设计模式 - 结构型 - 装饰者模式

介绍

意图：动态地给一个对象添加一些额外的职责。就增加功能来说，装饰器模式相比继承更为灵活。

主要解决：一般的，我们为了扩展一个类经常使用继承方式实现，由于继承为类引入静态特征，并且随着扩展功能的增多，子类会很膨胀。

何时使用：在不想增加很多子类的情况下扩展类。

如何解决：将具体功能职责划分，同时继承装饰者模式。

关键代码： 1、Component 类充当抽象角色，不应该具体实现。 2、修饰类引用和继承 Component 类，具体扩展类重写父类方法。

实现

饮料与调料

饮料

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

---

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;
	}
}

---

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

---

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

---

调料（装饰者，用来装饰饮料）

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

---

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 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();
	}
}

---

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();
	}
}

---

运行类

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());
	}
}

---

运行结果

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

JDK里的例子 —— IO

自建一个转换成小写的装饰者

public class LowerCaseInputStream extends FilterInputStream {

	public LowerCaseInputStream(InputStream in) {
		super(in);
	}
 
	public int read() throws IOException {
		int c = in.read();
		return (c == -1 ? c : Character.toLowerCase((char)c));
	}
		
	public int read(byte[] b, int offset, int len) throws IOException {
		int result = in.read(b, offset, len);
		for (int i = offset; i < offset+result; i++) {
			b[i] = (byte)Character.toLowerCase((char)b[i]);
		}
		return result;
	}
}

---

运行类

public class InputTest {
	public static void main(String[] args) throws IOException {
		int c;

		try {
			InputStream in = 
				new LowerCaseInputStream(
					new BufferedInputStream(
						new FileInputStream("D:\\eclipse-workspace\\common\\src\\designpattern\\decorator\\example3\\test.txt")));

			while((c = in.read()) >= 0) {
				System.out.print((char)c);
			}

			in.close();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
}

参考及引用

1.《Head First 设计模式》Eric Freeman 等 著

2.装饰者模式 | 菜鸟教程