Java 装饰者模式：混凝土构件的字段值是构件的字段值，但为什么？

我正在玩弄《头优先设计模式》一书中的装饰模式示例。我认为

beverage3

字段描述的值必须是“House Blend Coffee”，但它是“Unknown Beverage”

有人能解释一下吗？我们是根据接口编码的（在这个例子中，它是一个抽象类），但我仍然在实例化类HouseBlund，所以值应该是“HouseBludeCoffee”，但它不是

下面的代码并不包含示例的所有类

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();
    System.out.println("(added by me)return value of getDescription() without condiment: " + beverage3.getDescription());
    beverage3 = new Soy(beverage3);
    beverage3 = new Mocha(beverage3);
    beverage3 = new Whip(beverage3);
    System.out.println(beverage3.getDescription()
            + " $" + beverage3.cost());

    System.out.println("(added by me)value of field desciption: " + beverage3.description);
    }
}

饮料.java

public abstract class Beverage {

    String description = "Unknown Beverage";

    public String getDescription() {
        return description;
    }

    public abstract double cost();
}

调味品装饰器.java

public abstract class CondimentDecorator extends Beverage {

    public abstract String getDescription();
}

public class Mocha extends CondimentDecorator{

    Beverage beverage;

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

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

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

}

java

public class HouseBlend extends Beverage{

    public HouseBlend() {
        description = "House Blend Coffee";
    }
    @Override
    public double cost() {
        return .89;
    }

}

调味品装饰器.java

public abstract class CondimentDecorator extends Beverage {

    public abstract String getDescription();
}

public class Mocha extends CondimentDecorator{

    Beverage beverage;

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

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

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

}

---

问题是，您正在创建的摩卡对象本身就是一种有描述的饮料。你没有改变它的描述。您正在传递另一种饮料并增强对装饰对象的描述。但是，你仍然有你以前对摩卡咖啡的描述。当您直接访问它的描述时（这就是您不使用getDescription方法所做的），您正在访问未更改的字段，因此您将打印“未知饮料”

您创建的每种饮料都有其字段描述。如果你使用它而不改变它，你会得到“未知饮料”

使类Beverage的字符串描述受保护，并且仅通过Beverage实现的重写getDescription方法访问它。请查看：

public class StarbuzzCoffee {

    public static void main(String[] args) {
        Beverage beverage3 = new HouseBlend();
        System.out
            .println("(added by me)return value of getDescription() without condiment: " + beverage3.getDescription());
        beverage3 = new Mocha(beverage3);
        System.out.println(beverage3.getDescription() + " $" + beverage3.cost());

        System.out.println("(added by me)value of field desciption: " + beverage3.getDescription());
    }
}

abstract class Beverage {

  protected String description = "Unknown Beverage";

  public String getDescription() {
      return description;
  }

  public abstract double cost();
}

abstract class CondimentDecorator extends Beverage {

  public abstract String getDescription();
}

class HouseBlend extends Beverage{

  public HouseBlend() {
    description = "House Blend Coffee";
  }
  @Override
  public double cost() {
      return .89;
  }

}

class Mocha extends CondimentDecorator{

  Beverage beverage;

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

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

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

}

关于设计，将饮料的getDescription抽象化并强制每个实现覆盖它会更安全。这样你将来就可以避免这种问题了。请查看：

public class StarbuzzCoffee {

    public static void main(String[] args) {
        Beverage beverage3 = new HouseBlend();
        System.out
            .println("(added by me)return value of getDescription() without condiment: " + beverage3.getDescription());
        beverage3 = new Mocha(beverage3);
        System.out.println(beverage3.getDescription() + " $" + beverage3.cost());

        System.out.println("(added by me)value of field desciption: " + beverage3.description);
    }

}

abstract class Beverage {

  protected String description = "Unknown Beverage";

  public abstract String getDescription();

  public abstract double cost();
}

abstract class CondimentDecorator extends Beverage {
    // You don't need this code here. Because Beverage already 
    // provides a getDescription
  // public abstract String getDescription();
}

class HouseBlend extends Beverage{

  public HouseBlend() {
    description = "House Blend Coffee";
  }

  @Override
  public String getDescription() {
    return description;
  }

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

}

class Mocha extends CondimentDecorator{
  // Also make sure that this guy here is private
  // so other objects can't change or access 
  // its state without going through Mocha
  private Beverage beverage;

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

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

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

}

---

我在答案中添加了更多的解释以帮助您。如果你还有疑问，尽管问。我以前被这个问题困扰过。。。干杯