VB Classes - 2
ISYS 573
Creating an Array of ObjectsDim emps(2) As empDim i As IntegerFor i = 0 To emps.GetUpperBound(0) emps(i) = New emp()Nextemps(0).Eid = "e1"emps(0).Ename = "peter"emps(0).salary = 5000
Implementing a 1:M Relationship With Object Array
Public did As StringPublic dname As StringPublic emps(2) As Employee
Public eid As StringPublic ename As StringPublic salary As Double
Class Department
Class Employee
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)
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
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.
Using Object Array and Nested ClassPublic 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 SubEnd 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.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
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”.
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.
Iterating Through a CollectionDim Customer as clsCustomerDim Indx as LongFor Indx = 1 to Customers.Count
set Customer = Customers.Item(Indx)… class operations …
Next Indx
For Each Customer In Customers
class operations
Next Customer
Implementing a 1:M Relationship With Class Hierarchy
Public did As StringPublic dname As StringPublic emps As New Collection
Public eid As StringPublic ename As StringPublic salary As Currency
Class Department
Class Employee
Collection and Nested Class ExamplePublic 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
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
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”)
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.
Collection Class Example
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 ClassEnd Class
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
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
Public Class dep
Public depID As Integer
Public depName As String
End 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
Code using collection class
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.
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
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
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
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.
Inheritance ExamplePublic 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
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.
Overriding a MethodPublic 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
Overriding a PropertyPublic 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 PropertyEnd ClassPublic 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 PropertyEnd Class
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.
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.
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
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.
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 PropertyEnd ClassPublic Class employee Inherits Person Overrides ReadOnly Property age() As Integer Get age = CInt(DateDiff(DateInterval.Year, BirthDate, Now)) End Get End PropertyEnd 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.
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.
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.
Test ScopePublic 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.
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.
Abstract Class ExamplePublic 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
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.
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.
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