VB Classes - 2

1. VB Classes - 2 ISYS 573

2. Creating an Array of Objects Dim emps(2) As emp Dim i As Integer For i = 0 To emps.GetUpperBound(0) emps(i) = New emp() Next emps(0).Eid = "e1" emps(0).Ename = "peter" emps(0).salary = 5000

3. Implementing a 1:M Relationship With Object Array Class Department Public did As String Public dname As String Public emps(2) As Employee Class Employee Public eid As String Public ename As String Public salary As Double

4. Code Example Dim tempDep As New department() tempDep.did = "D1" tempDep.dname = "Accounting" tempDep.emps(0) = New emp() tempDep.emps(0).Eid = "E1" tempDep.emps(0).Ename = "Peter" MessageBox.Show(tempDep.emps(0).Ename)

5. Implementing a 1:M Relationship With ArrayList Class Department Public did As String Public dname As String Public emps As New ArrayList Class Employee Public eid As String Public ename As String Public salary As Currency

6. Example Public Class Dept Public did As String Public dname As String Public emps As New ArrayList Public Sub addemp(ByVal id As String, ByVal name As String) Dim e As New Emp e.eid = id e.ename = name emps.Add(e) End Sub End Class Public Class Emp Public eid As String Public ename As String End Class

7. Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim test As New Dept test.did = "d1" test.dname = "MIS" test.addemp("e1", "peter") test.addemp("e2", "paul") MessageBox.Show(test.emps.Item(0).eid) MessageBox.Show(test.emps.Item(1).eid)

8. Problem with Using ArrayList to Model the Entity on the Many Side of the Relationship • ArrayList can store different types of objects. • Because the property is a collection, user may use collection’s Add method to add a object of different type. • Test.Emps.Add(“Other Type”)

9. Collection Class • A collection class holds references for a series of objects created from the same class. • Create a hidden Private collection to hold data. • Create methods to simulate collection’s Add, Count, Items,RemoveAt, … etc. • Example:

10. Public Class Dept Public did As String Public dname As String Public emps As New depEmps End Class Public Class Emp Public eid As String Public ename As String End Class Public Class depEmps Private hiddenList As New ArrayList Public Sub add(ByVal id As String, ByVal name As String) Dim e As New Emp e.eid = id e.ename = name hiddenList.Add(e) End Sub Public ReadOnly Property items() As ArrayList Get items = hiddenList End Get End Property Public ReadOnly Property count() Get count = hiddenList.Count End Get End Property End Class

11. Code Using the Collection Class Dim test As New Dept test.did = "d1" test.dname = "MIS" test.emps.add("e1", "peter") test.emps.add("e2", "paul") MessageBox.Show(test.emps.items(0).eid) MessageBox.Show(test.emps.items(1).eid)

12. What If We Only Allow Two Employees in Each Dept? Public Sub add(ByVal id As String, ByVal name As String) Dim e As New Emp e.eid = id e.ename = name Static eCount As Integer If eCOunt > 1 Then MessageBox.Show("too many") Else eCount += 1 hiddenList.Add(e) End If End Sub

13. How to implement the Remove method? • How to implement the RemoveAt method?

14. Nested Classes • VB .Net lets you nest class definitions: • Class Outer • … • Class Inner • … • End Class • End Class • The inner class can be declared as: • Dim obj As New Outer.Inner

15. Public Class Emp Public Eid As String Public Ename As String Public salary As Double Public dependents As New deps() Public Class deps Private dcol As New arraylist Public Sub add(ByVal did As Integer, ByVal dname As String) Dim d As New dep() d.depID = did d.depName = dname dcol.Add(d) End Sub Public ReadOnly Property items() As ArrayList Get items = dcol End Get End Property Public ReadOnly Property count() Get count = dcol.Count End Get End Property End Class End Class

16. Public Class dep Public depID As Integer Public depName As String End Class Code using collection class Dim test1 As New Emp() Dim test2 As dep test1.dependents.add(1, "peter") test1.dependents.add(2, "paul") Dim i As Integer For i = 1 To test1.dependents.count MessageBox.Show(test1.dependents.items(i).depname) Next For Each test2 In test1.dependents.items MessageBox.Show(test2.depID.ToString & test2.depName) Next

17. Store Objects in Storage • Comma delimited file • Database • Serialization

18. Classes and Files • Two files with 1:M relationship • Dept.dat, and Emp.dat • Create a Dept class to model the relationship. • Create a form that: • Display department Ids in a listbox. • Display selected department info and its employees in textboxes.

