实验 25：访问者模式
本次实验属于模仿型实验，通过本次实验学生将掌握以下内容：
1、理解访问者模式的动机，掌握该模式的结构；
2、能够利用访问者模式法解决实际问题。
[实验任务]：打包员
在我们课堂上的“购物车”的例子中，增加一个新的访问者：打包员，负责对购物车中货物装包。
源代码
/*** 抽象元素类 商品*/
public interface Product
{/*** 用于接受访问者的访问*/void accept(Visitor visitor);
}/*** 抽象访问者类Visitor*/
public abstract class Visitor
{protected String name;public void setName(String name){this.name = name;}public abstract void visit(Apple apple);public abstract void visit(Book book);
}/*** 具体元素类 苹果类*/
public class Apple implements Product
{/*** 用于接受访问者的访问*/@Overridepublic void accept(Visitor visitor){visitor.visit(this);}
}/*** 具体元素类 书籍类*/
public class Book implements Product
{@Overridepublic void accept(Visitor visitor){visitor.visit(this);}
}import java.util.ArrayList;
import java.util.Iterator;/*** 购物车类*/
public class BuyBasket
{private ArrayList list = new ArrayList();public void accept(Visitor visitor){Iterator i = list.iterator();while (i.hasNext()){((Product) i.next()).accept(visitor);}}//添加商品public void addProduct(Product product){list.add(product);}//移除商品public void removeProduct(Product product){list.remove(product);}
}/*** 具体访问者类 顾客*/
public class Customer extends Visitor
{@Overridepublic void visit(Apple apple){System.out.println("顾客" + name + "选苹果");}@Overridepublic void visit(Book book){System.out.println("顾客" + name + "买书");}
}/*** 具体访问者类 打包员*/
public class Packager extends Visitor
{@Overridepublic void visit(Apple apple){System.out.println("打包员" + name + "将苹果打包");}@Overridepublic void visit(Book book){System.out.println("打包员" + name + "将书打包");}
}/*** 具体访问者类 收银员*/
public class Saler extends Visitor
{@Overridepublic void visit(Apple apple){System.out.println("收银员" + name + "给苹果过秤，然后计算价格。");}@Overridepublic void visit(Book book){System.out.println("收银员" + name + "直接计算书的价格。");}
}import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import java.io.File;public class XMLUtil
{public static Object getBean(){try{//创建DOM文档对象DocumentBuilderFactory dFactory = DocumentBuilderFactory.newInstance();DocumentBuilder builder = dFactory.newDocumentBuilder();Document doc = builder.parse(new File("src/main/resources/config25.xml"));//获取包含类名的文本节点NodeList nl = doc.getElementsByTagName("className");Node classNode = nl.item(0).getFirstChild();String cName = classNode.getNodeValue();//通过类名生成实例对象并将其返回Class c = Class.forName(cName);Object obj = c.newInstance();return obj;}catch (Exception e){e.printStackTrace();return null;}}
}/*** 测试类*/
public class Client
{public static void main(String[] args){//实例化商品Product b1 = new Book();Product b2 = new Book();Product a1 = new Apple();//实例化购物车BuyBasket basket = new BuyBasket();//添加商品basket.addProduct(b1);basket.addProduct(b2);basket.addProduct(a1);//实例化访问者Visitor visitor;//根据XML文件动态获取访问者visitor = (Visitor) XMLUtil.getBean();visitor.setName("张三");basket.accept(visitor);}
}
config25.xml
<?xml version="1.0" encoding="UTF-8"?>
<config><className>Tutorial_25.Packager</className>
</config>
