Sunny 軟件公司開(kāi)發(fā)人員使用訪問(wèn)者模式對(duì) OA 系統(tǒng)中員工數(shù)據(jù)匯總模塊進(jìn)行重構(gòu)，使得系統(tǒng)可以很方便地增加新類型的訪問(wèn)者，更加符合“單一職責(zé)原則”和“開(kāi)閉原則”，重構(gòu)后的基本結(jié)構(gòu)如圖所示：
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在圖中，F(xiàn)ADepartment 表示財(cái)務(wù)部，HRDepartment 表示人力資源部，它們充當(dāng)具體訪問(wèn)者角色，其抽象父類 Department 充當(dāng)抽象訪問(wèn)者角色；EmployeeList 充當(dāng)對(duì)象結(jié)構(gòu)，用于存儲(chǔ)員工列表；FulltimeEmployee 表示正式員工，ParttimeEmployee 表示臨時(shí)工，它們充當(dāng)具體元素角色，其父接口 Employee 充當(dāng)抽象元素角色。完整代碼如下所示：
import java.util.*;
//員工類：抽象元素類
interface Employee
{
public void accept(Department handler); //接受一個(gè)抽象訪問(wèn)者訪問(wèn)
}
//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); //調(diào)用訪問(wèn)者的訪問(wèn)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); //調(diào)用訪問(wèn)者的訪問(wèn)方法
}
}
//部門類：抽象訪問(wèn)者類
abstract class Department
{
//聲明一組重載的訪問(wèn)方法，用于訪問(wèn)不同類型的具體元素
public abstract void visit(FulltimeEmployee employee);
public abstract void visit(ParttimeEmployee employee);
}
//財(cái)務(wù)部類：具體訪問(wèn)者類
class FADepartment extends Department
{
//實(shí)現(xiàn)財(cái)務(wù)部對(duì)全職員工的訪問(wèn)
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(shí)際工資為：" + weekWage + "元。");
}
//實(shí)現(xiàn)財(cái)務(wù)部對(duì)兼職員工的訪問(wèn)
public void visit(ParttimeEmployee employee)
{
int workTime = employee.getWorkTime();
double hourWage = employee.getHourWage();
System.out.println("臨時(shí)工" + employee.getName() + "實(shí)際工資為：" + workTime * hourWage + "元。");
}
}
//人力資源部類：具體訪問(wèn)者類
class HRDepartment extends Department
{
//實(shí)現(xiàn)人力資源部對(duì)全職員工的訪問(wèn)
public void visit(FulltimeEmployee employee)
{
int workTime = employee.getWorkTime();
System.out.println("正式員工" + employee.getName() + "實(shí)際工作時(shí)間為：" + workTime + "小時(shí)。");
if(workTime > 40)
{
System.out.println("正式員工" + employee.getName() + "加班時(shí)間為：" + (workTime - 40) + "小時(shí)。");
}
else if(workTime < 40)
{
System.out.println("正式員工" + employee.getName() + "請(qǐng)假時(shí)間為：" + (40 - workTime) + "小時(shí)。");
}
}
//實(shí)現(xiàn)人力資源部對(duì)兼職員工的訪問(wèn)
public void visit(ParttimeEmployee employee)
{
int workTime = employee.getWorkTime();
System.out.println("臨時(shí)工" + employee.getName() + "實(shí)際工作時(shí)間為：" + workTime + "小時(shí)。");
}
}
//員工列表類：對(duì)象結(jié)構(gòu)
class EmployeeList
{
//定義一個(gè)集合用于存儲(chǔ)員工對(duì)象
private ArrayList<Employee> list = new ArrayList<Employee>();
public void addEmployee(Employee employee)
{
list.add(employee);
}
//遍歷訪問(wèn)員工集合中的每一個(gè)員工對(duì)象
public void accept(Department handler)
{
for(Object obj : list)
{
