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通过关联机制给类增加行为，其行为的扩展由修饰对象来决定；
与继承相似，不同点在于继承是在编译期间扩展父类，而装饰器模式在运行期间动态扩展原有对象；
或者说，继承是对类进行扩展，装饰模式是对对象进行扩展；
抽象构件
具体构件
抽象装饰类
具体装饰类
说明：具体构件、抽象装饰类、具体装饰类的共同父类是抽象构件，具体装饰类继承抽象装饰类并在运行期间装饰具体构件；
例子说明：
画家接口Painter，为抽象构件，有两个方法，获取画家描述信息及绘画；
PaintBeginner实现Painter接口，为具体构件；
PainterDecorator实现Painter接口，为抽象装饰类，其内部关联一个Painter对象，通过构造函数获取；
HillPainterDecorator、RiverPainterDecorator、TreePainterDecorator为具体装饰类，表明被装饰的画家能够绘画Hill、River、Tree；
类图：
代码实现：
Painter.java
package com.pichen.dp.decorator; public interface Painter { public abstract String getDescription(); public abstract String painting(); }
PaintBeginner.java
package com.pichen.dp.decorator; public class PaintBeginner implements Painter{ @Override public String getDescription() { return ""; } @Override public String painting() { /* do nothing */ return ""; } }
PainterDecorator.java
package com.pichen.dp.decorator; public abstract class PainterDecorator implements Painter{ private Painter decoratedPainter; public PainterDecorator(Painter decoratedPainter) { this.decoratedPainter = decoratedPainter; } public Painter getPainter(){ return this.decoratedPainter; } }
HillPainterDecorator.java
package com.pichen.dp.decorator; public class HillPainterDecorator extends PainterDecorator{ public HillPainterDecorator(Painter paper) { super(paper); } @Override public String getDescription() { return this.getPainter().getDescription() + "can paint hill, "; } @Override public String painting() { /* painting the hill */ return this.getPainter().painting() + paintingHill(); } public String paintingHill(){ return "Hill, "; } }
RiverPainterDecorator.java
package com.pichen.dp.decorator; public class RiverPainterDecorator extends PainterDecorator{ public RiverPainterDecorator(Painter paper) { super(paper); } @Override public String getDescription() { return this.getPainter().getDescription() + "can paint river, "; } @Override public String painting() { /* painting the river */ return this.getPainter().painting() + paintingRiver(); } public String paintingRiver(){ return "River, "; } }
TreePainterDecorator.java
package com.pichen.dp.decorator; public class TreePainterDecorator extends PainterDecorator{ public TreePainterDecorator(Painter paper) { super(paper); } @Override public String getDescription() { return this.getPainter().getDescription() + "can paint tree, "; } @Override public String painting() { /* painting the tree */ return this.getPainter().painting() + paintingTree(); } public String paintingTree(){ return "Tree, "; } }
Main.java
package com.pichen.dp.decorator; public class Main { public static void main(String[] args) { Painter p0 = new PaintBeginner(); System.out.println("Painter description:" + p0.getDescription()); System.out.println("Painting:" + p0.painting() + "\n"); Painter p1 = new HillPainterDecorator(new RiverPainterDecorator(new TreePainterDecorator(new PaintBeginner()))); System.out.println("Painter description:" + p1.getDescription()); System.out.println("Painting:" + p1.painting() + "\n"); Painter p2 = new RiverPainterDecorator(new HillPainterDecorator(new PaintBeginner())); System.out.println("Painter description:" + p2.getDescription()); System.out.println("Painting:" + p2.painting() + "\n"); } }
执行结果如下，PaintBeginner类的对象未装饰前，无行为；在被装饰器装饰后，其行为可动态扩展：
这里简单的以Reader、BufferedReader、FileReader举个例子，如下代码：
BufferedReader br = new BufferedReader(new FileReader(new File("test.txt"))); br.readLine();
说明：
其中BufferedReader与BufferedReader有一个共同抽象父类Reader，Reader为抽象构件；
new FileReader(new File("test.txt"))为具体构件，运行期间被修饰的对象；
BufferedReader为具体修饰类，运行期间修饰具体构件；
装饰后，被修饰的对象新增的行为是拥有readLine方法；
ps：查看源码，没发现BufferedReader对应的抽象装饰类，个人觉得没有抽象装饰类，装饰模式也是可以正常工作的，抽象构件（Reader）可以由具体修饰类关联；
另外，具体修饰类也可以作为基类，被其它类继承的，继承后的类同样也是具体修饰类，如LineNumberReader就是继承BufferedReader；
所以，上面语句还可以这样写（ps：只是举例，其实没必要用BufferedReader修饰，直接LineNumberReader装饰下就可以）：
BufferedReader br = new LineNumberReader(new BufferedReader(new FileReader(new File("test.txt")))); br.readLine();
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原文地址：http://www.cnblogs.com/chenpi/p/5173818.html
