Objects andVB Classes

1. Objects andVB Classes ISYS 350

2. What Is an Object? • Objects are key to understanding object-oriented technology. There are many examples of real-world objects: your dog, your desk, your television set, your bicycle. • Real-world objects share two characteristics: They all have state and behavior. Dogs have state (name, color, breed, hungry) and behavior (barking, wagging tail). Bicycles also have state (current gear, current speed) and behavior (changing gear, applying brakes). • Identifying the state and behavior for real-world objects is the key to model an object.

3. What Is a Class? • In the real world, you'll often find many individual objects all of the same kind. There may be thousands of other bicycles in existence, all of the same make and model. • Each bicycle was built from the same set of blueprints and therefore contains the same components. • In object-oriented terms, we say that your bicycle is an instance of the class of objects known as bicycles. • A class is the blueprint from which individual objects are created.

4. Entities • An entity is a person, place, object, event, or concept in a business environment about which the organization wishes to maintain data. • Person: Employee, Student, patient • Place: Warehouse, Store • Object: Product, Machine. • Event: Registration, Sale, Renewal • Concept: Account, Course • Physical existence: • Customer, student, product, etc. • Conceptual existence: • Bank accounts, sale

5. Entity Type • A collection of entities that share common properties or characteristics. • An entity type represents a collection of entities. • A business environment may involve many entity types. • University: Faculty, Student, Course • Department, Employee, Dependent • Sales person, Customer, Order

6. Adding a Class to a Project • Project/Add Class • *** MyClass is a VB keyword. • Steps: • Adding properties • Declare Public variables in the General Declaration section • Property procedures: Set / Get • Adding methods • Function • Procedure

7. Procedures . Sub procedure: Sub SubName(Arguments) … End Sub • To call a sub procedure SUB1 • CALL SUB1(Argument1, Argument2, …)

8. Call by Reference Call by Value • ByRef • The address of the item is passed. Any changes made to the passing variable are made to the variable itself. • ByVal • Default • Only the variable’s value is passed.

9. Demo: call a procedure; Input argument; output argument Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim mySum As Double Call myProcedure() Call AddTwoNum(InputBox("Enter num1:"), InputBox("Enter num2:")) Call ReturnSumByArgument(InputBox("Enter num1:"), InputBox("Enter num2:"), mySum) MessageBox.Show("The sum is: " + mySum.ToString) End Sub Private Sub myProcedure() MessageBox.Show("You are calling a procedure!") End Sub Private Sub AddTwoNum(ByVal Num1 As Double, ByVal Num2 As Double) Dim Sum As Double Sum = Num1 + Num2 MessageBox.Show("The sum is: " + Sum.ToString) End Sub Private Sub ReturnSumByArgument(ByVal Num1 As Double, ByVal Num2 As Double, ByRef Sum As Double) Sum = Num1 + Num2 End Sub

10. Function • Private Function tax(Bval salary as Double) As Double • tax = salary * 0.1 • End Function • Or • Private Function tax(ByVal salary as Double) As Double • Return salary * 0.1 • End Function

11. Using a Function Dim Salary As Double Salary = InputBox("Enter salary: ") MessageBox.Show("Tax is" + tax(Salary).ToString) Private Function tax(ByVal salary) As Double tax = salary * 0.1 End Function

12. Class Code Example Public Eid As String Public Ename As String Public salary As Double Public Function tax() As Double tax = salary * 0.1 End Function

13. Using a Class • Define a class variable using New • Example: Dim MyEmp As New Emp

14. Creating Property with Property Procedures • Implementing a property with a public variable the property value cannot be validated by the class. • We can create read-only, write-only, or write-once properties with property procedure. • Steps: • Declaring a private class variable to hold the property value. • Writing a property procedure to provide the interface to the property value.

15. Private pvEid As String Private pvEname As String Private pvSalary As Double Public Property eid() As String Get eid = pvEid End Get Set(ByVal Value As String) pvEid = Value End Set End Property Public Property eName() As String Get eName = pvEname End Get Set(ByVal Value As String) pvEname = Value End Set End Property Public Property Salary() As Double Get Salary = pvSalary End Get Set(ByVal Value As Double) pvSalary = Value End Set End Property

16. Property Procedure Code Example Public Class Emp2 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 hiddenJobCode = 1 Else hiddenJobCode = Value End If End Set Get JobCode = hiddenJobCode End Get End Property End Class

17. How the Property Procedure Works? • When the program sets the property, the property procedure is called and the code between the Set and End Set statements is executed. The value assigned to the property is passed in the Value argument and is assigned to the hidden private variable. • When the program reads the property, the property procedure is called and the code between the Get and End Get statements is executed.

18. Implementing a Read-Only Property • Declare the property procedure as ReadOnly with only the Get block. • Ex. Create a YearsEmployed property from the DateHired property: Public ReadOnly Property YearsEmployed() As Long Get YearsEmployed = Now.Year - DateHired.Year End Get End Property • Note: It is similar to a calculated field in database.

19. Implementing a Write-Only Property • Declare the property procedure as WriteOnly with only the Set block. • Ex. Create a PassWord property: Private hiddenPassword as String Public WriteOnly Property Password() As String Set(ByVal Value As String) hiddenPassword=Value End Set End Property

20. Anatomy of a Class Module Class Module Exposed Part Hidden Part Public Variables & Property Procedures Private Variables Public Procedures & Functions Private Procedures & Functions • Private variables and procedures can be created for internal use.

21. Overloading A class may have more than one methods with the same name but a different argument list (with a different number of parameters or with parameters of different data type), different parameter signature.

