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VB.Net中文教程(5)程序多重定义 (转)

原创 IT综合 作者:worldblog 时间:2007-12-06 09:52:14 0 删除 编辑
VB.Net中文教程(5)程序多重定义 (转)[@more@]

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主题:  程序多重定义(Overloading)


??????  内容  ??????
v 1. 程序成员的多重定义
 

 

 

1. 程序成员的多重定义

  「程序多重定义」(Function Overloading)又称为「程序重复定义」。它让对象更具弹性﹐能处理多样化之讯息。这观念源于日常生活经验。例如﹐我们常说﹕
  ◎  猫  玩  绣球
  ◎  猫  玩  老鼠
  ◎  猫  玩  鱼

猫玩绣球与玩老鼠之玩法不尽相同。但何以使用同一动词──「玩」呢﹖也许人们认为其目的是一致的﹕猫获得快乐。上述的「猫」为类别﹐而某只猫是对象。例如﹕加菲猫是对象﹐它可接受讯息──

 

  其中﹐「玩」代表着动作和过程﹐而绣球、老鼠及鱼则是「玩」之对象。回想﹐在程序中﹐「程序」代表一项动作及过程﹐而「自变量值」则为程序之处理对象。因之﹐上图可表示为──
 
 
  图1、 play()之多重定义

  oop 程序设计之理想为﹕让程序之写法与人们日常生活经验吻合﹐于是设计个Play()程序﹐让它能接受不同型态之资料做为处理对象。上述Play()已具「多重定义」﹐其特点是──
  1.  程序名称相同﹐例如﹕Play()。
  2.  自变量不同﹐例如﹕老鼠和鱼。

因猫玩绣球和玩老鼠的方法略有不同﹐例如老鼠是活的而绣球是死的﹐其玩的过程亦不尽相同。为了表示动作与过程之不同﹐Play()程序内之指令也有所不同。例如﹕

 

写VB程序时﹐其格式必须是──

  Class Cat
  Public Overloads Sub Play(绣球)
  指令
  .......
  End Sub
  Public Overloads Sub Play(老鼠)
  指令
  .......
  End Sub
  Public Overloads Sub Play(鱼)
  指令
  .......
  End Sub
  End Class

这就是「程序成员多重定义」了。Cat 类别含有三种Play()之定义﹐其自变量不同而且内部指令亦不相同。于是Play()程序能接受不同之自变量﹐并执行不同之指令﹐使得Play()具弹性了。请看个程序──

'ex01.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-------------------------------------------------------
Class Example
  Public Overloads Sub Display()
  MessageBox.Show("****")
  End Sub
  Public Overloads Sub Display(ByVal r As Integer)
  MessageBox.Show(str(r))
  End Sub
  Public Overloads Sub Display(ByVal f As Double)
  MessageBox.Show(str(f + 2))
  End Sub
  Public Overloads Sub Display(ByVal s As String)
  MessageBox.Show(s)
  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 a As New Example()
  a.Display()
  a.Display("Taiwan")
  a.Display(80)
  a.Display(100.5)
  End Sub
End Class

此程序输出如下﹕  ****
Taiwan
80
102.5

  这Example类别比较特殊﹐没有资料成员﹔但含有一个程序成员叫Display() 。而Display()有 4个不同之版本(定义)﹐可任君(计算机)挑选。计算机藉比对自变量来挑选「最相配」之Display()程序。
  例如﹕计算机执行到指令──
  a.Display("Taiwan")

由于自变量── "Taiwan"是字符串﹐其型态应配上String﹐所以计算机挑选并且执行第 4个程序──  Display( ByVal s As String ) 。同理﹐当计算机执行到指令──
  a.Display(100.5)

由于自变量──100.5之型态为Double﹐所以计算机选上并执行第 3个Display()程序── Display(ByVal f As Double )。同一程序名称但有数个不同之定义﹐各有不同之自变量及内部指令﹐此种现象就是「程序的多重定义」。

 

请再看个例子──

'ex02.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'--------------------------------------------------
Class Sum
  Private s As Integer
  Public Overloads Sub Add()
  s = 3 + 5
  End Sub
  Public Overloads Sub Add(ByVal x As Integer)
  s = x + 5
  End Sub
  Public Overloads Sub Add(ByVal x As Integer, ByVal y As Integer)
  s = x + y
  End Sub
  Public Sub Show()
  MessageBox.Show("Sum = " + str(s))
  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 a As New Sum()
  a.Add()
  a.Show()
  a.Add(80)
  a.Show()
  Dim b As New Sum()
  b.Add(100, 27)
  b.Show()
  End Sub
End Class

