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VB.Net中文教程(1) 类别与封装性 (转)

原创 IT综合 作者:worldblog 时间:2007-12-06 10:04:54 0 删除 编辑
VB.Net中文教程(1) 类别与封装性 (转)[@more@]

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主题:  类别与封装性(Ecapsulation)

 

????????????  内容  ????????????
v 1. 类别的「程序成员」
v 2. 「封装性」概念

 


1. 类别的「程序成员」(Procedure Member)

  类别 (Class)之任务是把资料(Data)和程序(Procedure)组织并封装起来。类别告诉计算机﹕「其对象应含有那些资料、应含有那些程序裨处理外界传来之讯息」。类别须详细说明它的资料及程序﹐我们称此资料是类别之「资料成员」(Data Member) ﹔而称此程序是类别之「程序成员」(Procedure Member)。有关类别内容之叙述﹐就是所谓的类别定义(Class Definition)。类别定义之格式为──
 
类别之用途为﹕宣告对象。例如﹕

'ex01.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Class Tree
  Public varity As String
  Public age As Integer
  Public height As Single
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 New Tree()
  MsgBox("Object a Is Created.")
  End Sub
 End Class

此程序定义了类别Tree﹐它只含资料而无程序﹐为一「阳春型」之类别。当计算机执行到Form1_Click()程序内之宣告指令──
  Dim a As New Tree()

就分配足够存放这 3项资料的内存空间给予对象 a。然而﹐此Tree类别只有资料而无程序。所以﹐对象 a无法接受外来之讯息。此时﹐可加入程序成员﹐使Tree类别含有程序、具有动力﹐对象就有能力来处理讯息了。例如﹕

'ex02.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'------------------------------------------------------------
Class Tree
  Public varity As String
  Public age As Integer
  Public height As Single
  Public Sub input(ByVal hei As Single)
  height = hei
  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 New Tree()
  a.input(2.1)
  Messagebox.Show("Set a.Height to 2.1", "Hello!")
  End Sub
End Class

此程序输出:Set a.Height to 2.1
  现在﹐Tree类别已拥有程序成员 input()。程序成员的写法与一般VB程序相同﹐只是它应宣告于类别内﹐成为类别之专属程序。此刻﹐对象 a含有 3项资料及 1个程序﹕
 
计算机执行到指令──
  a.input(2.1)

就将讯息──input(2.1)传给对象 a。此时计算机呼叫并执行对象 a内之input() 程序。对象 a内之 input()就是定义于Tree类别内之input() ﹔于是Form1_Click()就把自变量──2.1 传给 input()内之 hei变量。

 
 
接下来﹐叙述──
  height = hei

把 hei变量值存入对象 a之资料成员──height中。

 
 
此刻﹐对象 a对讯息之处理完成了﹐其内部资料改变了﹐亦即对象 a之内部状态(Internal State)改变了﹔这是对象的行为之一。上述您已经会加入一个程序了﹐依同样方法﹐继续加入其它程序﹐让对象的兴为更多采多姿。例如﹕

'ex03.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-----------------------------------------------------------------------
Class Tree
  Public varity As String
  Public age As Integer
  Public height As Single
  Public Sub input(ByVal hei As Single)
  height = hei
  End Sub
  Public Function inquireHeight() As Single
  inquireHeight = height
  End Function
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 Tree = New Tree()
  Dim h As Single
  a.input(2.1)
  h = a.inquireHeight()
  Messagebox.Show("height = " + str(h) + "公尺", "HI!")
  End Sub
End Class

此程序输出如下﹕height = 2.1公尺
  Tree类别有2个程序成员──input() 和inquireHeight()。类别之程序成员必须与其对象配合使用。格式为﹕
 
亦即﹐必须以讯息之形式出现。例如﹕
 
 如果程序成员不与对象相配合时﹐计算机会如何处理呢﹖例如﹕

'ex04.bas
'Some Error Here !
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'--------------------------------------------------------------
Class Tree
  Public varity As String
  Public age As Integer
  Public height As Single
  Public Sub input(ByVal hei As Single)
  height = hei
  End Sub
  Public Function inquireHeight() As Single
  inquireHeight = height
  End Function
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 Tree = New Tree()
  Dim h As Single
  a.input(2.1)
  h = inquireHeight()
  Messagebox.Show("height = " + str(h) + "公尺", "HI!")
  End Sub
End Class

