CSDN.NET 
2004-8-11 
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我们已经学会了简单工厂模式(Simple Factory Pattern)，工厂（factory）思想贯穿于整个面向对象编程（OOP）以及其他一些设计模式的始终。如：生成器(Builder)模式。其间，一个单一的类扮演类似交通警察的角色，决定哪一个单一层次上的子类将被实例化。 工厂方法模式（Factory Method pattern）是对工厂（factory）思想进行了巧妙的延伸，它使得将超类的实例化延迟到它的每一个子类。这个模式没有具体的指出延迟到哪一个子类，而是定义一个抽象类创建对象，让其子类决定创建哪一个对象。 下面是一个简单的例子，在一个游泳比赛中如何确定游泳运动员的泳道。按照运动员的成绩排列决赛事的分组，速度越快所分的小组的决赛的次序越靠后，反之，速度越慢就越先比赛，而且在每一个小组中成绩越好、速度越快的选手也就越靠近中间的泳道。这被称为straight seeding。 当游泳运动员在锦标赛比赛过程中，他们通常要游两次。 通过在预赛中相互竞争，前12名或者16名游泳运动员将继续在决赛中继续彼此竞争。 为了预赛工作更公平， 采用circle seeded，这样使得速度最快的3名选手分别处于最迅速3个小组的中心泳道。在剩下的选手中再选出速度最好的3名选手，等等。 我们要实现这个选拔模式并且使用工厂方法。 首先，设计抽象事件类： Public Class Events Protected numLanes As Integer Protected swmmers As Swimmers '----- Public Sub New(ByVal Filename As String, ByVal lanes As Integer) MyBase.New() Dim s As String Dim sw As Swimmer Dim fl As vbFile fl = New vbFile(Filename) '打开一个文本文件 fl.OpenForRead() numLanes = lanes '保存泳道数量信息 swmmers = New Swimmers '读取游泳选手信息 s = fl.readLine While Not fl.fEOF sw = New Swimmer(s) '建立对象 swmmers.Add(sw) 'add to list s = fl.readLine End While fl.closeFile() End Sub '----- Public Function getSwimmers() As ArrayList getSwimmers = swmmers End Function '----- Public Overridable Function isPrelim() As Boolean End Function '----- Public Overridable Function isFinal() As Boolean End Function '----- Public Overridable Function isTimedFinal() As Boolean End Function '----- Public Overridable Function getSeeding() As Seeding End Function End Class 因为所有的派生类都要从文本文件读取数据，所以，我们把Events类作为基类。其中所定义的方法均为虚方法，可以通过继承Events类来实现具体的类（PrelimEvent类、TimedFinalEvent类），这两个类之间唯一的不同就是返回的选拔的类别不同。我们也定义了一个包含以下方法的抽象选拔类： Public MustInherit Class Seeding Protected numLanes As Integer Protected laneOrder As ArrayList Protected numHeats As Integer Private asw() As Swimmer Protected sw As Swimmers '----- Public Function getSeeding() As Swimmers getSeeding = sw End Function '----- Public Function getHeat() As Integer End Function '----- Public Function getCount() As Integer getCount = sw.Count End Function '----- Public MustOverride Sub seed() '----- Public Function getSwimmers() As ArrayList getSwimmers = sw End Function '----- Public Function getHeats() As Integer Return numHeats End Function '----- Public Function odd(ByVal n As Integer) As Boolean odd = (n \ 2) * 2 < > n End Function '----- Public Function calcLaneOrder(ByVal lns As Integer) As ArrayList numLanes = lns Dim lanes(numLanes) As Integer Dim i As Integer Dim mid, incr, ln As Integer mid = (numLanes \ 2) If (odd(numLanes)) Then mid = mid + 1 End If incr = 1 ln = mid For i = 0 To numLanes - 1 lanes(i) = ln ln = mid + incr incr = -incr If (incr > 0) Then incr = incr + 1 End If Next i laneOrder = New ArrayList For i = 0 To numLanes - 1 laneOrder.Add(lanes(i)) Next i calcLaneOrder = laneOrder End Function Public Sub New(ByVal swmrs As Swimmers, ByVal lanes As Integer) MyBase.New() sw = swmrs numLanes = lanes End Sub '------------------- Public Function sort(ByVal sw As Swimmers) As Swimmers Dim i, j, max As Integer Dim tmp As Swimmer Try max = sw.Count Dim asw(max) As Swimmer For i = 0 To max - 1 asw(i) = CType(sw.Item(i), Swimmer) Next i For i = 0 To max - 1 For j = i To max - 1 If asw(i).getTime > asw(j).getTime Then tmp = asw(i) asw(j) = asw(i) asw(i) = tmp End If Next j Next i sw = New Swimmers For i = 0 To max - 1 sw.Add(asw(i)) Next i sort = sw Catch e As Exception Console.WriteLine("Caught " + i.ToString + " " + j.ToString + " " + max.ToString + " " + e.ToString()) Console.WriteLine(e.StackTrace) End Try End Function End Class | |
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