此程序输出如下﹕  Sum = 8
  Sum = 85
  Sum = 127

当计算机执行到指令──  b.Add( 100, 27 ),由于有两个自变量﹐且型态皆为Integer﹔计算机就选上并执行第三个Add() 程序。此时计算机把100传给x﹐而27传给y。这多重定义之观念﹐也常用于建构者程序上。例如﹕

'ex03.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'---------------------------------------------------
Class Rectangle
  Private height As Integer, Width As Integer
  Public Overloads Sub New()
  height = 0
  width = 0
  End Sub
  Public Overloads Sub New(ByVal k As Integer)
  height = k
  width = k
  End Sub
  Public Overloads Sub New(ByVal h As Integer, ByVal w As Integer)
  height = h
  width = w
  End Sub
  Public Sub ShowArea()
  MessageBox.Show("Area = " + str(height * width))
  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 r1 As New Rectangle()
  Dim r2 As New Rectangle(8)
  Dim r3 As New Rectangle(5, 6)
  r1.ShowArea()
  r2.ShowArea()
  r3.ShowArea()
  End Sub
End Class

此程序输出﹕ Area = 0
  Area = 64
  Area = 30

宣告对象时﹐若未给予自变量值﹐计算机呼叫New()﹔若给一个自变量值── 8﹐就呼叫 New(ByVal k As Integer) ﹔若给二个自变量值──5 及 6﹐则呼叫New(ByVal h As Integer, ByVal w As Integer)。请再看一个例子:

'ex04.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-------------------------------------------------------------------------------------------------
Class Rectangle
  Private height As Integer, Width As Integer
  Public Sub New(ByVal h As Integer, ByVal w As Integer)
  height = h
  width = w
  End Sub
  Public Function Area() As Integer
  Area = height * width
  End Function
  Public Overloads Function CompareWith(ByVal a As Integer) As Integer
  Dim d As Integer
  d = Area() - a
  If d <> 0 Then
  CompareWith = 1
  Else
  CompareWith = 0
  End If
  End Function
  Public Overloads Function CompareWith(ByVal r As Rectangle) As Integer
  Dim d As Integer
  d = Area() - r.Area()
  If d <> 0 Then
  d = 1
  End If
  CompareWith = d
  End Function
  Public Overloads Function CompareWith( ByVal x As Rectangle, ByVal
  y As Rectangle) As Integer
  Dim d As Integer
  d = x.Area() - y.Area()
  If d <> 0 Then
  d = 1
  End If
  CompareWith = d
  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 r1 As New Rectangle(10, 50)
  Dim r2 As New Rectangle(20, 25)
  If r1.CompareWith(400) = 0 Then
  MessageBox.Show("EQUAL")
  Else
  MessageBox.Show("NOT EQUAL")
  End If
  If r1.CompareWith(r2) = 0 Then
  MessageBox.Show("EQUAL")
  Else
  MessageBox.Show("NOT EQUAL")
  End If
  If r1.CompareWith(r1, r2) = 0 Then
  MessageBox.Show("EQUAL")
  Else
  MessageBox.Show("NOT EQUAL")
  End If
  End Sub
End Class

此程序输出﹕ NOT EQUAL
  EQUAL
  EQUAL

如此﹐CompareWith()程序就有三种用途了﹔如果您想增加其它用途﹐可尽情地再定义它。r1.CompareWith(400)呼叫第1个CompareWith(),比比看r1面积是否大于400;r1.ComapreWith(r2) 呼叫第2个CompareWith(),比比看r1面积是否大于r2的面积;r1.ComapreWith(r1, r2) 比比看r1面积是否大于r2的面积。如果没有使用多重定义方法,这三个程序名称不能相同。例如﹕上述程序可改写为──