当计算机看到Form1_Click()内之指令──
  h = inquireHeight( )

它视inquireHeight()为一独立之程序﹐与Tree类别内之inquireHeight()无关﹔于是计算机去找此inquireHeight()之定义﹐但找不着﹔所以程序错了。因之﹐您要掌握个原则── 程序成员之唯一任务是支持对象之行为﹐必须与对象配合使用。
2. 「封装性」概念

  对象把资料及程序组织并「封装」(Encapsulate) 起来﹐只透过特定的方式才能使用类别之资料成员和程序成员。对象如同手提袋﹐只从固定的开口才能存取东西﹐否则您一定不敢把钱放在手提袋中。对象像一座「防火墙」保护类别中的资料﹐使其不受外界之影响。想一想我国的万里长城可保护关内的人民﹐避免受胡人侵犯﹐但长城并非完全封闭﹐而有山海关、玉门关等出入口。对象和万里长城之功能是一致的﹐它保护其资料成员﹐但也有正常的资料存取管道﹕以程序成员来存取资料成员。请看个程序﹕

'ex05.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-------------------------------------------------------
Class Tree
  Public varity As String
  Public age As Integer
  Public height As Single
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 New Tree()
  a.height = 2.1
  Messagebox.Show("height = " + str(a.height) + "公尺")
  End Sub
End Class

此程序输出如下﹕height = 2.1公尺
  此程序中﹐Tree类别含有 3个资料成员﹐即对象内含有3个资料值,此类别之程序成员能直接存取之。同时,也允许其它程序来存取资料成员之值﹐其存取格式为﹕
 
例如﹕
  a.height = 2.1

此指令把 2.1存入对象 a之height变量中。于是对象 a之内容为﹕
 
 
 
  请看个常见错误如下﹕

'ex06.bas
'Some Error Here!
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-------------------- ------------------------------------
Class Tree
  Public varity As String
  Public age As Integer
  Public height As Single
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 New Tree()
  height = 2.1
  Messagebox.Show("height = " + str(a.height) + "公尺")
  End Sub
End Class

Form1_Click()程序内之指令── height = 2.1,此height变量并未与对象配合使用﹐计算机不认为它是Tree类别之height变量。计算机视其为Form1_Click()之自动变量(Automatic Variable)﹐但却未见到它的宣告﹐因之程序错了﹗这是对象对其资料成员保护最松的情形﹐因为对象所属类别(即Tree)之外的程序(如Form1_Click()程序)尚能存取资料成员的内容。就像一颗炸弹﹐除了引信管外﹐尚有许多管道可使炸弹内之化学药品爆炸﹔您将不敢把炸弹摆在飞机上﹐因何时会爆炸将无法控制。同理﹐Tree类别之资料──height变量﹐连外部的Form1_Click()皆可随意改变它﹔那么有一天height之内容出问题了﹐将难以追查出错之缘故﹐这种程序将让您大伤脑筋﹐因为您已无法掌握状况了。
  现在的VB程序中﹐能采取较严密之保护措施﹐使您较能控制类别内资料的变化状况。例如﹐

'ex07.bas
'Some Error Here!
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-----------------------------------------
Class Tree
  Private varity As String
  Private age As Integer
  Private height As Single
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
  Public Sub Form1_Click( ByVal sender As Object, ByVal
  e As System.EventArgs)
  Dim a As New Tree()
  a.height = 2.1
  MessageBox.Show("height = " + str(a.height))
  End Sub
End Class

此程序将原来的Public专用字改为Private﹐对Tree类别之资料成员采取严格之保护措施。Public 与Private之区别为──

  Public  表示此类别外之程序可来存取资料成员。
  Private  表示此类别外之程序绝无法直接存取资料成员﹐只有程序成员才能存取资料成员。

所以﹐计算机看到指令── a.height = 2.1,因Tree类别采取严格保护措施(private)﹐则Form1_Click()程序不能使用height变量名称。所以指令── a.height = 2.1错了。也许您问道﹕这样岂不是无法存取类别内之资料成员吗﹖答案是﹕「类别内之程序成员(Member Function) 可存取资料成员﹐而类别外之程序能藉程序成员代之存取资料成员。」
 
