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主题： 类别与封装性(Ecapsulation)
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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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