VB.Net中文教程(2) 继承与封装性(3)

 

#End Region

   Protected Sub Form1_Click( ByVal sender As Object,

                           ByVal e As System.EventArgs)

       Dim p1 As New Person("David", 25)

       p1.Show()

       p1.modify("David Smith", 28)

       p1.Show()

   End Sub

End Class

 

 

此程序输出：

              Name: David         Age: 25

              Name: David Smith    Age: 45

 

   modifyAge()为私有程序﹐New()及modify()为公用程序。modifyAge()为Person类别之程序成员﹐所以modify()能呼叫modifyAge()。Person类别外之函数不能呼叫modifyAge()。例如﹕在Form1_Click()中﹐可写着 p1.modify()﹐因为modify()为公用程序。但于Form1_Click()内﹐就不可写着 p1.modifyAge()﹐因为modifyAge()为 Private函数。如果放宽对modifyAge()之限制﹐使Person之子类别能呼叫modifyAge()﹐就须定义modifyAge()为 Protected函数﹐如下﹕

 

"ex05.bas

Imports System.ComponentModel

Imports System.Drawing

Imports System.WinForms

"----------------------------------------------------------------------

Class Person

   Protected name As String

   Private age As Integer

   Protected Sub modifyAge(ByVal a As Integer)

       age = a

   End Sub

   Public Sub New(ByVal na As String, ByVal a As Integer)

       name = na

       age = a

   End Sub

   Public Sub Show()

       Messagebox.Show("Name: " + name + "   Age: " + str(age))

   End Sub

End Class

 

Class Teacher

   Inherits Person

  

   Public Sub New(ByVal na As String, ByVal a As Integer)

       MyBase.New(na, a)

   End Sub

   Public Sub modify(ByVal na As String, ByVal a As Integer)

       MyBase.name = na

       MyBase.modifyAge(a)

   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 p1 As New Teacher("David", 25)

       p1.Show()

       p1.modify("Kent Smith", 45)

       p1.Show()

   End Sub

End Class

 

 

此程序输出：

                Name: David        Age: 25

                Name: Kent Smith    Age: 45

 

此时﹐子孙类别之函数能呼叫modifyAge()﹐但家族外之类别仍不可呼叫它。总结归纳为简单规则﹕

   ◎如果允许任何类别使用﹐就宣告为Public。

   ◎如果允许子类别使用﹐就宣告为Protected 。

   ◎如果允许本类别使用﹐就宣告为Private 。

 请在看个例子：

 

"ex06.bas

Imports System.ComponentModel

Imports System.Drawing

Imports System.WinForms

"----------------------------------------------------------------------

Class Person

   Protected name As String

   Private age As Integer

   Protected Sub modifyAge(ByVal a As Integer)

       age = a

   End Sub

   Public Sub New(ByVal na As String, ByVal a As Integer)

       name = na

       age = a

   End Sub

   Public Sub Show()

       Messagebox.Show("Name: " + name + "   Age: " + str(age))

   End Sub

End Class

 

Class Teacher

   Inherits Person

  

   Public Sub New(ByVal na As String, ByVal a As Integer)

       MyBase.New(na, a)

   End Sub

   Protected Sub modify(ByVal na As String, ByVal a As Integer)

       MyBase.name = na

       MyBase.modifyAge(a)

   End Sub

End Class

 

Class FullTime_Teacher

   Inherits Teacher

  

   Public Sub New(ByVal na As String, ByVal a As Integer)

       MyBase.New(na, a)

   End Sub

   Public Sub modifyValue(ByVal na As String, ByVal a As Integer)

       MyBase.modify(na, a)

   End Sub

   Public Sub modifyData(ByVal na As String, ByVal a As Integer)

       MyBase.name = na

       MyBase.modifyAge(a)

   End Sub

End Class

"-------------------------------------------------------------------

Public Class Form1

   Inherits System.WinForms.Form