设计模式03之 装饰者模式

装饰者模式 Decorator Pattern

设计原则 五：

开放-关闭原则，类应该对拓展开放，对修改关闭。

定义装饰者模式

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。

Java 装饰者者模式的应用，Java I/O 流的

1. 装饰者和被装饰对象有相同的超类型。

2. 你可以用一个或多个装饰者包装一个对象。

3. 既然装饰者和被装饰对象有相同的超类型，所以在任何需要原始对象（被包装的）的场合， 可以用装饰过的对象代替它。

4. 装饰者可以在所委托被装饰者的行为之前与/或之后，加上自己的行为，以达到特定的目的。（重要）

5. 对象可以在任何时候被装饰，所以可以在运行时动态地、不限量地用你喜欢的装饰者来装饰 对象。

类图

例图

装饰者类解析

package headfirst.decorator.starbuzz;

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

package headfirst.decorator.starbuzz;

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

package headfirst.decorator.starbuzz;

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

package headfirst.decorator.starbuzz;

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

package headfirst.decorator.starbuzz;

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

package headfirst.decorator.starbuzz;

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

package headfirst.decorator.starbuzz;

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();
	}
}

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();
	}
}

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();
	}
}

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();
	}
}

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());
	}
}