设计模式――装饰者模式（Decorator Pattern）

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。

类图

（图片源于网络）

代码实现（Java）

// Beverage.java
public abstract class Beverage {
    String description = "Unknown Beverage";
    public String getDescription() {
        return description;
    }
    public abstract double cost();
}

// HouseBlend.java
public class HouseBlend extends Beverage {
    public HouseBlend() {
        description = "House Blend Coffee";
    }
    public double cost() {
        return .89;
    }
}

// DarkRoast.java
public class DarkRoast extends Beverage {
    public DarkRoast() {
        description = "Dark Roast Coffee";
    }
    public double cost() {
        return .99;
    }
}

// Espresso.java
public class Espresso extends Beverage {
    public Espresso() {
        description = "Espresso";
    }
    public double cost() {
        return 1.99;
    }
}

// Decaf.java
public class Decaf extends Beverage {
    public Decaf() {
        description = "Decaf Coffee";
    }
    public double cost() {
        return 1.05;
    }
}

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

// Milk.java
public class Milk extends CondimentDecorator {
    Beverage beverage;
    public Milk(Beverage beverage) {
        this.beverage = beverage;
    }
    public String getDescription() {
        return beverage.getDescription() + ", Milk";
    }
    public double cost() {
        return .10 + beverage.cost();
    }
}

// Mocha.java
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 .20 + beverage.cost();
    }
}

// Soy.java
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();
    }
}

// Whip.java
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();
    }
}

测试代码

// StarbuzzCoffee.java
public class StarbuzzCoffee {
      
    public static void main(String args[]) {
        Beverage beverage = new Espresso();
        System.out.println(beverage.getDescription()
                + " $" + beverage.cost());
      
        Beverage beverage2 = new DarkRoast();
        beverage2 = new Mocha(beverage2);
        beverage2 = new Mocha(beverage2);
        beverage2 = new Whip(beverage2);
        System.out.println(beverage2.getDescription()
                + " $" + beverage2.cost());
      
        Beverage beverage3 = new HouseBlend();
        beverage3 = new Soy(beverage3);
        beverage3 = new Mocha(beverage3);
        beverage3 = new Whip(beverage3);
        System.out.println(beverage3.getDescription()
                + " $" + beverage3.cost());
    }
}

运行效果