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();}

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

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

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

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

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调料（装饰者，用来装饰饮料）

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

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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();	}}

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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();	}}

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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();	}}

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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();	}}

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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());	}}

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运行结果

Espresso $1.99Dark Roast Coffee, Mocha, Mocha, Whip $1.49House 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;	}}

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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