VB.Net中文教程(4) 类别继承(Inheritance)关系

1. 类型之接受 类型之间﹐有些互为独力﹐有些具备出色联系。兹引见类型间罕见的联系──「爷儿俩」联系﹔因为后代常接受双亲之心理或情绪特性﹐以是又称此联系为「接受」(inheritance) 联系。类型间之出色联系﹐把关系的类型构造起来﹐而且构造步调内之东西。若步调内之东西毫无构造﹔表露一片散沙﹐就不是好步调。完备之vb步调﹐必需关心类型间之联系﹐东西是有构造的。 即使 a类型「接受」 b类型﹐则称 a为「子类型」(subclass)﹐也称b 为「父类型」(superclass)﹐亦即 b为 a之父类型﹐a 为 b之子类型。在 c++中﹐父类型又称为「普通类型」(base class)﹐子类型又称为「派生类型」(derived class) 。大概您感触「接受」之看法很生疏﹐不知怎样看出类型间之接受联系。别担忧﹐有个简片面法﹕下列两报告之意旨沟通── (1) a 为 b之子类型。 (2) a 为 b之一种(a kind of) 特出类型。按照报告 (2)能简单找到爷儿俩联系。比方﹕肯尼士(kennex)消费高品德拍子﹐拍子分两种﹕网拍子与羽拍子。此后句子得悉﹕网拍子为一种(a kind of) 拍子﹐羽拍子亦为一种拍子。因之﹐网拍子为拍子之子类型﹐羽拍子亦为拍子之子类型﹐亦即拍子是父类型。以次图标之﹕ 图1、 普通类型与派生类型 即使安排步调来记载拍子之消费景象﹐则步调应设置普通类型──拍子﹐以及两派生类型──网拍子及羽拍子。步调应藉接受联系将三类型构造起来。除去货色（如拍子、公共汽车等）外﹐人也有接受联系。比方﹕书院职员囊括弟子、教授及人员﹐教授又分为现任教授及兼任教授。书院的人事软硬件体例﹐应设置类型联系如次﹕ 图2、 三代接受联系 步调不只要设置类型﹐也要设置其接受联系。2. 设置接受联系 前方各章里﹐已引见怎样设置类型﹔本节将报告您怎样设置类型之接受联系。兹举例证明之﹕步调的安排进程是﹕ step 1. 设置普通类型（父类型）。如﹕ class person ‥‥ end class step 2. 设置派生类型（子类型）。如﹕ class teacher inherits person ‥‥ end class class student inherits person ‥‥ end class inherits字眼之后为父类型称呼。它表白了：teacher 为person之子类型﹐且 student为person之子类型。此刻﹐已领会怎样表白接受联系了﹔但是﹐子类型从父类型接受什么货色呢﹖类型包括「材料」及「步调」。因之﹐子类型接受父类型之材料及步调。此刻﹐请看看怎样接受材料及步调。下述步调设置 person 类型﹐含有 2项材料及 3个步调﹕'ex01.basimports system.componentmodelimports system.drawingimports system.winforms'-----------------------------------------------------------------------------------------class person private name as string private age as integer public sub setvalue(byval na as string, byval a as integer) name = na age = a end sub public function birthyear() as integer birthyear = 2001 - age end function public sub display() messagebox.show("name: " + name + " age: " + str(age)) end subend 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 mike as person mike = new person() mike.setvalue("mike", 45) mike.display() messagebox.show("birthyear: " + str(mike.birthyear())) end subend class此步调输入如次﹕ name: mike age: 45birthyear: 1956 所谓接受材料﹐表白接受材料分子之设置﹐而不是接受材料之值﹐请详加辨别之。类型设置材料分子（含型态及称呼）﹐东西出生后﹐东西内才有材料值。以是「类型接受」乃接受类型之设置﹐不是接受东西之值。也即是说﹕若父类型设置name及 age两个材料分子﹐则子类型天才就具有此两个材料分子﹐以是子类型不需设置它们。所谓接受步调﹐表白子类型天才就具有父类型设置之步调分子。比方﹕person的子类型天才就具备 setvalue()、birthyear()及display()步调。此刻﹐就来设置person之子类型。'ex02.basimports system.componentmodelimports system.drawingimports system.winforms'-----------------------------------------------------------------------------------------public class person private name as string private age as integer public sub setvalue(byval na as string, byval a as integer) name = na age = a end sub public function birthyear() as integer birthyear = 2001 - age end function public sub display() messagebox.show("name: " + name + " age: " + str(age)) end subend classpublic class teacher inherits personend 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 steven as teacher steven = new teacher() steven.setvalue("steven", 35) steven.display() end subend class此步调输入如次﹕ name: steven age: 35 外表上teacher 类型中﹐不决义数据项及步调﹐但究竟上已具有name及 age两个材料分子﹐也具有 setvalue()、birthyear()及display() 三个步调分子。因之﹐steven为teacher类型之东西﹐它能呼唤setvalue()及display()步调。在运用上﹐子类型常常具有本人的数据项及步调﹐使其有别于父类型。