此程序输出：
Order777's cost = 198.75

3. 递归式Whole-part关系

Whole-part关系内含别的Whole-part关系﹐且允许有多层次的Whole-part关系﹐通称为递归式的Whole-part关系。在自然界中常见这种关系﹐例如﹐树叶是树的一部分﹐但树叶又是个整体﹐其内含着叶脉、叶绿素等「部分」对象。

图6、自然界的多层次Whole-part关系

在企业界﹐最典型的例子是「对象结构表」（bill of material简称BOM）﹐如下﹕

图7、企业物料表(BOM)的Whole-part关系

乍看之下，这些结构似乎很复杂﹐但从这些图形中﹐可看出这些对象可依其角色而分为两类﹕

1. Leaf对象。如上图里的「白色」类别之对象﹐它们不具有Whole 之角色﹐只具有part之角色。这通称为「基本组件」(primitive component) 。

2. Composite 对象。如上图中的「灰色」类别之对象﹐它们具有Whole之角色﹐也可能具有part之角色。这通称为「复合组件」(composite component) 。

因之﹐只需定义两个类别──Leaf及Composite 类别即行。

4. 以VB落实Composite样式

上述递归Whole-part关系是很常见的﹐是软件设计师惯用的手艺。因之﹐在Gamma 的"Design Patterns" 一书〔注1 〕中﹐也将之收录为重要的「样式」(Pattern) 之1。 该书所画的样式结构图如下图：

图8、Composite样式

这样式建议我们应定义Add() 、Remove()和GetChild()三个基本的可再定义的(overridable) 程序﹐以及其它的程序。专家们把这表示法视为样式﹐就意谓着﹕这是专家们所认为最理想的表达方式。现在﹐实际以依循这个样式来表达上述BOM 结构，必须定义下述类别：
u Part类别 ----- 对应到Component
u PiecePart类别 ----- 对应到Leaf
u AssemblyPart类别 ----- 对应到Composite

再将之落实为VB程序，如下：
'ex03.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
Imports System.Collections
'----------------------------------------------------
Interface IPart
Sub Add(ByVal p As IPart)
Function GetChild(ByVal n As Integer) As IPart
Function Cost() As Double
Function Name() As String
End Interface

Class Part
Private pname As String
Protected Sub New(ByVal na As String)
pname = na
End Sub
Protected Function getName() As String
getName = pname
End Function
End Class

Class PiecePart
Implements IPart
Inherits Part
Private pcost As Double

Public Sub New(ByVal na As String, ByVal c As Double)
MyBase.New(na)
pcost = c
End Sub
Public Sub Add(ByVal p As IPart) Implements IPart.Add
MessageBox.Show("Parts do not have Subparts")
End Sub
Public Function SubPart(ByVal n As Integer) As IPart Implements IPart.GetChild
SubPart = Nothing
End Function
Public Function Cost() As Double Implements IPart.Cost
Cost = pcost
End Function
Public Function Name() As String Implements IPart.Name
Name = MyBase.getName()
End Function
End Class

Class AssemblyPart
Implements IPart
Inherits Part

Private children As ArrayList

Public Sub New(ByVal na As String)
MyBase.New(na)
children = New ArrayList()
End Sub
Public Sub Add(ByVal p As IPart) Implements IPart.Add
children.Add(p)
End Sub
Public Function SubPart(ByVal n As Integer) As IPart Implements IPart.GetChild
Dim obj As Object
obj = children.Item(n)
SubPart = CType(obj, IPart)
End Function
Public Function Cost() As Double Implements IPart.Cost
Dim sum As Double
Dim ps As IPart
sum = 0
For Each ps In children
sum = sum + ps.Cost()
Next
Cost = sum
End Function
Public Function Name() As String Implements IPart.Name
Name = MyBase.getName()
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 aly1, aly2, p1, p2 As IPart
Dim ps As IPart

(编辑:aniston)