VB Classes - 2 ISYS 573. Creating an Array of Objects Dim emps(2) As emp Dim i As Integer For i = 0...
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Transcript of VB Classes - 2 ISYS 573. Creating an Array of Objects Dim emps(2) As emp Dim i As Integer For i = 0...
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)
Implementing a 1:M Relationship With ArrayList
Public did As StringPublic dname As StringPublic emps As New ArrayList
Public eid As StringPublic ename As StringPublic salary As Currency
Class Department
Class Employee
ExamplePublic 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 SubEnd ClassPublic Class Emp Public eid As String Public ename As StringEnd Class
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)
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”)
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:
Public Class Dept Public did As String Public dname As String Public emps As New depEmpsEnd ClassPublic Class Emp Public eid As String Public ename As StringEnd ClassPublic 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 PropertyEnd Class
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)
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
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
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
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.
<Serializable()> Public Class Dept Public did As String Public dname As String Public emps As New depEmpsEnd Class<Serializable()> Public Class Emp Public eid As String Public ename As StringEnd Class
Imports System.IOImports System.Runtime.Serialization.Formatters.BinaryPrivate 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
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.
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 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 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.
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.
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
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.
Structured Error HandlingTry
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
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
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
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
User-Defined Exception Class
Public Class JobCodeException
Inherits System.ApplicationException
Sub New(ByVal strMessage As String)
MyBase.New(strMessage)
End Sub
End Class
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