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VB.Net中文教程(9) 重新定义(Overriding)程序 (转)

原创 IT综合 作者:worldblog 时间:2007-12-06 10:05:55 0 删除 编辑
VB.Net中文教程(9) 重新定义(Overriding)程序 (转)[@more@]

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  主题:  重新定义(Overriding)程序


??????????  内容  ??????????
v 1. 重新定义程序

 

 

 


1. 重新定义(Override)程序

  在应用上﹐常见如下之情况﹕子类别从父类别继承之程序﹐并不合乎子类别之需要。此时可设计新程序取代之。

 
  图1、程序成员之重新定义

  例如﹐SalesPerson类别含有Bonus()程序﹔SalesManager由 SalesPerson继承而得Bonus()程序。Bonus()能计算销售人员之红利。然而﹐一般销售员与销售经理之红利计算方法不同﹔所以﹐SalesPerson之Bonus()显然并不适用于SalesManager。这SalesManager类别必须定义适用的Bonus()求算销售经理之红利。子类别不满意继承之程序而自行定义程序取代之﹐此情形称为「再定义」(Redefinition)或「重新定义」(Overriding)。自行定义程序与继承程序之名称相同。请看个程序﹕

'ex01.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-----------------------------------------------------------------------------------------
Class SalesPerson
  Protected totalSales As Double
  Public Sub New(ByVal t As Double)
  totalSales = t
  End Sub
  Public Overridable Function Bonus() As Double
  Bonus = totalSales * 0.008
  End Function
End Class

Class SalesManager
  Inherits SalesPerson
  Public Sub New(ByVal t As Double)
  MyBase.New(t)
  End Sub
  Public Overrides Function Bonus() As Double
  Bonus = totalSales * 0.008 + 1000
  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 Jim As New SalesPerson(50000)
  Messagebox.Show("Jim's Bonus: " + str(Jim.Bonus()))
  Dim Tom As New SalesManager(45000)
  Messagebox.Show("Tom's Bonus: " + str(Tom.Bonus()))
  End Sub
End Class

此程序输出如下﹕  Jim's bonus: 400
  Tom's bonus: 1360

  因Jim为SalesPerson类别之对象﹐则Jim.Bonus()所指的是SalesPerson之Bonus()程序。SalesManager类别继承此Bonus()﹐且自己定义新的Bonus()程序。此新程序取代了继承之程序。因之﹐Alvin.Bonus()是指SalesManager自己定义之Bonus()程序。
  「重新定义」(Overriding)的过程中﹐必定也产生「重复定义」(Overloading) 之现象。前者是针对「父子」类别之间﹐子类别有「修正」(Refinement)或「取代」(Replacement) 之意味时﹐才定义同名称之程序来取代父类别之程序。至于重复定义则范围较广﹐在类别内也能重复定义程序﹐父子类别之间也能重复定义程序﹐甚至互为独立的两类别之间也能发生重复定义之现象﹔重复定义之后新程序与原程序之间﹐在「涵义」(Semantic)上仅类似,但并不必然相同,例如Bonus(ByVal y As Year)与Bonus(ByVal y As Employee)两者涵义不尽相同。
  然而重新定义程序﹐例如上述的 SalesPerson例子中﹐父类别SalesPerson.Bonus()和子类别SalesManager.Bonus()皆表示同一涵义──求算红利﹐只是计算方法不同罢了。因之﹐「重新定义」着眼于以不同的执行过程来取代父类别之程序﹐但新旧程序之间﹐具有一致的意图。盼您能区分这易于混淆的概念。请再看个程序:

'ex02.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms

'--------------------------------------------------------------------------------------
Public Class Person
  Private name As String
  Private age As Integer
  Public Sub New(ByVal na As String, ByVal a As Integer)
  name = na
  age = a
  End Sub
  Public Function birthDay() As Integer
  birthDay = 2001 - age
  End Function
  Public Overridable Sub Display()
  Messagebox.Show("Name: " + name + "  Age: " + str(age))
  End Sub
End Class

Public Class Teacher
  Inherits Person
 
  Private salary As Decimal
  Public Sub New(ByVal na As String, ByVal a As Integer, ByVal sa As Decimal)
  MyBase.New(na, a)
  salary = sa
  End Sub
  Public Overrides Sub Display()
  MyBase.Display()
  Messagebox.Show("Salary: " + str(salary))
  End Sub
End Class

Public Class Student
  Inherits Person
 
  Private student_number As Integer
  Public Sub New(ByVal na As String, ByVal a As Integer, ByVal no As Integer)
  MyBase.New(na, a)
  student_number = no
  End Sub
  Public Overrides Sub Display()
  MyBase.Display()
  Messagebox.Show("StudNo: " + str(student_number))
  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 Teacher("Linda", 33, 50500)
  Dim y As New Student("Tom", 36, 11138)
  x.Display()
  y.Display()
  End Sub
End Class


此程序输出:
  Name: Linda  Age: 33
  Salary: 50500
  Name: Tom  Age: 36
  StudNo: 11138

Person的Display()不能显示出Teacher的salary资料,所以Teacher必须「修正」Person原有的Display()程序。同样地,Student类别也「修正」Person原有的Display()程序。n


 


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