VB.Net中文教程(13) Whole-Part关系

Class Car
    Class Engine
        Public model As String
        Public Sub New(ByVal mdl As String)
            model = mdl
        End Sub
    End Class
   
    Private e As Engine
    Private dr As Driver
    Public Sub New()
        e = New Engine("Honda")
    End Sub
    Public Sub assignTo(ByVal d As Driver)
        dr = d
    End Sub
    Public Sub Show()
        MessageBox.Show("Engine: " + e.model)
        dr.Show()
    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 civic As New Car()
        Dim d1 As New Driver("Wang")
        Dim d2 As New Driver("Kao")
        civic.assignTo(d1)
        civic.Show()
        civic.assignTo(d2)
        civic.Show()
    End Sub
End Class

此程序输出﹕
               Model: Honda
               Driver: Wang
               Model: Honda
               Model" Kao

Car之对象诞生后﹐也诞生Engine之对象e ﹔同时立即指定司机﹐如下指令﹕
     
        Dim civic As New Car()
        Dim d1 As New Driver("Wang")
           .....
        civic.assignTo(d1)
           .....

日常生活中的常见情况﹕汽车对象诞生时﹐不须立即指定司机对象。例如﹐汽车出厂时或闲置时并无司机﹐且汽车经常更换司机。此情形下﹐应先诞生civic对象和d1对象，如下：

此时，未立即指定司机﹔而必要时才以assignTo()程序指定司机。例如，将d1指定给civic对象﹐就令civic内之参考变量dr指向d1 对象，如下：

    上述程序里，d1、d2及civic 对象之间﹐谁先诞生并无关紧要﹐各独立存在。指令──civic.assignTo(d1) 将d1司机指定给civic 对象﹔另一指令──civic.assignTo(d2)表示﹕改由d2担任civic 之司机。
    此Car 类别以参考变量e来指向Engine之对象。现在﹐兹拿上节的Person类别做为例子﹐如果某人(Person 之对象）结婚了﹐就有配偶﹔反之尚未结婚时﹐则无配偶。此时﹐应将Person类别之定义修改如下﹐以表达这种「配偶」关系﹕

''''ex04.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
''''----------------------------------------------------
Class Person
    Private p_name As String
    Private p_age As Integer
    Private p_spouse As Person
   
    Public Sub New(ByVal na As String, ByVal a As Integer)
        p_name = na
        p_age = a
    End Sub
    Public Sub spouse(ByVal sp As Person)
        p_spouse = sp
        sp.p_spouse = Me
    End Sub
    Public Sub Show()
        Messagebox.Show(Me.p_name + ", " + str(Me.p_age)
                        + ",  sp: " + Me.p_spouse.p_name)
    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("David", 32)
        Dim y As New Person("Hellen", 24)
        x.spouse(y)
        x.Show()
        y.Show()
    End Sub
End Class

此程序输出﹕
David, 32, sp: Hellen
Hellen, 24, sp: David

     资料成员p_spouse指向配偶﹐而配偶亦为Person之对象。所以p_spouse之型态应为Person。一个人刚诞生时并无配偶﹐到结婚时才有配偶﹐所以藉spouce()来建立配偶关系。对象x 与y 结婚之后﹐互为对方之配偶。所以x.p_spouse指向y ﹐而y.p_spouse则指向x 。此时﹐x 和y 之内容如下﹕

 
   
于是这Person类别表达了「婚姻」关系。

4. 集合／成员关系

     集合意谓着「团体」(Group) ﹐由其成员(Member)组成的群体。例如﹐学校里的社团内有团员﹔公司的销售部含有推销人员。这团体并不像汽车实际包含着司机﹐而只是其成员之集合而已。这情形﹐统称为「集合／成员」(Collection-members)关系。
    有趣的是﹕在企业活动中﹐人们规划的方案﹐含许多小方案﹔则大方案是由小方案所组成。例如﹐东北亚旅行团的行程表包括在日本的观光行程、在韩国的观光行程和在香港的观光行程。这总行程表图标如下﹕

    总行程是次行程（或称段行程）之集合﹐这是「集合／成员」关系。

此外﹐棒球队是由经理、教练和球员组成﹕订单中含若干产品项目﹐皆为集合／成员关系。实际写程序时﹐不需明确划分「包含者／内容」和「集合／成员」两种关系。其原因是﹕集合与成员之间亦可互为独立﹐不具「生死与共」之亲蜜关系﹔例如﹐「香港观光行程」是独立存在的﹐它既可含于东北亚总行程中﹐又可含于东南亚旅行团的总行程中。因之﹐「集合／成员」关系是一种特殊的「组合」(Composition) 结构。
    兹拿上节Person类别做为例子﹐如果Person之对象会加入Club（俱乐部）成为俱乐部的会员﹐则Club与Person之关系为「集合／成员」关系。兹定义Club类别如下﹕

''''ex05.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
Imports System.Collections
''''----------------------------------------------------
Class Person
    Private p_name As String
    Private p_age As Integer
   
    Public Sub New(ByVal na As String, ByVal a As Integer)
        p_name = na
        p_age = a
    End Sub
    Public Sub display()
        Messagebox.Show(Me.p_name + ", " + str(Me.p_age))
    End Sub
End Class

Class Club
    Private c_name As String
    Private pa As ArrayList
   
    Public Sub New(ByVal na As String)
        c_name = na
        pa = New ArrayList()
    End Sub
    Public Sub join(ByVal p As Person)
        pa.Add(p)
    End Sub
    Public Sub display()
        Messagebox.Show("Club: " + Me.c_name + "  has member:")
        Dim p As Person
        For Each p In pa
            p.display()
        Next
    End Sub
En

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