Decorator模式设计模式

装饰者模式定义：动态地将责任附加到对象上。

若要扩展功能。装饰者提供了比继续更有弹性的替代方案。

简单定义：包装一个对象。以提供新的行为。

装饰者模式能够有效应对类爆炸问题。

OO原则：

对扩展开放，对改动关闭。

以书中的StarbuzzCoffee为例：

Beverage.java(饮料抽象类)

package headfirst.decorator.starbuzz;

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

  public String getDescription() {
    return description;
  }

  public abstract double cost();
}

CondimentDecorator.java(调料抽象类)

package headfirst.decorator.starbuzz;

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

DarkRoast.java(饮料详细类)

package headfirst.decorator.starbuzz;

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

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

Espresso.java

package headfirst.decorator.starbuzz;

public class Espresso extends Beverage {

  public Espresso() {
    description = "Espresso";
  }

  public double cost() {
    return 1.99;
  }
}

HouseBlend.java

package headfirst.decorator.starbuzz;

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

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

Mocha.java(调料详细类)

package headfirst.decorator.starbuzz;

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

package headfirst.decorator.starbuzz;

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

package headfirst.decorator.starbuzz;

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();
  }
}

測试类：StarbuzzCoffee.java

package headfirst.decorator.starbuzz;

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());
  }
}

输出：

Espresso $1.99
Dark Roast Coffee, Mocha, Mocha, Whip $1.49
House Blend Coffee, Soy, Mocha, Whip $1.34