面向对象设计模式：结构型模式之装饰器模式

一、引入

• 咖啡馆订购系统

• Initial 初始

  • 4 种咖啡
    • House blend (混合咖啡)
    • Dark Roast (深度烘培)
    • Decaf (低咖啡因咖啡)
    • Espresso (意式浓缩咖啡)
      
      

• 需求变更：客户可以加料（咖啡、牛奶、糖等）

  • steamed milk
  • soy
  • mocha
  • Whip

使用继承：类爆炸 NO

修改父类：Beverage

需求倘若变更：料种类增加、料价格变化，需要修改 Beverage 类

Classes should be open for extension, but closed for modification：开放封闭原则，对修改封闭

No

Decorator…

二、装饰器模式

aka：Wrapper (包装器)

2.1 Intent 意图

• Attach additional responsibilities to an object dynamically. 可动态地将其他职责附加到对象上.
• Decorators provide a flexible alternative to subclassing for extending functionality. 装饰器为扩展功能提供了一种替代子类化的灵活替代方案.
  • Dynamically extension
  • Better than inheritance

2.2 Applicability 适用性

• To add responsibilities to individual objects dynamically without affecting other object.
• When extension by subclassing is impractical. 当子类化扩展是不切实际的.
  • 如，子类数量爆炸

2.3 类图

• Component: defines the interface for objects that can have responsibilities added to them dynamically. 为可以动态地添加职责的对象定义接口
• ConcreteComponent: defines an object to which additional responsibilities can be attached. 定义一个可以附加其他责任的对象.
• Decorator: maintains a reference to a Component object and defines an interface that conforms to Component’s interface. 维护对 Component 对象的引用，并定义一个符合 Component 接口的接口
• ConcreteDecorator: adds responsibilities to the component. 向组件添加职责

2.4 优缺点

• 优点
  • More flexibility than static inheritance. 比静态继承更灵活
    • With Decorators, responsibilities can be added and removed at runtime simply by attaching and detaching them.
  • 避免"类爆炸"
  • 通过排列和组合，可以创建许多行为组合
  • 装饰类和被装饰类可以独立发展，不会相互耦合，装饰模式是继承的一个替代模式，装饰模式可以动态扩展一个实现类的功能
• 缺点
  • Lots of little objects
  • A decorator and its component are not same
  • 多层装饰比较复杂

2.5 应用实例：Java IO 类

InputStream and OutputStream 字节流
Reader and Writer 字符流

FilterInputStream：protected volatile InputStream in;

FilterInputStream 与 InputStream 为组合和被组合关系

与装饰器设计模式类图相符！

public class JavaIO {public static void main(String[] args) throws FileNotFoundException {// Open an InputStream.FileInputStream in = new FileInputStream("test.dat");// Create a buffered InputStream.BufferedInputStream bin = new BufferedInputStream(in);// Create a buffered, data InputStream.DataInputStream dbin = new DataInputStream(bin);// Create an unbuffered, data InputStream.DataInputStream din = new DataInputStream(in);// Create a buffered, pushback, data InputStream.PushbackInputStream pbdbin = new PushbackInputStream(dbin);}
}

2.6 应用实例：咖啡馆订购系统

• 饮品抽象类：Beverage

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

• 咖啡类：继承 Beverage

public class DarkRoast extends Beverage {public DarkRoast() {description = "DarkRoast";}public double cost() {return .99;}
}

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

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

• 装饰器：CondimentDecorator

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

• 装饰器子类：ConcreteDecorator

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();}
}

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();}
}

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();}
}

• 使用与测试：

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());}
}