'ex05.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-------------------------------------------------------------------------------------------
Class Rectangle
  Private height As Integer, Width As Integer
  Public Sub New(ByVal h As Integer, ByVal w As Integer)
  height = h
  width = w
  End Sub
  Public Function Area() As Integer
  Area = height * width
  End Function
  Public Function CompareWithInteger(ByVal a As Integer) As Integer
  Dim d As Integer
  d = Area() - a
  If d <> 0 Then
  d = 1
  End If
  CompareWithInteger = d
  End Function
  Public Function CompareWithRectangle(ByVal r As Rectangle) As Integer
  Dim d As Integer
  d = Area() - r.Area()
  If d <> 0 Then
  d = 1
  End If
  CompareWithRectangle = d
  End Function
  Public Function CompareTwoRectangle( ByVal x As Rectangle, ByVal
  y As Rectangle) As Integer
  Dim d As Integer
  d = x.Area() - y.Area()
  If d <> 0 Then
  d = 1
  End If
  CompareTwoRectangle = d
  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 r1 As New Rectangle(10, 50)
  Dim r2 As New Rectangle(20, 25)
  If r1.CompareWithInteger(400) = 0 Then
  MessageBox.Show("ggg EQUAL")
  Else
  MessageBox.Show("NOT EQUAL")
  End If
  If r1.CompareWithRectangle(r2) = 0 Then
  MessageBox.Show("EQUAL")
  Else
  MessageBox.Show("NOT EQUAL")
  End If
  If r1.CompareTwoRectangle(r1, r2) = 0 Then
  MessageBox.Show("EQUAL")
  Else
  MessageBox.Show("NOT EQUAL")
  End If
  End Sub
End Class

此程序输出﹕
NOT EQUAL
EQUAL
EQUAL

由于各程序名称不相同,您就得记忆各程序之名字﹐徒增记忆负担而且易于犯错﹐并不合乎人们生活习惯。因之﹐VB的多重定义观念﹐能增加程序之弹性及亲切感。
  程序多重定义情形并不限于单一类别之内,也可以发生于父子类别之间。例如:

'ex06.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'------------------------------------------------------------------------------------------
Public Class Person
  Private name As String
  Private age As Integer
  Public Sub New()
  End Sub
  Public Sub SetValue(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 Sub Display()
  Messagebox.Show("Name: " + name + "  Age: " + str(age))
  End Sub
End Class

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

Public Class Student
  Inherits Person
 
  Private student_number As Integer
  Public Overloads Sub SetValue( ByVal na As String, ByVal a As Integer, ByVal
  no As Integer)
  SetValue(na, a)
  student_number = no
  End Sub
  Public Sub pr()
  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 Person()
  x.SetValue("Alvin", 32)
  Dim y As New Student()
  y.SetValue("Tom", 36, 11138)
  x.Display()
  y.pr()
  End Sub
End Class

Teacher类别从Person继承了SetValue() ──
  SetValue(ByVal na As String, ByVal a As Integer)

自己又重复定义一个新的SetValue()程序──
  SetValue(ByVal na As String, ByVal a As Integer, ByVal no As Integer)

共有两个SetValue()可用。指令x.SetValue("Alvin", 32)呼叫第1个SetValue();指令y.SetValue("Tom", 36, 11138)呼叫第1个SetValue()。
  兹在扩充一个子类别如下:
 
'ex07.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'------------------------------------------------------------------------------------------
Public Class Person
  Private name As String
  Private age As Integer
  Public Sub New()
  End Sub
  Public Sub SetValue(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 Overloads Sub SetValue( ByVal na As String, ByVal a As Integer, ByVal
  sa As Decimal)
  SetValue(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 Overloads Sub SetValue( ByVal na As String, ByVal a As Integer, ByVal
  no As Integer)
  SetValue(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 Person()
  x.SetValue("Alvin", 32)
  Dim y As New Student()
  y.SetValue("Tom", 36, 11138)
  x.Display()
  y.Display()
  End Sub
End Class

此程序输出﹕
  Name: Alvin  Age: 32
  Name: Tom  Age: 36
  StudNo: 11138

  此时﹐Student 类别含有两个SetValue()程序,一个是从Person类别继承而来,另一个是自行定义的。如果上述Form1_Click()内的指令更改如下:


  Dim y As New Student()
  y.SetValue("Tom", 36, 5000.25)  'Error!
  y.Display()

虽然SetValue("Tom", 36, 5000.25)合乎Teacher的SetValue()程序的参数,但是Student并非Person的子类别,没有继承Student的SetValue(),所以错了。n


 


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