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: "
                         + str(crag.salary))
    End Sub
End Class

因为name是Person类别的私有资料，在子类别Teacher的modifyName()里不能使用此资料名称，所以错了。如果将Person类别定义为﹕

'ex03.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'---------------------------------------------------------
Class Person
    Protected name As String
    Public age As Integer

    Public Sub New(ByVal na As String, ByVal a As Integer)
        name = na
        age = a
    End Sub
    Public Function GetName() As String
        GetName = name
    End Function
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
    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) + ",     NAME: "
                         + crag.GetName())
    End Sub
End Class

此程序输出：
              AGE: 42,   NAME: Crag Clinton

    此时﹐name为家族公用之资料﹐凡是Person之子孙类别皆可取用之。但家族外之程序（如 Form1_Click()程序）仍不得直接使用之。如果上述定义改为﹕

Class Person
    Protected name As String
    Private salary As Decimal
    Public age As Integer

    Public Sub New(ByVal na As String, ByVal a As Integer)
        name = na
        age = a
    End Sub
End Class

此时﹐salary为类别私有﹐其它类别不得使用。name为家族私有﹐家族外之类别不得使用。age为公用﹐任何类别皆可用。

1.2 公用与私有程序
    上节介绍过﹕资料成员有 Private、Protected 及Public之分。同样地﹐程序成员也可分为 Private、Protected 及Public。虽然在应用上﹐程序成员大多定义为Public﹐但必要时﹐也能将过程定义为Private 或 Protected。私有程序和私有资料一样﹐只限于该类别之程序才能呼叫它。例如﹕

'ex04.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------------------------
Class Person
    Private name As String
    Private age As Integer
    Private 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 modify(ByVal na As String, ByVal a As Integer)
        name = na
        modifyAge(a)
    End Sub
    Public Sub Show()
        Messagebox.Show("Name: " + name + "   Age: " + str(age))
    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 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
   
    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 FullTime_Teacher("David", 25)
        p1.Show()
        p1.modifyValue("Kent Smith", 45)
        p1.Show()
    End Sub
End Class

此程序输出：
              Name: David         Age: 25
              Name: Kent Smith    Age: 45

    Show()是Person类别的Public程序，孙子类别FullTime_Teacher继承之，成为FullTime_Teacher类别的Public程序，所以Form1_Click()程序能写着指令：p1.Show()。name和modifyAge()是Person的Protected成员，Teacher的modify()程序里能直接使用它们。而且FullTime_Teacher的modifyData()程序里能直接使用它们。但是Form1_Click()就无法使用它们。modify()是Teacher的Protected成员，FullTime_Teacher的modifyValue()程序里能直接使用它们，但Form1_Click()就不行使用。所以若将上述Form1_Click()的指令改为如下，就不对了：

 Protected Sub Form1_Click( ..... )
        Dim p1 As New FullTime_Teacher("David", 25)
        p1.Show()
        p1.modify("Kent Smith", 45)   'Error!
        p1.modifyAge(24)   'Error!
        p1.Show()
    End SubEnd Class
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