1 Working with Objects

2 Defining a Class in VB.NET A class is a user-defined data type You can declare it as part of a module, but usually in a separate file (also.vb) Ingredients –Data members (fields) –Constructors –Properties –Methods Access Modifiers –Public: accessible by all –Private: accessible by the class itself only –Protected: accessible by the class itself or its sub-classes –Friend: accessible by all in the current assembly

3 ClassExample1 (Horse.vb) Public Class Horse Public m_name As String‘ data members (fields) Public m_color As String Friend m_height As Single = 1.5 ' in meters Public Sub New()‘ default constructors MyClass.New("", "", 0)‘ MyClass (i.e. this in Java) End Sub Public Sub New(ByVal name As String, ByVal color As String) MyClass.New(name, color, 0) End Sub Private Sub New(ByVal name As String, ByVal color As String, ByVal height As Single) m_name = name m_color = color If 1 <= height And height <= 2.5 Then m_height = height End If End Sub End Class

4 ClassExample1 (Module1.vb) Module Module1 Sub Main() Dim h As Horse = New Horse("Lightning", "Brown") Console.Out.WriteLine("Name is {0}", h.m_name) Console.Out.WriteLine("Color is {0}", h.m_color) Console.Out.WriteLine("Height is {0}", h.m_height) End Sub End Module

5 Properties If data members are public, they can be altered directly (horse.m_name), violates Data Hiding principle (Encapsulation) If private, need accessor methods (set_ & get_), so Properties are introduced Fields and properties are similar, but properties are accessed through its accessor methods Get and Set Normally use private or protected fields to hold the data

6 Defining Properties Public Class Horse Public m_name As String Public m_color As String Private m_height As Single = 1.5 Public Property height() As Single Get‘ Get block Return m_height End Get Set‘ Set block, note the keyword Value If 1 <= Value And Value <= 2.5 Then m_height = Value End If End Set End Property Public Sub New(ByVal name As String, ByVal color As String, ByVal height As Single) m_name = name m_color = color MyClass.height = height‘ MyClass.height Is a Property!! End Sub End Class

7 Using Properties Module Module1 Sub Main() Dim h As Horse = New Horse("Lightning", "Brown“, 2) h.height = 1.9 Console.Out.WriteLine("Name is {0}", h.m_name) Console.Out.WriteLine("Color is {0}", h.m_color) Console.Out.WriteLine("Height is {0}", h.height) End Sub End Module

8 ReadOnly Properties With Get block only Does not allow changes Public Class Person Private m_name As String Private m_dob As Date Private m_balances() As Decimal Public ReadOnly Property name() As String Get Return m_name End Get End Property Public ReadOnly Property dob() As Date Get Return m_dob End Get End Property … End Class

9 WriteOnly Properties With Set block only, rarely used Public Class OutTextStream Public WriteOnly Property outText() As String Set (ByVal Value As String) Console.Out.WriteLine(Value) End Set End Property End Class Module Module1 Sub Main() Dim o As OutTextStream = New OutTextStream() o.outText = “This goes to the console!” End Sub End Module

10 Parameterized Properties Property “Array”, can be multi-dimensional too Public Class Person Private m_name As String Private m_dob As Date Private m_balances() As Decimal … Public Property balances(ByVal index As Integer) As Decimal Get Return m_balances(index) End Get Set m_balances(index) = Value End Set End Property Public ReadOnly Property noOfBalances() As Integer Get Return m_balances.Length End Get End Property Public Sub New(ByVal name As String, ByVal dob As Date) m_name = name m_dob = dob m_balances = New Decimal(5) {} End Sub End Class ‘ Using it Dim p As Person = New person("John G.", #7/4/1960#) p.balances(0) = 2100 p.balances(1) = 10000

11 ClassExample2 Lab 7

12 Methods (1) Behavior, operations that an object can do A function or subroutine defined inside the class body that can be called on an object Can have modifiers Public, Private, Protected, or Friend Public Class Horse Private m_name As String Private m_color As String Private m_height As Single … ' We assume that a horse can jump 75% of its height Public Function Jump() As Single Return CSng(0.75 * m_height) End Function End Class

13 Methods (2) ‘ Calling Method Sub Main() Dim h As Horse = New Horse("Bucefalus", "black", 2) Console.Out.WriteLine("The horse (0) jumps {1} meters!", _ h.name, h.Jump()) End Sub Example ClassExample3

14 Method Overloading (1) To achieve the same task but with different arguments E.g. a Add Function that can be applied to Short, Integer, and Long Must have different argument signatures Public Class Calculator Public Overloads Function Add(ByVal x As Short, ByVal y As Short) As Long Return x + y End Function Public Overloads Function Add(ByVal x As Integer, ByVal y As Integer) As Long Return x + y End Function Public Overloads Function Add(ByVal x As Long, ByVal y As Long) As Long Return x + y End Function End Class

15 Method Overloading (2) Module Module1 Sub Main() Dim result As Long Dim calc As New Calculator() Dim s1 As Short = 3, s2 As Short = 5 result = calc.Add(s1, s2)‘ using Add(Short, Short) Dim i1 As Integer = 3, i2 As Integer = 5 result = calc.Add(i1, i2) ‘ using Add(Integer, Integer) Dim j1 As Long = 3, j2 As Long = 5 result = calc.Add(j1, j2) ‘ using Add(Long, Long) End Sub End Module Example ClassExample4

16 ClassExample5 Account id: String balance: Decimal New(id:String, balance:Decimal) Person name: String accounts: Account() balance: Decimal New(name:String) Deposit(ac:String, amt:Decimal) Withdraw(ac:String, amt:Decimal): Boolean Transfer(acFrom:String, acTo:String, amt:Decimal) GetAccountById(ac:String): Account

17 Shared Class Members Similar concepts of “static” in other OO languages Shared data member Shared Methods

18 Shared Data Members (1) Can be fields or properties Belong to the class, shared by all instances Also known as class variable All instances get the same value Shared property CANNOT access non-shared (instance) fields or properties Accessed through class name, e.g. Product.taxRate Save memory Define constants (no instantiation of class need)

19 Shared Data Members (2) Public Class Product Private m_name As String ' product name (instance variable) Private m_listPrice As Decimal ' the list price (instance variable) Private Shared s_taxRate As Double‘ shared variable Public Shared Property taxRate() As Double‘ shared property Get Return s_taxRate End Get Set s_taxRate = Value End Set End Property Public Sub New(ByVal name As String, ByVal listPrice As Decimal) m_name = name m_listPrice = listPrice End Sub... End Class

20 Shared Methods (1) Actions performed on class level, not object level Do not require the existence of an object (no instantiation need) E.g. some math functions CANNOT access non-shared (instance) members Accessed through class name, e.g. Product.GetUnknownPrice(10)

21 Shared Methods (2) Public Class Product Private m_name As String ' product name Private m_listPrice As Decimal ' the list price Private Shared s_taxRate As Double... Public Sub New(ByVal name As String, ByVal listPrice As Decimal) m_name = name m_listPrice = listPrice End Sub Public Function GetPrice(ByVal discount As Double) As Decimal Dim price As Double price = m_listPrice + m_listPrice * s_taxRate - m_listPrice * discount Return CDec(price) End Function Public Shared Function GetPriceUnknown(ByVal price As Decimal) As Decimal Return CDec(price + price * s_taxRate) End Function End Class

22 ClassExample6