VB.Net中文教程(9) 重新定义(Overriding)程序

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主题：重新定义(Overriding)程序

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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("Jims Bonus: " + str(Jim.Bonus()))
        Dim Tom As New SalesManager(45000)
        Messagebox.Show("Toms Bonus: " + str(Tom.Bonus()))
    End Sub
End Class

此程序输出如下﹕ Jims bonus: 400
Toms 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()

   &nbs

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