26.3 完整解決方案

      Sunny軟體公司開發人員使用訪問者模式對OA系統中員工資料彙總模組進行重構，使得系統可以很方便地增加新型別的訪問者，更加符合“單一職責原則”和“開閉原則”，重構後的基本結構如圖26-3所示：

import java.util.*;

//員工類：抽象元素類
interface Employee
{
	public void accept(Department handler); //接受一個抽象訪問者訪問
}

//全職員工類：具體元素類
class FulltimeEmployee implements Employee
{
	private String name;
	private double weeklyWage;
	private int workTime;

	public FulltimeEmployee(String name,double weeklyWage,int workTime)
	{
		this.name = name;
		this.weeklyWage = weeklyWage;
		this.workTime = workTime;
	}	

	public void setName(String name) 
    {
		this.name = name; 
	}

	public void setWeeklyWage(double weeklyWage) 
    {
		this.weeklyWage = weeklyWage; 
	}

	public void setWorkTime(int workTime) 
    {
		this.workTime = workTime; 
	}

	public String getName() 
    {
		return (this.name); 
	}

	public double getWeeklyWage() 
    {
		return (this.weeklyWage); 
	}

	public int getWorkTime() 
    {
		return (this.workTime); 
	}

	public void accept(Department handler)
    {
		handler.visit(this); //呼叫訪問者的訪問方法
	}
}

//兼職員工類：具體元素類
class ParttimeEmployee implements Employee
{
	private String name;
	private double hourWage;
	private int workTime;

	public ParttimeEmployee(String name,double hourWage,int workTime)
	{
		this.name = name;
		this.hourWage = hourWage;
		this.workTime = workTime;
	}	

	public void setName(String name) 
    {
		this.name = name; 
	}

	public void setHourWage(double hourWage) 
    {
		this.hourWage = hourWage; 
	}

	public void setWorkTime(int workTime) 
    {
		this.workTime = workTime; 
	}

	public String getName() 
    {
		return (this.name); 
	}

	public double getHourWage() 
    {
		return (this.hourWage); 
	}

	public int getWorkTime() 
    {
		return (this.workTime); 
	}

	public void accept(Department handler)
    {
		handler.visit(this); //呼叫訪問者的訪問方法
	}
}

//部門類：抽象訪問者類
abstract class Department
{
    //宣告一組過載的訪問方法，用於訪問不同型別的具體元素
	public abstract void visit(FulltimeEmployee employee);
	public abstract void visit(ParttimeEmployee employee);	
}

//財務部類：具體訪問者類
class FADepartment extends Department
{
    //實現財務部對全職員工的訪問
	public void visit(FulltimeEmployee employee)
	{
		int workTime = employee.getWorkTime();
		double weekWage = employee.getWeeklyWage();
		if(workTime > 40)
		{
			weekWage = weekWage + (workTime - 40) * 100;
		}
		else if(workTime < 40)
		{
			weekWage = weekWage - (40 - workTime) * 80;
			if(weekWage < 0)
			{
				weekWage = 0;
			}
		}
		System.out.println("正式員工" + employee.getName() + "實際工資為：" + weekWage + "元。");			
	}

    //實現財務部對兼職員工的訪問
	public void visit(ParttimeEmployee employee)
	{
		int workTime = employee.getWorkTime();
		double hourWage = employee.getHourWage();
		System.out.println("臨時工" + employee.getName() + "實際工資為：" + workTime * hourWage + "元。");		
	}		
}

//人力資源部類：具體訪問者類
class HRDepartment extends Department
{
    //實現人力資源部對全職員工的訪問
	public void visit(FulltimeEmployee employee)
	{
		int workTime = employee.getWorkTime();
		System.out.println("正式員工" + employee.getName() + "實際工作時間為：" + workTime + "小時。");
		if(workTime > 40)
		{
			System.out.println("正式員工" + employee.getName() + "加班時間為：" + (workTime - 40) + "小時。");
		}
		else if(workTime < 40)
		{
			System.out.println("正式員工" + employee.getName() + "請假時間為：" + (40 - workTime) + "小時。");
		}						
	}

