设计模式之装饰者模式

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。

装饰者模式可以有效应对类爆炸问题。

OO原则：

对扩展开放，对修改关闭。

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(饮料具体类)

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(调料具体类)

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();
	}
}

测试类：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