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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