比方﹕'ex03.basimports system.componentmodelimports system.drawingimports system.winforms'------------------------------------------------------------------------------------------------------public class person private name as string private age as integer public sub setvalue(byval na as string, byval a as integer) name = na age = a end sub public function birthyear() as integer birthyear = 2001 - age end function public sub display() messagebox.show("name: " + name + " age: " + str(age)) end subend classpublic class teacher inherits person private salary as decimal public sub tr_setvalue( byval na as string, byval a as integer, byval sa as decimal) setvalue(na, a) salary = sa end sub public sub pr() mybase.display() messagebox.show("salary: " + str(salary)) end subend 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 steven as teacher steven = new teacher() steven.tr_setvalue("steven", 35, 35000) steven.pr() end subend class此步调输入如次﹕ name: steven age: 35salary: 35000此刻﹐teacher 类型含有三个材料分子﹕ 1. name ──从person类型接受而来。 2. age ──从person类型接受而来。 3. salary ──本人设置的。其余﹐也含有五个步调分子﹕ 1. setvalue() ──从person接受而来。 2. birthyear()──从person接受而来。 3. display() ──从person接受而来。 4. tr_setvalue()──本人设置的。 5. pr()──本人设置的。 因为setvalue()为 teacher之步调分子﹐以是tr_setvalue()能径直呼唤setvalue()来设定name及age值﹔之后﹐tr_setvalue()本人设定salary之值。同理﹐pr()能径直呼唤 display()来表露name及age之实质﹔之后﹐pr()本人输入salary之值。大概﹐您会问及﹕子类型本人设置之步调﹐能否能与父类型之步调同称呼呢﹖谜底是确定的﹐并且是罕见之写法。即使称呼沟通然而参数不普遍（像参数个数各别或参数型态各别），即是爷儿俩类型之间的步调多重设置(procedure overloading)了，必需写上overloads字眼。比方﹕上述步调十分于──'ex04.basimports system.componentmodelimports system.drawingimports system.winforms'------------------------------------------------------------------------------------------class person private name as string private age as integer public sub new() end sub public sub setvalue(byval na as string, byval a as integer) name = na age = a end sub public function birthday() as integer birthday = 2001 - age end function public sub display() messagebox.show("name: " + name + " age: " + str(age)) end subend classclass teacher inherits person private salary as decimal public overloads sub setvalue( byval na as string, byval a as integer, byval sa as decimal) setvalue(na, a) salary = sa end sub public sub pr() mybase.display() messagebox.show("salary: " + str(salary)) end subend 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 steven as teacher steven = new teacher() steven.setvalue("steven", 35, 35000) steven.pr() end subend class此步调输入如次﹕ name: steven age : 35 salary: 35000此种情景﹐teacher 类型具有两个 setvalue()步调﹐一个由person接受而来﹐一个是本人设置的。同声﹐也具有两个display()步调。此时﹐计划机怎样辨别它们呢﹖计划机按照步调的参数来确定呼唤那一个步调，当您写成 steven.setvalue("steven", 35, 35000)时，此setvalue()是子类型本人的setvalue()。而写成setvalue(na,a)时，则是由person类型接受而来之setvalue()步调。此vb步调已设置如次之类型联系﹕接下来﹐再为person设置一个子类型── student。