VB.Net中文教程(3) 继承与封装性

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主题：继承与封装性

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v 1. 继承与封藏性

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 "
    .......
#End Region
    Protected Sub Form1_Click( ByVal sender As Object,
                            ByVal e As System.EventArgs)
        Dim crag As New Teacher("Crag", 38, 45000)
        crag.modifyName("Crag Clinton")
        MessageBox.Show( "AGE: " + str(crag.age) + ",     SALARY: "
            &n

补充：软件开发 , Vb ,