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得比Account 类别简单﹐但它总共却含有deposit() 、withdraw()等6 个程序﹐功能比Account 类别强大许多。对于指令──account.deposit(5000) 而言﹐其有两种意义﹕
I. Saving_Account继承Account ﹐所以Saving_Account具有deposit() 程序﹐可将5000存入account 对象。
II. Saving_Account为Account 之子类别﹐它能借用（再使用）父类别之deposit() 程序﹐来将5000存入account 对象。
这两者是一体的两面﹐继承功能让软件师能不断新增子类别﹐不断再使用现有类别之程序﹐来简化子类别之复杂度。继承使得子类别拥有祖父类别之各种功能﹐且再增加新功能﹔再使用使得子类别尽量委托祖父类别来担任工作﹐自己以逸待劳。因而﹐善用继承功能﹐时时心存再使用观念﹐便能设计简单却功能强大的子类别了。而程序员的效率提升了﹐软件之成本下降了﹐此乃OOP 的理想目标之一。兹再扩充一个子类别CD_Account如下﹕
''''ex09.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
Class CD_Account
Inherits Saving_Account
Public Sub New(ByVal na As String, ByVal rate As Double, ByRef b As Decimal)
MyBase.New( na, rate + 0.003 )
MyBase.deposit(b)
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 a As CD_Account
a = New CD_Account("Linda Lin", 0.008, 2500)
a.display()
a.close()
End Sub
End Class
CD_Account除了在定义自己的New()之外，皆继承Saving_Account的成员。请您仔细阅读下述程序，看看跟上一个程序有何微妙差异？
''''ex10.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
Class CD_Account
Private acc As Saving_Account
Public Sub New(ByVal na As String, ByVal rate As Double, ByRef b As Decimal)
acc = New Saving_Account(na, rate + 0.003)
acc.deposit(b)
End Sub
Public Function comp_interest() As Double
comp_interest = acc.comp_interest()
End Function
Public Sub display()
acc.display()
End Sub
Public Sub Close()
acc.close()
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 a As CD_Account
a = New CD_Account("Linda Lin", 0.008, 2500)
''''a.deposit(5000) ''''Error here!!
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