设计模式之工厂方法（FACTORY METHOD））（一）
2024-07-21 02:23:37
供稿：网友
工厂方法
the factory method
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
