《VB2008从入门到精通(PDF格式英文版)》第119章

deferred　execution　code。　The　advantage　of　deferred　execution　is　that　the　code　can　contain
a　state　whenever　it　is　executed。　
Some　Things　for　You　to　Do　
The　following　are　some　things　for　you　to　do　to　start　applying　your　budding　knowledge　of　soft
ware　engineering　to　improving　the　code　base。　
1。　Collection　classes　before　Visual　Basic　2005　allowed　you　to　mix　types。　With　Visual　Basic
2005　and　later；　the generics　classes　do　not　allow　you　to　mix　types。　Provide　a
solution　where　you　could　mix　types　with　Visual　Basic　2005　and　later　collections。　
2。　Create　a　list　that　contains　the　numbers　1　to　20。　Remove　the　numbers；　15；　10；　and　3　to　7。　
3。　Create　a　list　with　an　object　that　is　defined　as　follows：　
Class　MyType
Public　Value　as　String　
End　Class　
4。　Add　ten　elements　to　the　list；　and　then　sort　the　list　alphabetically　from　A　to　Z。　Hint：　look
at　the　method　Sort（）　and　implement　a　custom　Iparer（Of　）。　As　part　of　this　exercise；
you　need　to　investigate　and　figure　out　how　to　use Iparer（Of　）。　My　suggestion　is　to
search　the　MSDN　and　Code　Project　web　sites。　
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C H A P T E R 1 0　
■　■　■　
Learning　About　Persistence　
Your　programs　will　probably　need　to　read　and/or　write　data　to　some　type　of　storage　device。
That　storage　device　might　be　a　hard　disk；　USB　drive；　or　even　the　network。　The　key　concept　is
that　you　are　taking　information　from　memory　and　transferring　it　to　some　other　location。　Later；
you　will　retrieve　that　information　and　use　it　to　execute　some　task。
Taking　data　from　memory　and　transferring　it　to　another　place　is　referred　to　as　persistence。
Most　examples　of　persistence　involve　creating　an　object；　and　then　saving　that　object　via　a　file
to　the　hard　disk。　However；　reading　and　writing　an　object　is　not　just　saving　data　to　the　hard　disk；
even　though　that　is　often　the　result。　Reading　and　writing　data　to　the　hard　disk　is　about　reading
and　writing　to　data　streams。　This　chapter　focuses　on　the　process　of　reading　and　writing　data
to　streams。　
This　chapter’s　example　is　a　set　of　applications　for　a　lottery…prediction　system。　You’ll　see
how　streams　are　generic　concepts　that　can　apply　to　files；　the　console；　or　even　the　network。　
Throughout　this　book；　the　examples　use　console　applications　to　test　some　code；　and　this
chapter’s　example　also　includes　console　applications。　Although　Visual　Basic　is　typically　used　as
a　GUI　development　tool；　the　point　of　using　the　console　is　to　help　you　understand　the　Visual
Basic　programming　language。　One　of　the　biggest　criticisms　of　Visual　Basic　developers　has　been
that　they　can’t　develop　like　real　developers。　This　book　is　intended　for　real　developers　who　happen
to　use　Visual　Basic。　
Organizing　the　Lottery…Prediction　System　
Let’s　say　we　want　to　predict　the　next　set　of　lottery　numbers。　We　have　a　program　that　saves　the
numbers　drawn；　and　each　week；　we　run　a　program　that　retrieves　the　drawn　numbers　and　predicts
the　next　set　of　numbers。　Many　will　argue　that　lottery　numbers　are　random　and　thus　cannot　be
predicted。　But　that　doesn’t　mean　that　we　can’t　write　a　program　to　generate　the　probabilities；
and　that　usually　entails　knowing　which　numbers　have　been　drawn　previously。　
The　lottery…prediction　example　involves　three　applications：　TextProcessor；　which　is　used
to　read　a　messed…up　lottery　number　file　that　will　be　cleaned　up；　Binary2Text；　which　converts　a
binary　stream　into　text；　and　Text2Binary；　which　converts　a　text　stream　into　binary。　Five　projects
are　defined　for　these　applications：　
o Binary2Text：　A　console　program　that　is　used　to　convert　a　binary　lottery　ticket　stream　into
a　text　stream。　
o LottoLibrary：　A　class　library　that　contains　the　definition　of　the　Ticket　type　that　repre
sents　a　lottery　ticket　in　memory。　
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250 CH　AP　T　E　R 1　0 ■ L　E　A　R　N　I　N　G A　B　OU　T P　E　R　S　IS　TE　N　CE　
o ReaderWriter：　A　class　library　that　contains　the　infrastructure　code　for　processing　streams
and　mand…line　arguments。　
o Text2Binary：　A　console　program　that　is　used　to　convert　a　text　lottery　ticket　stream　into　a
binary　stream。　
o TextProcessor：　A　console　application　that　will　read　and　write　a　text　file。　This　application
will　bee　a　prototype　example　of　how　to　write　a　console…based　application。　It
contains　a　reference　to　the　ReaderWriter　class　library。　
Piping　Data　Using　a　Console　
Console　applications　are　not　very　interactive；　for　the　most　part；　they　are　keyboard…based　appli
cations。　The　main　advantage　of　console　applications　is　their　ability　to　dynamically　string　data
stream　manipulations　together；　a　process　called　piping。
For　the　lottery…prediction　example；　TextProcessor　is　a　console　application　that　will　be　fed
data　by　a　pipe　and　generate　data　using　a　pipe；　as　illustrated　Figure　10…1。　A　file　feeds　a　pipe；　which
feeds　the　console　application　that　manipulates　the　data；　which　then　feeds　an　outgoing　pipe
that　could　be　used　to　feed　another　console　application。　
Figure　10…1。　Pipeline　approach　to　processing　
TextProcessor　will　read　a　file　of　lottery　numbers；　clean　them　up　（for　example；　by　removing
empty　lines　of　text）；　and　remove　any　duplicates。　The　console　program　wil