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                # 操作復雜對象結構——訪問者模式(三) 26.3 完整解決方案 Sunny軟件公司開發人員使用訪問者模式對OA系統中員工數據匯總模塊進行重構,使得系統可以很方便地增加新類型的訪問者,更加符合“單一職責原則”和“開閉原則”,重構后的基本結構如圖26-3所示: ![](http://my.csdn.net/uploads/201204/06/1333714391_6314.gif) 在圖26-3中,FADepartment表示財務部,HRDepartment表示人力資源部,它們充當具體訪問者角色,其抽象父類Department充當抽象訪問者角色;EmployeeList充當對象結構,用于存儲員工列表;FulltimeEmployee表示正式員工,ParttimeEmployee表示臨時工,它們充當具體元素角色,其父接口Employee充當抽象元素角色。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); //調用訪問者的訪問方法 } } //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); //調用訪問者的訪問方法 } } //部門類:抽象訪問者類 abstract class Department { //聲明一組重載的訪問方法,用于訪問不同類型的具體元素 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:對象結構 class EmployeeList { //定義一個集合用于存儲員工對象 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對象 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展性,我們將具體訪問者類的類名存儲在配置文件中,并通過工具類XMLUtil來讀取配置文件并反射生成對象,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,并返回一個實例對象 public static Object getBean() { try { //創建文檔對象 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點 NodeList nl = doc.getElementsByTagName("className"); Node classNode=nl.item(0).getFirstChild(); String cName=classNode.getNodeValue(); //通過類名生成實例對象并將其返回 Class c=Class.forName(cName); Object obj=c.newInstance(); return obj; } catch(Exception e) { e.printStackTrace(); return null; } } } ``` 配置文件config.xml中存儲了具體訪問者類的類名,代碼如下所示: ``` <?xml version="1.0"?> <config> <className>FADepartment</className> </config> ``` 編寫如下客戶端測試代碼: ``` 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); } } ``` 編譯并運行程序,輸出結果如下: ``` 正式員工張無忌實際工資為:3700.0元。 正式員工楊過實際工資為:2000.0元。 正式員工段譽實際工資為:2240.0元。 臨時工洪七公實際工資為:1600.0元。 臨時工郭靖實際工資為:1080.0元。 ``` 如果需要更換具體訪問者類,無須修改源代碼,只需修改配置文件,例如將訪問者類由財務部改為人力資源部,只需將存儲在配置文件中的具體訪問者類FADepartment改為HRDepartment,如下代碼所示: ``` <?xml version="1.0"?> <config> <className>HRDepartment</className> </config> ``` 重新運行客戶端程序,輸出結果如下: ``` 正式員工張無忌實際工作時間為:45小時。 正式員工張無忌加班時間為:5小時。 正式員工楊過實際工作時間為:40小時。 正式員工段譽實際工作時間為:38小時。 正式員工段譽請假時間為:2小時。 臨時工洪七公實際工作時間為:20小時。 臨時工郭靖實際工作時間為:18小時。 ``` 如果要在系統中增加一種新的訪問者,無須修改源代碼,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對象的方法。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類型為“退休人員”,由于原有系統并未提供相應的訪問接口(在抽象訪問者中沒有聲明任何訪問“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”的支持具有傾斜性,可以很方便地添加新的訪問者,但是添加新的元素較為麻煩。
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