设计模式-装饰者模式

1. 定义
   装饰者模式动态地将责任责任附加到对象上。若要扩展功能，装饰者提供了比继承更有弹性的替代方案。

2. 实现要点
   装饰器与被装饰的类需要继承自相同接口，来达到类型匹配。装饰器持有被装饰的类的实例。

3. 代码实例

   /**
    * 基础组件
    */
   abstract class Beverage {
   
       private String description = "Unknown Beverage";
   
       public String getDescription() {
           return description;
       }
   
       public void setDescription(String description) {
           this.description = description;
       }
   
       public abstract double cost();
   }
   
   /**
    * 装饰器基类
    */
   abstract class Decorator extends Beverage {
   
       @Override
       public abstract String getDescription();
   }
   
   /**
    * 被装饰的类
    */
   class Espresso extends Beverage {
   
       public Espresso() {
           setDescription("Espresso");
       }
   
       @Override
       public double cost() {
           return 1.99;
       }
   }
   
   /**
    * 装饰器
    */
   class Mocha extends Decorator {
   
       private Beverage beverage;
   
       public Mocha(Beverage beverage) {
           this.beverage = beverage;
       }
   
       @Override
       public double cost() {
           return beverage.cost() + 0.29;
       }
   
       @Override
       public String getDescription() {
           return beverage.getDescription() + ", Mocha";
       }
   }
   
   class CoffeeTest {
       public static void main(String[] args) {
           Beverage beverage = new Espresso();
           beverage = new Mocha(beverage);
   
           System.out.println(beverage.getDescription() + " " + beverage.cost());
       }
   }