19. Serializing Classes • Add a Serializable attribute: • <Serializable()> Public Class Dept

20. <Serializable()> Public Class Dept Public did As String Public dname As String Public emps As New depEmps End Class <Serializable()> Public Class Emp Public eid As String Public ename As String End Class

21. Imports System.IO Imports System.Runtime.Serialization.Formatters.Binary Private Sub Form9_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim test As New Dept test.did = "d1" test.dname = "MIS" test.emps.add("e1", "peter") test.emps.add("e2", "paul") Dim fs As New FileStream("c:\testSerializing.txt", FileMode.Create) Dim bf As New BinaryFormatter bf.Serialize(fs, test) fs.Close() End Sub Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim fs As New FileStream("c:\testSerializing.txt", FileMode.Open) Dim testS As New Dept Dim bf As New BinaryFormatter testS = CType(bf.Deserialize(fs), Dept) TextBox1.Text = testS.did TextBox2.Text = testS.dname End Sub

22. Inheritance • The process in which a new class can be based on an existing class, and will inherit that class’s interface and behaviors. The original class is known as the base class, super class, or parent class. The inherited class is called a subclass, a derived class, or a child class.

23. Inheritance Example Public Class Emp Public Eid As String Public Ename As String Public salary As Double Public Function tax() As Double tax = salary * 0.1 End Function End Class Public Class secretary Inherits Emp Public WordsPerMinute As Integer End Class

24. Overriding • When a property or method in the base class is not adequate for a derived class, we can override the base class property or method by writing one with the same name in the derived class. • The property or method in the base class must be declared with the Overridable keyword. • The overridden property or method must be declared with the Overrides keyword. • Note: Keywords Overridable and Overrides apply only to property procedure (not properties declared by public variables) or method.

25. Overriding a Method Public Class Emp Public Eid As String Public Ename As String Public salary As Double Public Overridable Function tax() As Double tax = salary * 0.1 End Function End Class Public Class secretary Inherits Emp Public WordsPerMinute As Integer Public Overrides Function tax() As Double If salary > 3000 Then tax = salary * 0.1 Else tax = salary * 0.05 End If End Function End Class

26. Overriding a Property Public Class Emp Public Eid As String Public Ename As String Private hiddenSal As Double Public Overridable Property salary() As Double Get salary = hiddenSal End Get Set(ByVal Value As Double) hiddenSal = Value End Set End Property End Class

27. Public Class secretary Inherits Emp Private sal As Double Public WordsPerMinute As Integer Public Overrides Property salary() As Double Get salary = sal End Get Set(ByVal Value As Double) If Value > 5000 Then sal = 5000 Else sal = Value End If End Set End Property End Class

28. MyBase • The MyBase keyword refers to the base class. It is useful when you want to reference a field, property, or method of the base class.

29. Public Overridable Function tax() As Double tax = salary * 0.1 End Function Public Class secretary Inherits Emp Public WordsPerMinute As Integer Public Overrides Function tax() As Double If salary > 3000 Then tax = salary * 0.1 Else tax = salary * 0.05 End If End Function End Class Public Overrides Function tax() As Double If salary > 3000 Then tax = MyBase.tax Else tax = salary * 0.05 End If End Function Note: With MyBase, we can reuse the code in the base class.