步调如次﹕'ex05.basimports system.componentmodelimports system.drawingimports system.winforms'------------------------------------------------------------------------------------------public class person private name as string private age as integer public sub new() end sub public sub setvalue(byval na as string, byval a as integer) name = na age = a end sub public function birthday() as integer birthday = 2001 - age end function public sub display() messagebox.show("name: " + name + " age: " + str(age)) end subend classpublic class teacher inherits person private salary as decimal public overloads sub setvalue( byval na as string, byval a as integer, byval sa as decimal) setvalue(na, a) salary = sa end sub public sub pr() mybase.display() messagebox.show("salary: " + str(salary)) end subend classpublic class student inherits person private student_number as integer public overloads sub setvalue( byval na as string, byval a as integer, byval no as integer) setvalue(na, a) student_number = no end sub public sub pr() mybase.display() messagebox.show("studno: " + str(student_number)) end subend 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() x.setvalue("alvin", 32) dim y as new student() y.setvalue("tom", 36, 11138) x.display() y.pr() end subend class此步调输入﹕ name: alvin age: 32 name: tom age: 36 studno: 11138 此时﹐student 类型含有name、age 及student_number三个材料分子。并且具有 setvalue()、person.setvalue() 、display()、pr() 及birthyear()五个步调分子。所以创造了如次之接受联系﹕ x 东西含name及age 两项材料﹐训令──x.setvalue ("alvin", 32)﹐将材料惠存x 东西中。因student接受person﹐以是y 东西内含name、age 及student_number三项材料﹐训令──y.setvalue("tom", 36, 11138) 将材料惠存y 东西中。上述 setvalue()步调之功效是﹕设定东西之初值。可由建构者(constructor) 包办之。以是此student之设置十分于──'ex06.basimports system.componentmodelimports system.drawingimports system.winforms'----------------------------------------------------public class person private name as string private age as integer public sub new() end sub public sub setvalue(byval na as string, byval a as integer) name = na age = a end sub public function birthday() as integer birthday = 2001 - age end function public sub display() messagebox.show("name: " + name + " age: " + str(age)) end subend classpublic class teacher inherits person private salary as decimal public overloads sub setvalue(byval na as string, byval a as integer, byval sa as decimal) setvalue(na, a) salary = sa end sub public sub pr() mybase.display() messagebox.show("salary: " + str(salary)) end subend classpublic class student inherits person private student_number as integer public sub new(byval na as string, byval a as integer, byval no as integer) setvalue(na, a) student_number = no end sub public sub pr() mybase.display() messagebox.show("studno: " + str(student_number)) end subend 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() x.setvalue("alvin", 32) dim y as new student("tom", 36, 11138) x.display() y.pr() end subend class此步调输入﹕ name: alvin age: 32 name: tom age: 36 studno: 11138子类型之建构者呼唤父类型之setvalue()来设定东西初值。请提防看y 东西之出生进程﹕y 出生时﹐建构者── public sub new(byval na as string, byval a as integer, byval no as integer) setvalue(na, a) student_number = no end sub它先呼唤父类型的天然建构者person.new()一道出生y 东西。此时﹐person.new()出生如次﹕接受局部是person.new()出生的。person.new()工作实行了﹐轮到student的建构者自己出生（夸大）如次﹕ 出生结束﹐发端实行student建构者内之训令以设定y 之初值。开始呼唤父类型的setvalue() ﹐所以setvalue()将值惠存y 如次﹕结果﹐实行训令── student_number = no﹐设定student_number数据之初值﹕所以﹐student的建构者出生y 东西﹐也设定初值﹐功成完备。 上述的student建构者呼唤父类型之天然建构者（由于person类型不决义建构者﹐计划机机动爆发天然建构者）来扶助出生东西。即使person类型设置了建构者﹐student建构者便能径直呼唤person建构者了。