((Employee)obj).accept(handler);
}
}
}為了提高系統(tǒng)的靈活性和可擴(kuò)展性，我們將具體訪問(wèn)者類的類名存儲(chǔ)在配置文件中，并通過(guò)工具類 XMLUtil 來(lái)讀取配置文件并反射生成對(duì)象，XMLUtil 類的代碼如下所示：
import javax.xml.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.SAXException;
import java.io.*;
class XMLUtil
{
//該方法用于從XML配置文件中提取具體類類名，并返回一個(gè)實(shí)例對(duì)象
public static Object getBean()
{
try
{
//創(chuàng)建文檔對(duì)象
DocumentBuilderFactory dFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = dFactory.newDocumentBuilder();
Document doc;
doc = builder.parse(new File("config.xml"));
//獲取包含類名的文本節(jié)點(diǎn)
NodeList nl = doc.getElementsByTagName("className");
Node classNode=nl.item(0).getFirstChild();
String cName=classNode.getNodeValue();
//通過(guò)類名生成實(shí)例對(duì)象并將其返回
Class c=Class.forName(cName);
Object obj=c.newInstance();
return obj;
}
catch(Exception e)
{
e.printStackTrace();
return null;
}
}
}配置文件config.xml中存儲(chǔ)了具體訪問(wèn)者類的類名，代碼如下所示：
<?xml version="1.0"?>
<config>
<className>FADepartment</className>
</config>編寫(xiě)如下客戶端測(cè)試代碼：
class Client
{
public static void main(String args[])
{
EmployeeList list = new EmployeeList();
Employee fte1,fte2,fte3,pte1,pte2;
fte1 = new FulltimeEmployee("張無(wú)忌",3200.00,45);
fte2 = new FulltimeEmployee("楊過(guò)",2000.00,40);
fte3 = new FulltimeEmployee("段譽(yù)",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);
}
}編譯并運(yùn)行程序，輸出結(jié)果如下：
正式員工張無(wú)忌實(shí)際工資為：3700.0元。
正式員工楊過(guò)實(shí)際工資為：2000.0元。
正式員工段譽(yù)實(shí)際工資為：2240.0元。
臨時(shí)工洪七公實(shí)際工資為：1600.0元。
臨時(shí)工郭靖實(shí)際工資為：1080.0元。如果需要更換具體訪問(wèn)者類，無(wú)須修改源代碼，只需修改配置文件，例如將訪問(wèn)者類由財(cái)務(wù)部改為人力資源部，只需將存儲(chǔ)在配置文件中的具體訪問(wèn)者類 FADepartment 改為 HRDepartment，如下代碼所示：
<?xml version="1.0"?>
<config>
<className>HRDepartment</className>
</config>重新運(yùn)行客戶端程序，輸出結(jié)果如下：
正式員工張無(wú)忌實(shí)際工作時(shí)間為：45小時(shí)。
正式員工張無(wú)忌加班時(shí)間為：5小時(shí)。
正式員工楊過(guò)實(shí)際工作時(shí)間為：40小時(shí)。
正式員工段譽(yù)實(shí)際工作時(shí)間為：38小時(shí)。
正式員工段譽(yù)請(qǐng)假時(shí)間為：2小時(shí)。
臨時(shí)工洪七公實(shí)際工作時(shí)間為：20小時(shí)。
臨時(shí)工郭靖實(shí)際工作時(shí)間為：18小時(shí)。如果要在系統(tǒng)中增加一種新的訪問(wèn)者，無(wú)須修改源代碼，只要增加一個(gè)新的具體訪問(wèn)者類即可，在該具體訪問(wèn)者中封裝了新的操作元素對(duì)象的方法。從增加新的訪問(wèn)者的角度來(lái)看，訪問(wèn)者模式符合“開(kāi)閉原則”。
如果要在系統(tǒng)中增加一種新的具體元素，例如增加一種新的員工類型為“退休人員”，由于原有系統(tǒng)并未提供相應(yīng)的訪問(wèn)接口（在抽象訪問(wèn)者中沒(méi)有聲明任何訪問(wèn)“退休人員”的方法），因此必須對(duì)原有系統(tǒng)進(jìn)行修改，在原有的抽象訪問(wèn)者類和具體訪問(wèn)者類中增加相應(yīng)的訪問(wèn)方法。從增加新的元素的角度來(lái)看，訪問(wèn)者模式違背了“開(kāi)閉原則”。
綜上所述，訪問(wèn)者模式與抽象工廠模式類似，對(duì)“開(kāi)閉原則”的支持具有傾斜性，可以很方便地添加新的訪問(wèn)者，但是添加新的元素較為麻煩。
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