设计模式-装饰模式
本篇文章主要讲解装饰模式。
装饰模式:动态地给对象添加一些额外的职责,就增加功能来说,装饰模式比生产子类更为灵活。
装饰模式(Decorator)结构图
Component是定义一个对象接口,可以给这些对象动态地添加职责。ConcreteComponent是定义了一个具体的对象,也可以给这个对象添加一些职责。Decorator,装饰抽象类,继承了Component,从外类来扩展Component类的功能,但对于Component来说,是无需知道Decorator的存在的。至于ConcreteDecorator就是具体的装饰对象,起到给Component添加职责的功能。
来看看基本的代码实现。
package com.lwx.decorator; /** * Created with IntelliJ IDEA. * Description: * User: lwx * Date: 2019-02-27 * Time: 21:17 */ public interface Component { void operation(); }
package com.lwx.decorator; /** * Created with IntelliJ IDEA. * Description: * User: lwx * Date: 2019-02-27 * Time: 21:18 */ public class ConcreteComponent implements Component { public void operation() { System.out.println("具体的对象操作"); } }
package com.lwx.decorator; /** * Created with IntelliJ IDEA. * Description: * User: lwx * Date: 2019-02-27 * Time: 21:19 */ public class Decorator implements Component { protected Component component; public void setComponent(Component component) { this.component = component; } //重写operation方法,实际执行的是Component类中的operation方法 public void operation() { if (component != null) { component.operation(); } } }
package com.lwx.decorator; /** * Created with IntelliJ IDEA. * Description: * User: lwx * Date: 2019-02-27 * Time: 21:21 */ public class ConcreteDecoratorA extends Decorator { @Override public void operation() { super.operation(); System.out.println("具体装饰对象A的操作"); } }
package com.lwx.decorator; /** * Created with IntelliJ IDEA. * Description: * User: lwx * Date: 2019-02-27 * Time: 21:22 */ public class ConcreteDecoratorB extends Decorator { @Override public void operation() { super.operation(); addedBehavior(); System.out.println("具体装饰对象B的操作"); } private void addedBehavior() { System.out.println("装饰对象B独有方法"); } }
最后看下测试类和运行结果。
package com.lwx.decorator; /** * Created with IntelliJ IDEA. * Description: * User: lwx * Date: 2019-02-27 * Time: 21:24 */ public class ComponentTest { public static void main(String[] args) { ConcreteComponent c = new ConcreteComponent(); ConcreteDecoratorA d1 = new ConcreteDecoratorA(); ConcreteDecoratorB d2 = new ConcreteDecoratorB(); d1.setComponent(c); d2.setComponent(d1); d2.operation(); } }
下面是装饰模式在I/O库中的应用(图片来自http://www.zuoxiaolong.com/blog/article.ftl?id=115)
现在我们来编写一个装饰者把输入流内所有的数字去掉。
先在D盘新建一个记事本,内容随便写,只要加几个数字就行;然后我们开始编写装饰者。
package com.lwx.decorator.example; import java.io.FilterInputStream; import java.io.IOException; import java.io.InputStream; /** * Created with IntelliJ IDEA. * Description: * User: lwx * Date: 2019-03-03 * Time: 12:11 */ public class RemoveNumberInputStream extends FilterInputStream { protected RemoveNumberInputStream(InputStream in) { super(in); } @Override public int read() throws IOException { //获取返回的字节,如果是文件末尾返回-1 int c = super.read(); if (c != -1) { //判断字节是否为数字 if (Character.isDigit(c)) { // '\0' 代表空字节 char empty = '\0'; return empty; } } return c; } }
测试类
package com.lwx.decorator.example; import java.io.BufferedInputStream; import java.io.FileInputStream; import java.io.IOException; import java.io.InputStream; /** * Created with IntelliJ IDEA. * Description: * User: lwx * Date: 2019-03-03 * Time: 12:33 */ public class InputStreamTest { public static void main(String[] args) { try { int c; InputStream inputStream = new FileInputStream("D:\\test.txt"); RemoveNumberInputStream in = new RemoveNumberInputStream(inputStream); while ((c = in.read()) >= 0) { System.out.print((char) c); } in.close(); } catch (IOException e) { e.printStackTrace(); } } }
我记事本写的内容是:dasdsa1sadsa2dd5sad4sad2dsa9das3,然后运行结果为:
总结一下装饰模式的优点和缺点:
优点:
1.装饰模式比生成子类更为灵活
2.把类的核心职责和装饰功能分开,可以去除相关类中重复的装饰逻辑
缺点:
1.装饰模式会导致设计中出现许多小类,如果过度使用,会使程序变得复杂
2.这种比继承更加灵活机动的特性,也同时意味着装饰模式比继承更加易于出错,排错也很困难,对于多次装饰的对象,调试时寻找错误可能需要逐级排查,较为烦琐。
最后附上demo的githup地址:https://github.com/yijinqincai/design_patterns