装饰模式-Decorator Pattern

基本概念

装饰模式，动态地给一个对象添加一些额外的职责，就增加功能来说，装饰模式比使用子类继承父类更为灵活，装饰模式可以有效地把类的核心职责和附加功能区分开。

结构图

 


上图摘自《大话设计模式》

应用场景

当需要往旧的类中添加新的方法或新的逻辑的时候，如果在主类中添加，会增加主类的复杂度；这些需要加入的东西如果仅仅是为了满足某些特定行为的需要，可以作为装饰功能来添加，从而使用装饰模式，使主类不变，仅负责核心职责的实现。

源码示例（以绘制图形作为主类核心职责，以添加边框颜色作为装饰功能）

1.创建Shape接口

package com.spook.decorator;

/**
* Shape接口
*/
public interface Shape {
public void draw();

}
2.创建Circle类
package com.spook.decorator;

/**
* Circle类
*/
public class Circle implements Shape {

@Override
public void draw() {
// TODO Auto-generated method stub
System.out.println("draw circle");
}

}
3.创建Square类
package com.spook.decorator;

/**
* Square类
*/
public class Square implements Shape {

@Override
public void draw() {
// TODO Auto-generated method stub
System.out.println("draw square");
}

}
4.装饰者抽象类，实现Shape接口
package com.spook.decorator;

/**
* 装饰者抽象类
*/
public abstract class ShapeDecorator implements Shape {

protected Shape decoratedShape;

public ShapeDecorator(Shape shape) {
this.decoratedShape = shape;
}

@Override
public void draw() {
// TODO Auto-generated method stub
if (decoratedShape != null) {
decoratedShape.draw();
}
}

}
5.蓝色边框装饰功能实现类
package com.spook.decorator;

/**
* 蓝色边框装饰者类
*/
public class BlueShapeDecorator extends ShapeDecorator {

public BlueShapeDecorator(Shape shape) {
super(shape);
// TODO Auto-generated constructor stub
}

@Override
public void draw() {
// TODO Auto-generated method stub
super.draw();
setBlueBorder(decoratedShape);
}

private void setBlueBorder(Shape decoratedShape) {
System.out.println("Border Color: Blue");
}

}
6.红色边框装饰功能实现类
package com.spook.decorator;

/**
* 红色边框装饰者类
*/
public class RedShapeDecorator extends ShapeDecorator {

public RedShapeDecorator(Shape shape) {
super(shape);
// TODO Auto-generated constructor stub
}

@Override
public void draw() {
// TODO Auto-generated method stub
super.draw();
setRedBorder(decoratedShape);
}

private void setRedBorder(Shape decoratedShape) {
System.out.println("Border Color: Red");
}
}
7.测试类

package com.spook.decorator;

/**
* 测试类
*/
public class Test {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Shape blueCircle = new BlueShapeDecorator(new Circle());
blueCircle.draw();

Shape redSquare = new RedShapeDecorator(new Square());
redSquare.draw();

}

}

运行测试类输出如下内容：
draw circle
Border Color: Blue
draw square
Border Color: Red