的是：

1.拿一个深焙咖啡(DarkRoast)对象

2.以摩卡(Mocha)对象装饰它

3.以奶泡(Whip)对象装饰它

4.调用cost()方法，并依赖委托(delegate)将调料的价钱加上去。

创建的图如下：

结算时，最外圈装饰者(Whip)的cost()就可以得到：

装饰者模式类图：

参照此类图，得到星巴兹饮料类图：

具体实现代码：

Beverage(饮料)

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

    public String getDescription() {
        return description;
    }

    public abstract double cost();
}

装饰者类，Condiment(调料)

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

各种饮料

HouseBlend（综合咖啡）

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

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

Espresso(浓缩咖啡)

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;
    }
}

各种调料

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（豆浆）

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;
    }
}

测试：

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());
    }
}

运行结果：

Java I/O流中的装饰者模式

编写自己的I/O装饰者，实现大小写转换

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;
    }
}

测试

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();
        }
    }
}

运行结果

总结：

装饰者模式--动态的将责任附加到对象上，想要扩展功能，装饰者提供有别于继承的另一种选择。