Class and Interface Example
Before beginning this example make sure you have completed the Introduction to Classes and Introduction to Interfaces tutorials because I will not be giving an in depth explanation of how classes and interfaces work.
This example is just to give you a little more practice using classes and interfaces. The example will use a Bank Account interface that will be used in a Savings Account Class and Checking Account Class.
Open up Visual Studio .NET and create a new Console Application called Bank.
I will keep this simple so it will just have a Balance property, Debit function, Credit sub, and just because I need an example of a write once property it will have an Account Number. Again changing the Class to Interface.
We will just say AccountNumber has to be greater than zero. Balance is read only because it can only be changed through Debit and Credit. In Debit and Credit the Amount must be greater than zero. In a Savings Account you cannot take out more money than there is in your account. Since debit may not work lets declare it as a function that will return True if it works or False if it doesn't.
Public Interface IBankAccount
Property AccountNumber() As Integer
ReadOnly Property Balance() As Double
Sub Credit(ByVal Amount As Double)
Function Debit(ByVal Amount As Double) As Boolean
End Interface
AccountNumber will be zero when the object is first created. Write once properties aren't as common anymore since you can just have them supply the value inside the constructor. So when the object is created the AccountNumber is invalid but this isn't going to be used anywhere so it doesn't really matter. If AccountNumber is still zero that means they haven't changed it yet so we will allow them to change it.
Private mintNumber As Integer
Public Property AccountNumber() As Integer Implements IBankAccount.AccountNumber
Get
Return mintNumber
End Get
Set(ByVal Value As Integer)
If mintNumber = 0 And Value > 0 Then
mintNumber = Value
End If
End Set
End Property
It's up to you if you want to throw an exception if AccountNumber is invalid or has already been set. Balance will simply return the balance variable and Credit simply adds to the balance as long as the amount is positive.
Private mdblBalance As Double
Public ReadOnly Property Balance() As Double Implements IBankAccount.Balance
Get
Return mdblBalance
End Get
End Property
Public Sub Credit(ByVal Amount As Double) Implements IBankAccount.Credit
If Amount > 0 Then
mdblBalance += Amount
End If
End Sub
The Credit function works a little different. As I said in a savings account you cannot take out more money then there is in your account so we must determine if they can take out the money and also return whether the debit went through.
Public Function Debit(ByVal Amount As Double) As Boolean Implements IBankAccount.Debit
If Amount > mdblBalance Then
Return False
Else
mdblBalance -= Amount
Return True
End If
End Function
I would also like to add two constructors: one with no parameters and the other with one parameter which will be the starting balance of the account. The first constructor will simply set the balance to zero and the second will set the balance to the number they supply as long as it's not negative.
#Region "Constructor(s)"
Public Sub New()
mdblBalance = 0
End Sub
Public Sub New(ByVal StartingBalance As Double)
If StartingBalance >= 0 Then
mdblBalance = StartingBalance
Else
Throw New ArgumentException("Balance cannot be negative")
End If
End Sub
#End Region
Q: Why don't we just do Me.Balance = StartingBalance like we did in interface tutorial for the shapes? A: Balance is read only and therefore has no Set so you can't use it to validate the StartingBalance. That means you must do it yourself inside the constructor. I decided to throw an error if StartingBalance is invalid.
Now lets do the CheckingAccount class. This class is almost exactly like the SavingsAccount class but there is no need to check the balance because it can be negative. Add a new class and call it CheckingAccount. You can simply copy the code from SavingsAccount and make the slight modifications to it.
Public Class CheckingAccount
Implements IBankAccount
Private mdblBalance As Double
Private mintNumber As Integer
#Region "Constructor(s)"
Public Sub New()
mdblBalance = 0
End Sub
Public Sub New(ByVal StartingBalance As Double)
mdblBalance = StartingBalance
End Sub
#End Region
Public Property AccountNumber() As Integer Implements IBankAccount.AccountNumber
Get
Return mintNumber
End Get
Set(ByVal Value As Integer)
If mintNumber = 0 And Value > 0 Then
mintNumber = Value
End If
End Set
End Property
Public ReadOnly Property Balance() As Double Implements IBankAccount.Balance
Get
Return mdblBalance
End Get
End Property
Public Sub Credit(ByVal Amount As Double) Implements IBankAccount.Credit
If Amount > 0 Then
mdblBalance += Amount
End If
End Sub
Public Function Debit(ByVal Amount As Double) As Boolean _
Implements IBankAccount.Debit
mdblBalance -= Amount
Return True
End Function
End Class
You can now continue to the Class & Interface Example - Executable where I will make a simple menu driven console application that uses these to classes.
Author Information:
http://www.programmers-corner.com