  图1、类别之沟通管道──程序成员

  这如同﹐引信管才能引起炸弹内之化学药品爆炸﹐人们只能经由引信管引爆之﹐让人们觉得使用炸弹既安全又简单。同样地﹐对象经由程序成员和外界沟通﹐可减少外界无意中破坏对象内之资料(无意中引爆炸弹)。例如﹕

'ex08.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'--------------------------------------------------
Class Tree
  Private varity As String
  Private age As Integer
  Private height As Single
  Public Sub input(ByVal hei As Single)
  height = hei
  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 New Tree()
  a.input(2.1)
  MessageBox.Show("OK")
  End Sub
End Class

  将input()摆在Tree类别中﹐为Tree之程序成员﹐它能存取资料成员height之值。把input()程序宣告为Public表示类别外之程序可藉来呼叫它﹐其呼叫格式为──
 
  简单规则是﹕
  Public 表示授权给外界之程序藉由此格式呼叫程序成员。

如果此程序改写为﹕

 

'ex09.bas
'Some Error Here!
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------
Class Tree
  Private varity As String
  Private age As Integer
  Private height As Single
  Private Sub input(ByVal hei As Single)
  height = hei
  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 New Tree()
  a.input(2.1)
  MessageBox("OK")
  End Sub
End Class

这程序有问题﹐因为 input()是Tree类别之Private程序成员而非Public程序成员﹐所以不能使用如下格式──

 
 
所以此程序错了。  请再看个例子吧﹗

'ex10.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-------------------------------------------
Class Tree
  Private varity As String
  Private height As Single
  Public age As Integer
  Public Sub ShowAge()
  MessageBox.Show("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 a As New Tree()
  a.age = 8
  a.age = a.age + 2
  a.ShowAge()
  End Sub
End Class

Tree类别包含 2个Private成员── variety及height﹐且有 2个Public成员── age及 ShowAge()。由于age是Public资料成员﹐所以Fom1_Click()可使用格式──
 
来存取Tree内之age变量。指令── a.age = 8把8存入对象 a内之age 变量。指令──a.age = a.age + 2使对象a之age变量值加上2﹐成为10。由于ShowAge()程序是Public程序成员﹐也可使用格式──
 
来呼叫 ShowAge()程序。
由于类别(即对象)之目的是保护资料﹐并且提供程序成员来与外界沟通(接受、处理、并反应讯息)。通常﹐资料成员皆宣告为Private﹐而程序成员皆宣告为Public。亦即尽量少用格式──
 
而尽量多用格式──
 
例如﹕

'ex11.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'--------------------------------------------------------------------
Class Tree
  Private varity As String
  Private age As Integer
  Private height As Single
  Public Sub input(ByVal v As String, ByVal a As Integer, ByVal hei As Single)
  varity = v
  age = a
  height = hei
  End Sub
  Public Sub Show()
  MessageBox.Show(varity + ", " + str(age) + ", " + str(height))
  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, b As New Tree()
  a.input("peach", 8, 2.1)
  b.input("pinapple", 2, 0.5)
  a.Show()
  b.Show()
  End Sub
End Class

  这个VB程序﹐Tree内之资料成员──variety, age及height皆为Private成员,而input()及Show()程序是Public成员。Form1_Click()程序中﹐首先诞生对象── a及 b。接下来﹐指令──
 
 

把 3项资料分别存入对象 a之资料成员中﹐a 之内容为﹕

 
 
同样地﹐指令──
  b.input( "pineapple", 2, 0.5 )

也把 3项资料存入对象 b之中﹐则 b之内容为﹕

 
 
最后﹐呼叫Show()程序把对象 a和对象 b之内容显示出来﹕
 
  peach,  8,  2.1
  pineapple,  2,  .5
n

 


 


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