    //實現人力資源部對兼職員工的訪問
	public void visit(ParttimeEmployee employee)
	{
		int workTime = employee.getWorkTime();
		System.out.println("臨時工" + employee.getName() + "實際工作時間為：" + workTime + "小時。");
	}		
}

//員工列表類：物件結構
class EmployeeList
{
    //定義一個集合用於儲存員工物件
	private ArrayList<Employee> list = new ArrayList<Employee>();

	public void addEmployee(Employee employee)
	{
		list.add(employee);
	}

    //遍歷訪問員工集合中的每一個員工物件
	public void accept(Department handler)
	{
		for(Object obj : list)
		{
			((Employee)obj).accept(handler);
		}
	}
}
 

      為了提高系統的靈活性和可擴充套件性，我們將具體訪問者類的類名儲存在配置檔案中，並通過工具類XMLUtil來讀取配置檔案並反射生成物件，XMLUtil類的程式碼如下所示：

import javax.xml.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.SAXException;
import java.io.*;
class XMLUtil
{
    //該方法用於從XML配置檔案中提取具體類類名，並返回一個例項物件
    public static Object getBean()
    {
		try
		{
			//建立文件物件
			DocumentBuilderFactory dFactory = DocumentBuilderFactory.newInstance();
			DocumentBuilder builder = dFactory.newDocumentBuilder();
			Document doc;							
			doc = builder.parse(new File("config.xml")); 
		
			//獲取包含類名的文字節點
			NodeList nl = doc.getElementsByTagName("className");
            Node classNode=nl.item(0).getFirstChild();
            String cName=classNode.getNodeValue();
            
            //通過類名生成例項物件並將其返回
            Class c=Class.forName(cName);
	  	    Object obj=c.newInstance();
            return obj;
        }   
        catch(Exception e)
        {
           	e.printStackTrace();
           	return null;
       	}
    }
}
 

       配置檔案config.xml中儲存了具體訪問者類的類名，程式碼如下所示：

<?xml version="1.0"?>
<config>
	<className>FADepartment</className>
</config>

     編寫如下客戶端測試程式碼：

class Client
{
	public static void main(String args[])
	{
		EmployeeList list = new EmployeeList();
		Employee fte1,fte2,fte3,pte1,pte2;

		fte1 = new FulltimeEmployee("張無忌",3200.00,45);
		fte2 = new FulltimeEmployee("楊過",2000.00,40);
		fte3 = new FulltimeEmployee("段譽",2400.00,38);
		pte1 = new ParttimeEmployee("洪七公",80.00,20);
		pte2 = new ParttimeEmployee("郭靖",60.00,18);

		list.addEmployee(fte1);
		list.addEmployee(fte2);
		list.addEmployee(fte3);
		list.addEmployee(pte1);
		list.addEmployee(pte2);

		Department dep;
		dep = (Department)XMLUtil.getBean();
		list.accept(dep);
	}
}

      編譯並執行程式，輸出結果如下：

      如果需要更換具體訪問者類，無須修改原始碼，只需修改配置檔案，例如將訪問者類由財務部改為人力資源部，只需將儲存在配置檔案中的具體訪問者類FADepartment改為HRDepartment，如下程式碼所示：

<?xml version="1.0"?>
<config>
    <className>HRDepartment</className>
</config>

      重新執行客戶端程式，輸出結果如下：

      如果要在系統中增加一種新的訪問者，無須修改原始碼，只要增加一個新的具體訪問者類即可，在該具體訪問者中封裝了新的操作元素物件的方法。從增加新的訪問者的角度來看，訪問者模式符合“開閉原則”。

      如果要在系統中增加一種新的具體元素，例如增加一種新的員工型別為“退休人員”，由於原有系統並未提供相應的訪問介面（在抽象訪問者中沒有宣告任何訪問“退休人員”的方法），因此必須對原有系統進行修改，在原有的抽象訪問者類和具體訪問者類中增加相應的訪問方法。從增加新的元素的角度來看，訪問者模式違背了“開閉原則”。

      綜上所述，訪問者模式與抽象工廠模式類似，對“開閉原則”的支援具有傾斜性，可以很方便地新增新的訪問者，但是新增新的元素較為麻煩。