Visual Basic 2005 ?? Visual Basic .NET ??? ?????????? ?? ????? ???? ????

????? ?? ???? ????

??????????

• Visual Basic 2005 ?? Visual Basic .NET ????????
• ????????-??????? ?????????

????? ?? ???? ????

??? ??? ????? ????????? ?????

1. Microsoft Visual Studio 2005 ?? Microsoft Visual Studio .NET ?? ??????? ????, ?? ??? ??? Visual Basic ????? ????????? ????????? ??? ?????UseInheritance?? ???? ??????? ???????? ???
2. ????? ????????????????? ??,????? ??????.
3. ??????? ???? ??? ?????????? ????? ???, ??????Account.VB????????????? ??? ????? ????,?????.
   
   ???:Visual Studio 2005, ??? ????? ????add?? ?????????.
4. Visual Studio .NET displays the file Account.vb.
5. ????????? ???????

????? ?? ???? ????

Create an abstract base class

1. ?????MustInheritmodifier to the Account class, to make Account an abstract class:

   Public MustInherit Class Account
   
   End Class
   					

????? ?? ???? ????

Write code for the base class

1. Add two fields to the Account class:

   Private Name As String       ' Only accessible in base class
   Protected Balance As Double  ' Accessible in base class and derived class

2. Add a constructor to initialize these fields:

   Public Sub New(ByVal Nm As String, ByVal Bal As Double)
      Name = Nm
      Balance = Bal
   End Sub 

3. Add the following methods to the class. TheOverridablekeyword means these methods can be overridden in derived classes:

   Public Overridable Sub Credit(ByVal Amount As Double)
      Balance += Amount
   End Sub
   
   Public Overridable Sub Debit(ByVal Amount As Double)
      Balance -= Amount
   End Sub
   
   Public Overridable Sub Display()
      Console.WriteLine("Name=" & Name & ", " & "Balance=" & Balance)
   End Sub

4. Add the following method to the class. Because this method is not marked asOverridable, it cannot be overridden in derived classes. This method provides the capability to change the name of the account holder.

   Public Sub ChangeName(ByVal newName As String)
      Name = newName
   End Sub

5. Add the following method to the class. TheMustOverridekeyword means this method must be overridden in derived classes:

   Public MustOverride Function CalculateBankCharge() As Double

????? ?? ???? ????

Create a derived class

1. ??????????????? ??,????? ??????.
2. ??????? ???? ??? ?????????? ????? ???, ??????SavingsAccount.vb????????????? ??? ????? ????,?????.
3. Visual Studio .NET displays the file SavingsAccount.vb.
4. Change the SavingsAccount class definition as follows, so that SavingsAccount inherits from Account (note that theInheritskeyword must appear on a new line):

   Public Class SavingsAccount 
      Inherits Account
   
   End Class

????? ?? ???? ????

Write code for the derived class

1. Add a field to the SavingsAccount class:

   Private MinBalance As Double  ' If the balance drops below MinBalance, 
                                 ' the bank will charge a fee on the account

2. ????? ???? ???? ??? ?? ?? ???? ??? ??????? ???? ?? ??? ???????????, ?? constructor ??????:

   Public Sub New(ByVal Nm  As String, _
                  ByVal Bal As Double, _
                  ByVal Min As Double)
      MyBase.New(Nm, Bal)        ' Call base-class constructor first
      MinBalance = Min           ' Then initialize fields in this class
   End Sub

3. SavingsAccount ???? ?? ??? ????? ??????? ??? ??????? ?? ??????? ?? ???????Overridable???? ???? ?? ??????? ?? ?? ????:

   Public Overrides Sub Debit(Amount As Double)
      If Amount <= Balance Then  ' Use balance, inherited from base class
         MyBase.Debit(Amount)    ' Call Debit, inherited from base class
      End If
   End Sub
   
   Public Overrides Sub Display()
      MyBase.Display()           ' Call Display, inherited from base class
      Console.WriteLine("$5 charge if balance goes below $" & MinBalance)
   End Sub

4. ??? ??????? ???? ????MustOverride???? ???? ?? ???????? SavingsAccount ???? ?? ??? ????? ???? ??????:

   Public Overrides Function CalculateBankCharge() As Double
      If Balance < MinBalance Then
         Return 5.0
      Else
         Return 0.0
      End If
   End Function

????? ?? ???? ????

???????? ???? ?? ??? ???

1. ??? ????? ????? ??? Module1.vb ?? ??? ??? ????????? ?????
2. ?????????????? ??????, ?? SavingsAccount ???????? ?? ?? ??? ??? ?????:

   Dim sa As SavingsAccount = New SavingsAccount("Freda Smith", 100.00, 25)
   sa.Display()

3. ??? ???? ?? ??? ????? ??? ????????????????? ??? SavingsAccount ?? ???? ??:

   sa.Credit(100)
   sa.Debit(180)
   sa.ChangeName("Freda Jones")
   sa.Display()
   Console.WriteLine("Bank charge: $" & sa.CalculateBankCharge())
   					

4. ????????? ??????
5. ?????? ???????? ??,??????? ?? ???? ??????? ????????????? ?????? ????????? ????? ?? ????? ??????? ????????? ???? ??:

   Name=Freda Smith, balance=100
   $5 charge if balance goes below $25
   Name=Freda Jones, balance=20
   $5 charge if balance goes below $25
   Bank charge: $5
   						

6. ????????? ?? ???: ?????, ????? ?? ??? ????? ?? ????? ????? ??? breakpoint ?? ??????? ?? ??? ????????????? ?? ?? ??? ????????????????? ???????? ??.. ???????? ??? ??? ??? ?? ????????? ?? ????? ??? ?? ???? ?????? ??? ???? ????

????? ?? ???? ????