'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：
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