30. The Scope of Class Properties and Methods • Public: Available within its own class and to client code and subclasses. No restriction on access in the current or other projects. • Private: Available only within its own class, not accessible from a derived class. • Protected: Available only within its own class and derived subclasses, not available to client code. • Friend: Available within its own class and to client code and subclasses, but only within the current project.

31. Abstract Classes (Virtual Classes) • To prevent users from using your class as is and instead force them to inherit from it, you can create an abstract class. • An abstract class cannot be instantiated, but is designed to be used only as a base class. An abstract class is declared with the MustInherit keyword: • Public MustInherit Class Person • Abstract classes may have methods that are declared with the MustOverride keyword. Such methods are not implemented in the abstract class but must be implemented in any derived classes.

32. Abstract Class Example Public MustInherit Class clsEmp Public Eid As String Public Ename As String Public salary As Double MustOverride Function tax() As Double End Class Public Class clsEmpSecretary Inherits clsEmp Public Overrides Function tax() As Double If salary > 5000 Then tax = salary * 0.1 Else tax = salary * 0.1 End If End Function End Class

33. NonInheritable Classes (Sealed Classes) • A class declared with the NotInheritable keyword can be instantiated but cannot be subclassed: • Public NotInheritable Class Emp • Use NotInheritable when you want others to be able to use your class but not base their own classes on it.

34. Base Class and Derived Class Constructors • It is possible for both a base class and a derived class to have constructors. When an instance of the derived class is created, the base class constructor is called first, and then the derived class constructor is called.

35. Comparing Object Variables with the Is Operator • Multiple object variables can reference the same object. To determine whether two object variables reference the same object, use the Is operator, not =. • Dim emp1 as new emp() • Dim emp2 as emp • Emp2 = emp1 • If emp2 Is emp1 Then • Msgbox(“Same object”) • End if

36. Exception • Exceptions signal errors or unexpected events that occur while an application is running. • An error handler is a section of code that intercepts and responds to exceptions.

37. Structured Error Handling Try result = Val(TextBox1.Text) / Val(TextBox2.Text) TextBox3.Text = result.ToString Catch except As InvalidCastException MessageBox.Show(except.Message) Catch except As DivideByZeroException MessageBox.Show(except.Message) Catch except As Exception 'Handle everything else MessageBox.Show(except.Message) Finally MessageBox.Show("I get exdecuted, no matter what") End Try

38. User-Defined Application Exceptions • System.ApplicationException • Throw • Throw New ApplicationException("Test exception") • Use Try block to catch the excaption • Try • Statements • Catch err ApplicationException • MessageBox.Show(err.Message) • End Try

39. Application Exception Example Public Class emp Public SSN As String Public Ename As String Public DateHired As Date Private hiddenJobCode As Long Public Property JobCode() Set(ByVal Value) If Value < 1 Or Value > 4 Then Throw New ApplicationException("Invalide JobCode") Else hiddenJobCode = Value End If End Set Get JobCode = hiddenJobCode End Get End Property End Class

40. Catch Application Exception Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim myEmp As New emp() Try myEmp.JobCode = TextBox1.Text MessageBox.Show(myEmp.JobCode) Catch err As ApplicationException MessageBox.Show(err.Message) TextBox1.Focus() End Try End Sub

41. User-Defined Exception Class Public Class JobCodeException Inherits System.ApplicationException Sub New(ByVal strMessage As String) MyBase.New(strMessage) End Sub End Class

42. Using User-Defined Exception in Class Private hiddenJobCode As Long Public Property JobCode() Set(ByVal Value) If Value < 1 Or Value > 4 Then Throw New JobCodeException("Invalide JobCode") Else hiddenJobCode = Value End If End Set Get JobCode = hiddenJobCode End Get

43. Using User-Defined Exception in Program Dim myEmp As New Emp() Try myEmp.Eid = TextBox1.Text myEmp.Ename = TextBox2.Text myEmp.salary = CDbl(TextBox3.Text) myEmp.JobCode = TextBox4.Text Catch err As JobCodeException MessageBox.Show(err.Message) TextBox4.Focus() TextBox4.SelectAll() End Try