Lesson 25: Mastering Object-Oriented Programming in VB2022

Public Class Person
    ' Private fields (encapsulation)
    Private _firstName As String
    Private _lastName As String
    Private _birthDate As Date

    ' Public properties
    Public Property FirstName As String
        Get
            Return _firstName
        End Get
        Set(value As String)
            If Not String.IsNullOrWhiteSpace(value) Then
                _firstName = value
            Else
                Throw New ArgumentException("First name cannot be empty")
            End If
        End Set
    End Property

    Public Property LastName As String
        Get
            Return _lastName
        End Get
        Set(value As String)
            If Not String.IsNullOrWhiteSpace(value) Then
                _lastName = value
            Else
                Throw New ArgumentException("Last name cannot be empty")
            End If
        End Set
    End Property

    Public ReadOnly Property FullName As String
        Get
            Return $"{FirstName} {LastName}"
        End Get
    End Property

    Public Property BirthDate As Date
        Get
            Return _birthDate
        End Get
        Set(value As Date)
            If value <= Date.Today Then
                _birthDate = value
            Else
                Throw New ArgumentException("Birth date cannot be in the future")
            End If
        End Set
    End Property

    Public ReadOnly Property Age As Integer
        Get
            Dim today = Date.Today
            Dim age = today.Year - BirthDate.Year
            If BirthDate.Date > today.AddYears(-age) Then age -= 1
            Return age
        End Get
    End Property

    ' Constructor
    Public Sub New(firstName As String, lastName As String, birthDate As Date)
        Me.FirstName = firstName
        Me.LastName = lastName
        Me.BirthDate = birthDate
    End Sub

    ' Method to display information
    Public Overridable Sub DisplayInfo()
        Console.WriteLine($"Name: {FullName}")
        Console.WriteLine($"Birth Date: {BirthDate.ToShortDateString()}")
        Console.WriteLine($"Age: {Age}")
    End Sub
End Class

Public Class Student
    Inherits Person

    ' Additional properties specific to Student
    Public Property StudentId As String
    Public Property Major As String
    Public Property GPA As Double

    ' Constructor with base class parameters
    Public Sub New(firstName As String, lastName As String, 
                  birthDate As Date, studentId As String, 
                  major As String, gpa As Double)
        ' Call base class constructor
        MyBase.New(firstName, lastName, birthDate)
        Me.StudentId = studentId
        Me.Major = major
        Me.GPA = gpa
    End Sub

    ' Override DisplayInfo method
    Public Overrides Sub DisplayInfo()
        ' Call base class implementation
        MyBase.DisplayInfo()
        Console.WriteLine($"Student ID: {StudentId}")
        Console.WriteLine($"Major: {Major}")
        Console.WriteLine($"GPA: {GPA.ToString("F2")}")
    End Sub

    ' Additional method specific to Student
    Public Function IsHonorsStudent() As Boolean
        Return GPA >= 3.5
    End Function
End Class

' Define an interface
Public Interface IPayable
    Function CalculatePayment() As Decimal
    Sub ProcessPayment()
End Interface

' Employee class implements IPayable
Public Class Employee
    Inherits Person
    Implements IPayable

    Public Property EmployeeId As String
    Public Property HourlyRate As Decimal
    Public Property HoursWorked As Double

    Public Sub New(firstName As String, lastName As String, 
                  birthDate As Date, employeeId As String, 
                  hourlyRate As Decimal)
        MyBase.New(firstName, lastName, birthDate)
        Me.EmployeeId = employeeId
        Me.HourlyRate = hourlyRate
    End Sub

    ' Implement IPayable interface
    Public Function CalculatePayment() As Decimal Implements IPayable.CalculatePayment
        Return HourlyRate * CDec(HoursWorked)
    End Function

    Public Sub ProcessPayment() Implements IPayable.ProcessPayment
        Dim payment = CalculatePayment()
        Console.WriteLine($"Processing payment of {payment:C} for {FullName}")
        ' Actual payment processing logic would go here
    End Sub
End Class

' Vendor class also implements IPayable
Public Class Vendor
    Implements IPayable

    Public Property VendorName As String
    Public Property InvoiceAmount As Decimal

    Public Sub New(vendorName As String, invoiceAmount As Decimal)
        Me.VendorName = vendorName
        Me.InvoiceAmount = invoiceAmount
    End Sub

    ' Implement IPayable interface
    Public Function CalculatePayment() As Decimal Implements IPayable.CalculatePayment
        Return InvoiceAmount
    End Function

    Public Sub ProcessPayment() Implements IPayable.ProcessPayment
        Console.WriteLine($"Processing vendor payment of {InvoiceAmount:C} to {VendorName}")
    End Sub
End Class