VB.Net中文教程(2) 继承与封装性(1)

 

1. 继承与封装性(Encapsulation)

1.1 公用与私有数据

 

   前面已介绍「封藏性」(Encapsulation) 之观念。即是﹕类别内所定义之资料成员﹐只限于程序成员才能存取之。现在所面临之问题为﹕子类别能否直接存取父类别之资料呢﹖就如同﹕儿女从父母亲继承了财产﹐但能否取用或卖掉继承而来的财产呢﹖如果可以﹐显然违背了「封藏性」之理想。如果不行﹐显然带给程序员莫大之限制。理论上﹐百分之百的封藏性最为完美﹔但应用上﹐若给予子类别若干优待﹐能提高程序之弹性及效率。为了解决此鱼与熊掌不可兼得之困境﹐VB提供三种选择﹕

 

   (1) Public    ──指定某些数据为公用的﹐任何程序皆可直接取用之﹔此时并无任何封藏作用。

   (2) Protected ──指定某些资料为家族公用﹐亦即只有子孙类别内之程序可取用﹐非子孙类别之程序必须呼叫家族内之程序代为存取。

   (3) Private   ──指定某资料为类别私有﹐只限于该类别之程序才可取用。子类别之程序也必须呼叫父类别之程序代为存取﹐此时具百分之百封藏性。

 

   先前介绍「封装性」基本概念时，您已经认识了Public和Private的用意了，至于Protected则配合继承来使用，于是在此特别强调它。

   由于VB向现实妥协﹐开了方便之门﹐无百分之百封藏性﹔所以有些人认为 VB并非完美的 OOP语言。您认为如何呢﹖请看个程序﹕

 

"ex01.bas

Imports System.ComponentModel

Imports System.Drawing

Imports System.WinForms

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

Class Person

   Private name As String

   Public age As Integer

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

       name = na

       age = a

   End Sub

End Class

 

Class Teacher

   Inherits Person

  

   Public salary As Single

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

       MyBase.New(na, a)

       salary = sa

   End Sub

   Public Sub modifyAge(ByVal a As Integer)

       age = 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 crag As New Teacher("Crag", 38, 45000)

       crag.modifyAge(42)

       MessageBox.Show( "AGE: " + str(crag.age) + ",     SALARY: "

                        + str(crag.salary))

   End Sub

End Class

 

此程序输出如下﹕

               AGE: 42     SALARY: 45000

 

   Person之age资料成员定义为 Public﹐表示 age为公用数据﹐任何程序皆可存取之。因之﹐Teacher 之 modifyAge()可使用age这名称。相对地﹐于 Teacher类别中﹐就不得使用name这名称﹐因为name定义为Privatec﹐表示name为Person类别之私有资料。再看Teacher类别之salary资料﹐它定义为Public﹐表示公用之意。所以Form1_Click()可直接使用 crag.salary格式取得salary之值。然而﹐不能写成﹕crag.name ﹐因为name为私有资料。例如，下述程序是不对的：

 

"ex02.bas

"Some Error Here!

Imports System.ComponentModel

Imports System.Drawing

Imports System.WinForms

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

Class Person

   Private name As String

   Public age As Integer

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

       name = na

       age = a

   End Sub

End Class

 

Class Teacher

   Inherits Person

  

   Public salary As Single

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

       MyBase.New(na, a)

       salary = sa

   End Sub

   Public Sub modifyName(ByVal na As String)

       name = na   "Error Here!

   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 "