比方﹕'ex07.basimports system.componentmodelimports system.drawingimports system.winforms'-----------------------------------------------------------------------------------------public class person private name as string private age as integer public sub new(byval na as string, byval a as integer) name = na age = a end sub public function birthday() as integer birthday = 2001 - age end function public sub display() messagebox.show("name: " + name + " age: " + str(age)) end subend classpublic class teacher inherits person private salary as decimal public sub new(byval na as string, byval a as integer, byval sa as decimal) mybase.new(na, a) salary = sa end sub public sub pr() mybase.display() messagebox.show("salary: " + str(salary)) end subend classpublic class student inherits person private student_number as integer public sub new(byval na as string, byval a as integer, byval no as integer) mybase.new(na, a) student_number = no end sub public sub pr() mybase.display() messagebox.show("studno: " + str(student_number)) end subend 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("alvin", 32) dim y as new student("tom", 36, 11138) x.display() y.pr() end subend class此步调输入﹕ name: alvin age: 32 name: tom age: 36 studno: 11138子类型之建构者── public sub new(byval na as string, byval a as integer, byval sa as decimal) ...... end sub先呼唤父类型之建构者── public sub new(byval na as string, byval a as integer) ...... end sub扶助出生东西如次﹕ 接着﹐实行父类型person的建构者内之训令﹐设定初值如次﹕person建构者处事实行了﹐轮到student建构者的处事﹐夸大东西如次﹕出生结束﹐实行student建构者内之训令﹐设定初值﹕所以﹐y 东西出生结束了。 子类型建构者呼唤父类型建构者之形式﹕比方﹐student建构者内的mybase.new(na, a)训令将na及a值传播给person建构者﹐所以person建构者设定name及age之初值。除去呼唤person建构者除外﹐还实行本人的训令──student_number = no ﹐设定student_number之初值。student类型之东西含有三个材料分子﹐个中name及 age是由person类型接受而来﹐就藉person建构者设定其初值。至于student类型本人设置之材料分子student_number﹐就由本人的训令──student_number = no设定其初值。3. 藉「接受」夸大步调 步调之兴盛是循序渐进的﹐软硬件内之类型需随企业之生长而连接夸大。类型夸大常源于﹕(1) 企业生长了﹐爆发了新类型。 比方﹕某计划机公司﹐往日创造巨型及迷你计划机﹐此刻新推出部分计划机。就必需创造新子类型如次﹕ 图3、 步调夸大本领──派生子类型 再如﹐某钱庄往日供给空头支票帐户(checking account)及积聚帐户(saving account)给存户﹐此刻拟减少按期帐户(cd account)。就得新增子类型如次﹕ 这新类型cd_account接受父类型saving_account及account 的材料及步调。比方﹐account 之设置如次﹕'ex08.basimports system.componentmodelimports system.drawingimports system.winforms'-----------------------------------------------------------------------------------class account private shared next_number as integer protected acc_no as integer protected name as string protected balance as decimal shared sub new() next_number = 1000 end sub public sub new(byval na as string) name = na balance = 0 acc_no = next_number next_number = next_number + 1 end sub public sub deposit(byval money as decimal) balance = balance + money end sub public sub withdraw(byval money as decimal) balance = balance - money end sub public sub close() acc_no = -1 end subend classclass saving_account inherits account private interest_rate as double public sub new(byval na as string, byval rate as double) mybase.new(na) me.interest_rate = rate end sub public function comp_interest() as double comp_interest = balance * interest_rate end function public sub display() messagebox.show("acc_no: " + str(mybase.acc_no)) messagebox.show("余额: " + str(me.balance) + " 本钱: " + str(comp_interest())) end subend 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 saving_acc as new saving_account("tom bush", 0.085) dim account as saving_account account = new saving_account("linda lin", 0.008) account.deposit(5000) account.withdraw(2500) account.display() account.close() end subend class此步调输入﹕ acc_no: 1001 余额: 2500 本钱: 20 因saving_account接受account ﹐以是saving_account类型亦具有deposit() 、withdraw()及close() 三个步调。