Decorator模式設計模式

裝飾者模式定義：動態地將責任附加到對象上。

若要擴展功能。裝飾者提供了比繼續更有彈性的替代方案。

簡單定義：包裝一個對象。以提供新的行爲。

裝飾者模式可以有效應對類爆炸問題。

OO原則：

對擴展開放，對改動關閉。

以書中的StarbuzzCoffee爲例：

Beverage.java(飲料抽象類)

package headfirst.decorator.starbuzz;

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

CondimentDecorator.java(調料抽象類)

package headfirst.decorator.starbuzz;

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

DarkRoast.java(飲料詳細類)

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(調料詳細類)

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();
	}
}

測試類：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());
	}
}

輸出：

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