Book Notes

The Decorator Pattern: attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

  • Decorators have the same supertype as the objects they decorate.
  • You can use one or more decorators to wrap an object.
  • The decorator adds its own behavior before and/or after delegating to the object it decorates to do the rest of the job.

Design Pattern:

Example

咖啡店饮品案例：

Beverage 和 CondimentDecorator 是两个抽象类，Beverage 是基类，CondimentDecorator 是装饰者基类：

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

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

DarkRoast 和 Espresso 是具体的饮料类：

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

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

Milk 和 Mocha 是具体的装饰者类：

public class Milk extends CondimentDecorator {
	public Milk(Beverage beverage) { // 装饰者类需要接收一个饮料对象作为参数
		this.beverage = beverage;
	}

	public String getDescription() { // 装饰者类需要实现和父类一样的 getDescription 方法
		return beverage.getDescription() + ", Milk";
	}

	public double cost() { // 装饰者类需要实现和父类一样的 cost 方法
		return .10 + beverage.cost();
	}
}

public class Mocha extends CondimentDecorator {
	public Mocha(Beverage beverage) {
		this.beverage = beverage;
	}
 
	public String getDescription() {
		return beverage.getDescription() + ", Mocha";
	}
 
	public double cost() {
		return .20 + beverage.cost();
	}
}

test code:

装饰类可以无限嵌套，并且和父类有一样的类型。

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());
	}
}