22. Method Overloading Using the Overloads Keyword Public Overloads Function tax() As Double tax = salary * 0.1 End Function Public Overloads Function tax(ByVal sal As Double) As Double tax = sal * 0.1 End Function

23. 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.

24. Employee Super Class with Three SubClasses All employee subtypes will have emp nbr, name, address, and date-hired Each employee subtype will also have its own attributes

25. 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

26. Overriding • If a method in a base class is not appropriate for a derived class, we can override the base class method by adding a one with the same name to the derived class.

27. Overriding ToString Method Public Class Emp Public SSN As String Public FirstName As String Public LastName As String Public BirthDate As Date Public Overrides Function ToString() As String toString = FirstName & " " & LastName & "'s birthday is " & BirthDate.ToString End Function End Class

28. .Net Framework Class Library Structure • Assembly: • Basic unit of deployment. • Implemented as Dynamic Link Library, DLL. • May contain many Namespace • NameSpace: • Organize a group of related classes. • Class • Object Browser • Ex: System.Windows.Forms

29. Referencing Assemblies and Classes • Each project automatically references essential assemblies. • Project property/References • Or: Solution Explorer/Show All Files • Add additional reference: • Project/Add Reference

30. Constructors • A constructor is a method that runs when a new instance of the class is created. In VB .Net the constructor method is always named Sub New.

31. Constructor Example Public Sub New() Me.eid = "" ename = "" salary = 0.0 End Sub Public Sub New(ByVal empId As String, ByVal empName As String, ByVal empSal As Double) eid = empId ename = empName salary = empSal End Sub Note: Cannot use Overloads with the New.

32. Constructor of the Sub Class Public Sub New(ByVal empId As String, ByVal empName As String, ByVal empSal As Double, ByVal WPM As Integer) MyBase.New(empId, empName, empSal) WordsPerMinute = WPM End Sub Public Sub New() End Sub

33. Constructors and Read-Only Field • A constructor procedure is the only place from inside a class where you can assign a value to read-only fields. • Public ReadOnly currentDate As Date • Public Sub New() • Me.eid = "" • ename = "" • salary = 0.0 • currentDate = Today • End Sub

34. Working with Many Objects with Collections

35. .Net Collection Data Structures • System.Collection • Array • ArrayList • HashTable • Others:SortedList, Stack, Queue

36. ArrayList • More flexible than array: • No need to declare the number of objects in a collection. • Objects can be added, deleted at any position. • Object can be retrieved from a collection by a key. • Can store any types of data and objects.

37. ArrayList • Define an arraylist: • Dim myArrayList As New ArrayList() • Properties:Count, Item, etc. • To retrieve the first member: • myArrayList.Item(0) 0-based index • Methods: • Clear, Add, Insert, Remove, RemoveAt, Contains, IndexOf, etc.

38. ArrayList Demo Dim testArrayList As New ArrayList() Dim Fruits() As String = {"Apple", "orange", "Banana"} Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim f2 As New Form2() testArrayList.Add("David") testArrayList.Add(20) testArrayList.Add(Fruits) testArrayList.Add(f2) TextBox1.Text = testArrayList.Item(0) TextBox2.Text = testArrayList.Item(1).ToString TextBox3.Text = testArrayList.Item(2)(1) TextBox4.Text = testArrayList.Item(3).Age End Sub

39. For Each Loop with ArrayList Dim testArrayList As New ArrayList() Dim f2 As New DataForm2() Dim Fruits() As String = {"Apple", "orange", "Banana"} testArrayList.Add("David") testArrayList.Add(20) testArrayList.Add(Fruits) testArrayList.Add(f2) Dim myObj As Object For Each myObj In testArrayList MessageBox.Show(myObj.GetType.ToString) Next

40. Using ArrayList as a Parallel Array Example: Select an interest rate from listbox and return its value Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged MessageBox.Show(rateList.Item(ListBox1.SelectedIndex)) End Sub Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load rateList.Add(0.05) rateList.Add(0.06) rateList.Add(0.07) rateList.Add(0.08) rateList.Add(0.09) rateList.Add(0.1) End Sub

41. Data Binding with ArrayLists • Arraylists can be used as data source for a control. • Demo: ListBox DataSource property. • Dim myArrayList As New ArrayList() • myArrayList.Add("apple") • myArrayList.Add("banana") • myArrayList.Add("orange") • ListBox1.DataSource = myArrayList

42. Use arraylist to store many employee objects

43. Module Module Module1 Public empArrayList As New ArrayList End Module

44. Data Entry Form Dim newEmp As New emp newEmp.eid = TextBox1.Text newEmp.eName = TextBox2.Text newEmp.Salary = CDbl(TextBox3.Text) empArrayList.Add(newEmp)

45. Update Form

46. Creating ListBox and Display Selected Employee Data Private Sub Form5_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim obj As Object For Each obj In empArrayList ListBox1.Items.Add(obj.eid) Next End Sub Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged TextBox1.Text = empArrayList.Item(ListBox1.SelectedIndex).ename TextBox2.Text = empArrayList.Item(ListBox1.SelectedIndex).Salary.ToString End Sub

47. Update Employee Data Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click If box1Changed Then empArrayList.Item(ListBox1.SelectedIndex).ename = TextBox1.Text End If If box2Changed Then empArrayList.Item(ListBox1.SelectedIndex).salary = CDbl(TextBox2.Text) End If End Sub Dim box1Changed As Boolean = False Dim box2Changed As Boolean = False Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged box1Changed = True End Sub Private Sub TextBox2_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox2.TextChanged box2Changed = True End Sub

48. Binding DataGridView Private Sub Form6_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load DataGridView1.DataSource = empArrayList End Sub