VB.Net中文教程(1) 类别与封装性(3)
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。」
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