|
a.display()
a.close()
End Sub
End Class
(2) 环境改变了﹐必须修正旧类别。
例如﹕软件内含Sales 类别如下──
''''ex11.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
''''----------------------------------------------------------------------------------
Class Sales
Protected sum As Double
Public Sub New()
sum = 0
End Sub
Public Overridable Sub input(ByVal amount As Double)
sum = sum + amount
End Sub
Public Sub display()
Messagebox.Show("The Sum Is: " + str(sum))
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 x As New Sales()
x.input(3000.5)
x.input(7999.5)
x.input(15000)
x.display()
End Sub
End Class
此程序输出如下﹕The Sum Is: 26000
input() 输入销售金额﹐且将金额加总起来﹐存于 sum变量中。display() 显示出总金额。随着企业之成长﹐往往对软件之功能有新的需求。例如﹕希望程序输出平均销售额。此时﹐必须更新原来的软件﹐其方法有二﹕
(I) 改写 Sales类别之定义。
此为传统软件之维护方法﹐就像一座房屋﹐已不敷使用﹐于是用推土机推掉﹐重建之。通常这种方法并不佳﹐原因是﹕旧的软件有些功能仍然可用﹐废弃重建实在可惜。若不想重建﹐只想去修改它﹐但是软件若很庞大﹐得浪费人力去了解旧软件的结构﹐弄通了之后﹐才能加以修正﹐显然不经济。进一步想﹐于修正之过程中﹐不经意而更动旧软件内仍可用之部分﹐则原来正确之功能﹐反而有遭破坏之虞﹗
(II) 藉继承关系修正 Sales类别。
亦即定义 Sales之子类别﹐来修正及扩充 Sales之功能。此时﹐并未更动Sales 类别之定义﹐所以不会破坏 Sales之功能。既然保存原有的良好功能﹐又能增加新的功能﹐岂非两全其美呢﹖此「旧干发新枝」之景象即是千年神木得以茁壮且绿意盎然之道理﹐也正是 OOP魅力的源头。现在﹐就为 Sales定义个子类别叫 Sales_new如下﹕
''''ex12.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
''''------------------------------------------------------------------------------------
Class Sales
Protected sum As Double
Public Sub New()
sum = 0
End Sub
Public Overridable Sub input(ByVal amount As Double)
sum = sum + amount
End Sub
Public Sub display()
Messagebox.Show("The Sum Is: " + str(sum))
End Sub
End Class
Class Sales_new
Inherits Sales
Protected number As Integer
Public Sub New()
number = 0
End Sub
Public Overrides Sub input(ByVal amount As Double)
MyBase.input(amount)
number = number + 1
End Sub
Public Sub display_average()
Dim avg As Double
If number > 0 Then
avg = sum / number
MessageBox.Show("The Average Is : " + str(avg))
End If
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 x As New Sales_new()
x.input(3005.5)
x.input(5994.5)
x.input(15012.6)
x.display()
x.display_average()
End Sub
End Class
此程序输出﹕
The Sum Is: 24012.6
The Average Is: 8004.2
子类别增加了新数据项number﹐重新定义了input()﹐使新input()保存父类别内之input()功能﹐并加上number = number +1之运算。保留了父类别之display()﹐但增加了display_average()程序。依此方法﹐软件自然日益茁壮﹐且历久弥新﹗藉继承关系不断扩充乃为OOP之重要技术﹐盼您善用之。n
上一页 [1] [2] [3] [4] [5] [6]
没有相关教程
| 
注：本站部分文章源于互联网，版权归原作者所有！如有侵权，请原作者与本站联系，本站将立即删除！ 本站文章除特别注明外均可转载，但需注明出处！ [MinTao学以致用网] 