请换个观点﹐从saving_account类型之安排者来看﹐他透过接受联系来「滥用」父类型之步调。「滥用」又称为「再运用」(resue) ﹐能使得子类型大略但却强而有力。就如saving_account类型显得比account 类型大略﹐但它所有却含有deposit() 、withdraw()等6 个步调﹐功效比account 类型宏大很多。对于训令──account.deposit(5000) 而言﹐其有两种意旨﹕ i. saving_account接受account ﹐以是saving_account具备deposit() 步调﹐可将5000惠存account 东西。 ii. saving_account为account 之子类型﹐它能滥用（再运用）父类型之deposit() 步调﹐来将5000惠存account 东西。 这两者是一体的两面﹐接受功效让软硬件师能连接新增子类型﹐连接再运用现有类型之步调﹐来简化子类型之搀杂度。接受使得子类型具有太爷类型之百般功效﹐且再减少新功效﹔再运用使得子类型尽管委派太爷类型来控制处事﹐本人以逸待劳。所以﹐善用接受功效﹐常常心存再运用看法﹐便能安排大略却功效宏大的子类型了。而步调员的功效提高了﹐软硬件之本钱低沉了﹐此乃oop 的理念目的之一。兹再夸大一个子类型cd_account如次﹕'ex09.basimports system.componentmodelimports system.drawingimports system.winforms'--------------------------------------------------------------------------------class account private shared next_number as integer protected acc_no as integer protected name as string protected balance as decimal shared sub new() next_number = 1000 end sub public sub new(byval na as string) name = na balance = 0 acc_no = next_number next_number = next_number + 1 end sub public sub deposit(byval money as decimal) balance = balance + money end sub public sub withdraw(byval money as decimal) balance = balance - money end sub public sub close() acc_no = -1 end subend classclass saving_account inherits account private interest_rate as double public sub new(byval na as string, byval rate as double) mybase.new(na) me.interest_rate = rate end sub public function comp_interest() as double comp_interest = balance * interest_rate end function public sub display() messagebox.show("acc_no: " + str(mybase.acc_no)) messagebox.show("余额: " + str(me.balance) + " 本钱: " + str(comp_interest())) end subend classclass cd_account inherits saving_account public sub new(byval na as string, byval rate as double, byref b as decimal) mybase.new( na, rate + 0.003 ) mybase.deposit(b) end subend 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 a as cd_account a = new cd_account("linda lin", 0.008, 2500) a.display() a.close() end subend classcd_account除去在设置本人的new()除外，皆接受saving_account的分子。请您提防观赏下述步调，看看跟上一个步调有何巧妙分别？'ex10.basimports system.componentmodelimports system.drawingimports system.winforms'--------------------------------------------------------------------------------class account private shared next_number as integer protected acc_no as integer protected name as string protected balance as decimal shared sub new() next_number = 1000 end sub public sub new(byval na as string) name = na balance = 0 acc_no = next_number next_number = next_number + 1 end sub public sub deposit(byval money as decimal) balance = balance + money end sub public sub withdraw(byval money as decimal) balance = balance - money end sub public sub close() acc_no = -1 end subend classclass saving_account inherits account private interest_rate