一、什么是装饰模式
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；
二、补充说明
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；
三、角色
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；
四、例子
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：
五、JAVA IO流与装饰模式
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();