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. 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

6. Purposes to Have Nested Classes • They are useful for organizing your classes in groups of related classes, and help solve name ambiguity. • Animal.Mouse, Peripheral.Mouse • We can encapsulate one or more auxiliary classes inside the class that uses them and to avoid making them visible to other parts of the application. In this case, inner class should be declared as Private.

7. Using Object Array and Nested Class Public Class Dept Public did As String Public dname As String Public emps(2) As deptEmp Public Sub New() Dim i As Integer For i = 0 To emps.GetUpperBound(0) emps(i) = New deptEmp() Next End Sub Public Class deptEmp Public eid As String Public ename As String End Class Public Sub addemp(ByVal id As String, ByVal name As String) Static index As Integer If index > 2 Then MessageBox.Show("out of bound") Else emps(index).eid = id emps(index).ename = name index += 1 End If End Sub End Class

8. Dim test As New Dept() Private Sub FormTest_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load test.did = "d1" test.dname = "MIS" test.addemp("e1", "peter") test.addemp("e2", "paul") test.addemp("e3", "mary") test.addemp("e4", "Linda") Dim i As Integer For i = 0 To test.emps.GetUpperBound(0) MessageBox.Show(test.emps(i).eid & test.emps(i).ename) Next End Sub

9. Collections • Collections are used to store lists of objects. • More flexible than array: • No need to declare the number of objects in a collection, no need to ReDim. • Objects can be added, deleted at any position. • Object can be retrieved from a collection by a key. • A collection’s name usually end with a “s”.

10. Using Collections • Define a collection: • Ex. Private Customers as New Collection • Methods: • ADD: Add object to a collection • Dim Customer as New clsCustomer • Customers.Add(Customer) • Add an object with a key: • Customers.Add(Customer, Customer.CID) • Item: Retrieve an object from a collection with a position index (base 1) or with a key. • Set Customer = Customers.Item(1) • Set Customer = Customers.Item(“C101”) • Count: Return the number of objects in a collection. • Remove: Delete an object with a position index or key.

11. Iterating Through a Collection Dim Customer as clsCustomer Dim Indx as Long For Indx = 1 to Customers.Count set Customer = Customers.Item(Indx) … class operations … Next Indx For Each Customer In Customers class operations Next Customer

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

13. Collection and Nested Class Example Public Class Dept Public did As String Public dname As String Public emps As New Collection() Public Class deptEmp Public eid As String Public ename As String End Class Public Sub addemp(ByVal id As String, ByVal name As String) Dim e As Dept.deptEmp e.eid = id e.ename = name emps.Add(e) End Sub End Class

14. Dim test As New Dept() Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load test.did = "d1" test.dname = "MIS" test.addemp("e1", "peter") test.addemp("e2", "paul") Dim demp As Dept.deptEmp For Each demp In test.emps MessageBox.Show(demp.eid & demp.ename) Next

15. Problem with Using Collection to Model the Entity on the Many Side of the Relationship • Collection 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”)

16. 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,Remove, … etc.

17. Collection Class Example

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

19. Code Using the Collection Class Dim test As New Dept() Private Sub FormTest_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load test.did = "D1" test.dname = "MIS" test.emps.add("e1", "peter") test.emps.add("e2", "paul") test.emps.add("e3", "mary") test.emps.add("e4", "linda") Dim i As Integer For i = 1 To test.emps.count MessageBox.Show(test.emps.items(i).eid & test.emps.items(i).ename) Next End Sub

20. 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 Collection() 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 Collection Get items = dcol End Get End Property Public ReadOnly Property count() Get count = dcol.Count End Get End Property End Class End Class

21. 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

22. 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.

23. Code to Create the Listbox Dim fileNumber As Integer Dim did As String Dim dname As String Private Sub FormTest_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load fileNumber = FreeFile() FileOpen(fileNumber, "c:\dept.dat", OpenMode.Input) Do While Not EOF(fileNumber) Input(fileNumber, did) Input(fileNumber, dname) ListBox1.Items.Add(did) Loop FileClose(fileNumber) End Sub

24. Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged Dim selDep As New Dept() fileNumber = FreeFile() FileOpen(fileNumber, "c:\dept.dat", OpenMode.Input) Do While Not EOF(fileNumber) Input(fileNumber, did) Input(fileNumber, dname) If did = ListBox1.SelectedItem Then selDep.did = did selDep.dname = dname Dim fileNumber2 As Integer Dim eid As String Dim ename As String Dim empdid As String fileNumber2 = FreeFile() FileOpen(fileNumber2, "c:\emp.dat", OpenMode.Input) Do While Not EOF(fileNumber2) Input(fileNumber2, eid) Input(fileNumber2, ename) Input(fileNumber2, empdid) If empdid = ListBox1.SelectedItem Then selDep.emps.add(eid, ename) End If Loop FileClose(fileNumber2) Exit Do End If Loop

25. FileClose(fileNumber) TextBox1.Text = selDep.dname Dim result As String Dim i As Integer For i = 1 To selDep.emps.count result = result & selDep.emps.items(i).eid & selDep.emps.items(i).ename & vbCrLf Next TextBox2.Text = result TextBox3.Text = selDep.emps.count.ToString End Sub

26. 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. • Inherited classes should always have an “is a” relationship with the base class.

27. 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

28. 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.

29. 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

30. 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 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

31. 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.

32. 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.

33. Store the Overridden Property in the Base Class Property Public Overrides Property salary() As Double Get Return MyBase.salary End Get Set(ByVal Value As Double) If Value > 5000 Then MyBase.salary = 5000 Else MyBase.salary = Value End If End Set End Property

34. MyClass • The MyClass keyword ensures that a method in a base class always uses the property and methods in that class as opposed to their overridden version in the inherited class.

35. Public Class Person Public SSN As String Public Name As String Public BirthDate As Date Public Overridable ReadOnly Property age() As Integer Get age = CInt(DateDiff(DateInterval.Year, BirthDate, Now)) If Month(Now) < Month(BirthDate) Or (Month(Now) = Month(BirthDate) And Microsoft.VisualBasic. Day(Now) < Microsoft.VisualBasic. Day(birdate)) Then age = age - 1 End If End Get End Property Public ReadOnly Property CanVote() As Boolean Get Return (age >= 18) End Get End Property End Class Public Class employee Inherits Person Overrides ReadOnly Property age() As Integer Get age = CInt(DateDiff(DateInterval.Year, BirthDate, Now)) End Get End Property End Class Note: For an employee, the CanVote property will use the overridden Age in the Employee class to determine its value. To correct it, use MyClass.Age>=18 in the base class.

36. Public Variable or Public Property • To make a property visible outside the class we should use property procedure instead of public variable because we can always change the internal implementation of a property procedure without any impact on derived classes.

37. 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.

38. Test Scope Public Class ClassScope Public publicProperty As Integer Private privateProperty As Integer Protected protectedProperty As Integer Friend friendProperty As Integer End Class Public Class derivedClassScope Inherits ClassScope Public Property test() Get test = MyBase.protectedProperty End Get Set(ByVal Value) MyBase.protectedProperty = Value End Set End Property End Class Note: Test in a form.

39. 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.

40. 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

41. 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.

42. 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.

43. 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