as double public sub new(byval na as string, byval rate as double) mybase.new(na) me.interest_rate = rate end sub public function comp_interest() as double comp_interest = balance * interest_rate end function public sub display() messagebox.show("acc_no: " + str(mybase.acc_no)) messagebox.show("余额: " + str(me.balance) + " 本钱: " + str(comp_interest())) end subend classclass cd_account private acc as saving_account public sub new(byval na as string, byval rate as double, byref b as decimal) acc = new saving_account(na, rate + 0.003) acc.deposit(b) end sub public function comp_interest() as double comp_interest = acc.comp_interest() end function public sub display() acc.display() end sub public sub close() acc.close() end subend 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 a as cd_account a = new cd_account("linda lin", 0.008, 2500) 'a.deposit(5000) 'error here!! a.display() a.close() end subend class(2) 情况变换了﹐必需矫正旧类型。 比方﹕软硬件内含sales 类型如次──'ex11.basimports system.componentmodelimports system.drawingimports system.winforms'----------------------------------------------------------------------------------class sales protected sum as double public sub new() sum = 0 end sub public overridable sub input(byval amount as double) sum = sum + amount end sub public sub display() messagebox.show("the sum is: " + str(sum)) end subend 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 sales() x.input(3000.5) x.input(7999.5) x.input(15000) x.display() end subend class此步调输入如次﹕the sum is: 26000input() 输出出卖金额﹐且将金额加总起来﹐存于 sum变量中。display() 表露出总金额。跟着企业之生长﹐常常对软硬件之功效有新的需要。比方﹕蓄意步调输入平衡出卖额。此时﹐必需革新从来的软硬件﹐其本领有二﹕(i) 改写 sales类型之设置。 此为保守软硬件之保护本领﹐就像一座衡宇﹐已不敷运用﹐所以用推土机推掉﹐重修之。常常这种本领并不佳﹐因为是﹕旧的软硬件有些功效仍旧可用﹐废除重修简直怅然。若不想重修﹐只想去窜改它﹐然而软硬件若很宏大﹐得滥用人工去领会旧软硬件的构造﹐弄通了之后﹐本领加以矫正﹐明显不财经。进一步想﹐于矫正之进程中﹐不经意而变动旧软硬件内仍可用之局部﹐则从来精确之功效﹐相反有遭妨害之虞﹗(ii) 藉接受联系矫正 sales类型。 亦即设置 sales之子类型﹐来矫正及夸大 sales之功效。此时﹐并未变动sales 类型之设置﹐以是不会妨害 sales之功效。既是生存原有的杰出功效﹐又能减少新的功效﹐难道面面俱到呢﹖此「旧干发新枝」之场合即是千年神木得以繁茂且绿意盎然之原因﹐也恰是 oop魅力的泉源。此刻﹐就为 sales设置个子类型叫 sales_new如次﹕'ex12.basimports system.componentmodelimports system.drawingimports system.winforms'------------------------------------------------------------------------------------class sales protected sum as double public sub new() sum = 0 end sub public overridable sub input(byval amount as double) sum = sum + amount end sub public sub display() messagebox.show("the sum is: " + str(sum)) end subend classclass sales_new inherits sales protected number as integer public sub new() number = 0 end sub public overrides sub input(byval amount as double) mybase.input(amount) number = number + 1 end sub public sub display_average() dim avg as double if number > 0 then avg = sum / number messagebox.show("the average is : " + str(avg)) end if end subend 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 sales_new() x.input(3005.5) x.input(5994.5) x.input(15012.6) x.display() x.display_average() end subend class此步调输入﹕ the sum is: 24012.6 the average is: 8004.2 子类型减少了新数据项number﹐从新设置了input()﹐使新input()生存父类型内之input()功效﹐并加上number = number +1之演算。保持了父类型之display()﹐但减少了display_average()步调。依此本领﹐软硬件天然日益繁茂﹐且持久弥新﹗藉接受联系连接夸大乃为oop之要害本领﹐盼您善用之。n