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nd Sub
End Class
Public Class Teacher
Inherits Person
Private salary As Decimal
Public Sub New(ByVal na As String, ByVal a As Integer, ByVal sa As Decimal)
MyBase.New(na, a)
salary = sa
End Sub
Public Sub pr()
MyBase.Display()
Messagebox.Show("Salary: " + str(salary))
End Sub
End Class
Public Class Student
Inherits Person
Private student_number As Integer
Public Sub New(ByVal na As String, ByVal a As Integer, ByVal no As Integer)
MyBase.New(na, a)
student_number = no
End Sub
Public Sub pr()
MyBase.Display()
Messagebox.Show("StudNo: " + str(student_number))
End Sub
End Class
''''-----------------------------------------------------------------------------------------
Public Class Form1
Inherits System.WinForms.Form
Public Sub New()
MyBase.New()
Form1 = Me
''''This call is required by the Win Form Designer.
InitializeComponent()
''''TODO: Add any initialization after the InitializeComponent() call
End Sub
''''Form overrides dispose to clean up the component list.
Public Overrides Sub Dispose()
MyBase.Dispose()
components.Dispose()
End Sub
#Region " Windows Form Designer generated code "
........
#End Region
Protected Sub Form1_Click( ByVal sender As Object, ByVal
e As System.EventArgs)
Dim x As New Person("Alvin", 32)
Dim y As New Student("Tom", 36, 11138)
x.Display()
y.pr()
End Sub
End Class
此程序输出﹕
Name: Alvin Age: 32
Name: Tom Age: 36
StudNo: 11138
子类别之建构者──
Public Sub New(ByVal na As String, ByVal a As Integer, ByVal sa As Decimal)
......
End Sub
先呼叫父类别之建构者──
Public Sub New(ByVal na As String, ByVal a As Integer)
......
End Sub
协助诞生对象如下﹕
接着﹐执行父类别Person的建构者内之指令﹐设定初值如下﹕
Person建构者工作完成了﹐轮到Student建构者的工作﹐扩充对象如下﹕
诞生完毕﹐执行Student建构者内之指令﹐设定初值﹕
于是﹐y 对象诞生完毕了。
子类别建构者呼叫父类别建构者之模式﹕
例如﹐Student建构者内的MyBase.New(na, a)指令将na及a值传递给Person建构者﹐于是Person建构者设定name及age之初值。除了呼叫Person建构者之外﹐还执行自己的指令──student_number = no ﹐设定student_number之初值。Student类别之对象含有三个资料成员﹐其中name及 age是由Person类别继承而来﹐就藉Person建构者设定其初值。至于Student类别自己定义之资料成员student_number﹐就由自己的指令──student_number = no设定其初值。
3. 藉「继承」扩充程序
程序之发展是渐进的﹐软件内之类别需随企业之成长而不断扩充。类别扩充常源于﹕
(1) 企业成长了﹐产生了新类别。
例如﹕某计算机公司﹐过去制造大型及迷你计算机﹐现在新推出个人计算机。就必须建立新子类别如下﹕
图3、 程序扩充方法──衍生子类别
再如﹐某银行过去提供支票帐户(Checking Account)及储蓄帐户(Saving Account)给客户﹐现在拟增加定期帐户(CD Account)。就得新增子类别如下﹕
这新类别CD_Account继承父类别Saving_Account及Account 的资料及程序。例如﹐Account 之定义如下﹕
''''ex08.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
''''-----------------------------------------------------------------------------------
Class Account
Private Shared next_number As Integer
Protected acc_no As Integer
Protected name As String
Protected balance As Decimal
Shared Sub New()
next_number = 1000
End Sub
Public Sub New(ByVal na As String)
name = na
balance = 0
acc_no = next_number
next_number = next_number + 1
End Sub
Public Sub deposit(ByVal money As Decimal)
balance = balance + money
End Sub
Public Sub withdraw(ByVal money As Decimal)
balance = balance - money
End Sub
Public Sub close()
acc_no = -1
End Sub
End Class
Class Saving_Account
Inherits Account
Private interest_rate As Double
Public Sub New(ByVal na As String, ByVal rate As Double)
MyBase.New(na)
Me.interest_rate = rate
End Sub
Public Function comp_interest() As Double
comp_interest = balance * interest_rate
End Function
Public Sub display()
Messagebox.Show("ACC_NO: " + str(MyBase.acc_no))
Messagebox.Show("余额: " + str(Me.balance) + " 利息: "
+ str(comp_interest()))
End Sub
End Class
''''-----------------------------------------------------
Public Class Form1
Inherits System.WinForms.Form
Public Sub New()
MyBase.New()
Form1 = Me
''''This call is required by the Win Form Designer.
InitializeComponent()
''''TODO: Add any initialization after the InitializeComponent() call
End Sub
''''Form overrides dispose to clean up the component list.
Public Overrides Sub Dispose()
MyBase.Dispose()
components.Dispose()
End Sub
#Region " Windows Form Designer generated code "
.......
#End Region
Protected Sub Form1_Click( ByVal sender As Object, ByVal
e As System.EventArgs)
Dim saving_acc As New Saving_Account("Tom Bush", 0.085)
Dim account As Saving_Account
account = New Saving_Account("Linda Lin", 0.008)
account.deposit(5000)
account.withdraw(2500)
account.display()
account.close()
End Sub
End Class
此程序输出﹕
ACC_NO: 1001
余额: 2500
利息: 20
因Saving_Account继承Account ﹐所以Saving_Account类别亦拥有deposit() 、withdraw()及close() 三个程序。请换个角度﹐从Saving_Account类别之设计者来看﹐他透过继承关系来「借用」父类别之程序。「借用」又称为「再使用」(Resue) ﹐能使得子类别简单但却强而有力。就如Saving_Account类别显 上一页 [1] [2] [3] [4] [5] [6] 下一页
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