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In a true OO language, each object would
have its own properties and methods that you could call
against the object itself such as MyObject.MyMethod().
However, because eVB does not allow you to create instances
of a class as in VB, I had to come up with another way to
simulate the idea behind OO programming. If the object doesn’t
know what its properties are, then who will? The answer
is: the class file. From project to project, if you
are planning on reusing a class you don’t want to have
to add additional files and code to keep track of your objects.
You want to drop one file into your project and have the
functionality of that class immediately available. For each
property of a class, I create a private collection in my
class file which will hold all values for all instances
of the class. So rather than having the object know its
properties, it will simply need to know its “key”
in the classes collection’s. In order to achieve polymorphism
and inheritance, each object will need to know what type
of object it is, and what classes it inherits from. In order
to have our object know both its type and its key, I decided
to make our “object” a string that contains the
necessary information.
The example program
In today’s job market, executives
need to be up to date on the current payroll expenses, and
be able to quickly enact cost cutting measures. So I developed
a “Pocket Salary Cutter” application do
demonstrate the use of objects in eVB. I decided that I
would need an Employee class that would have properties
of First Name, Last Name, and Annual Income.
In addition to the general Employee, we need two specific
kinds of employees: Interns and Managers.
Interns are paid based on an hourly wage, and Managers have
an Annual Bonus in addition to their Salary. Because both
Interns and Managers ARE Employees, we can have the Intern
and Manager classes inherit from our Employee class. Using
the Pocket Salary Cutter application, lets add an employee
to our collection.
Private ccolEmployees As OSIUTIL.Collection
Private ccolInterns As OSIUTIL.Collection
Private ccolManagers As OSIUTIL.Collection
Private Sub cmdAddEmployee_Click()
Dim objNew
If Me.optEmployee.Value = True Then
objNew = Employee.Constructor()
Employee.setFirstName objNew, txtEmployeeFirstName.Text
Employee.setLastName objNew, txtEmployeeLastName.Text
Employee.setAnnualIncome objNew, txtEmployeeAnnualIncome.Text
ccolEmployees.Add (objNew)
.
.
.
End Sub
When the form is loaded, the collections
are created for use throught the form. When the user clicks
on cmdAddNewEmployee the Employee.Constructor
is called.
Option Explicit
'******************************
'* Employee Class is a Base Class
'******************************
Private Const UNIQUE_EMPLOYEE_STRING =
";Employee::"
Private ciUniqueNumber As Integer
Private ccolLastName As OSIUTIL.Collection
Private ccolFirstName As OSIUTIL.Collection
Private ccolAnnualIncome As OSIUTIL.Collection
Private Sub Form_Load()
'create Collections
Set ccolLastName = Factory.CreateObject("OSIUTIL.Collection")
Set ccolFirstName = Factory.CreateObject("OSIUTIL.Collection")
Set ccolAnnualIncome = Factory.CreateObject("OSIUTIL.Collection")
End Sub
Public Function Constructor()
ciUniqueNumber = ciUniqueNumber + 1
'Add another member for the collections
CreateCollectionMember (CStr(ciUniqueNumber))
'return the unique identifier to the caller
Constructor = UNIQUE_EMPLOYEE_STRING & CStr(ciUniqueNumber)
& ";"
End Function
Private Sub CreateCollectionMember(sNewKey)
ccolFirstName.Add "Undefined", CStr(sNewKey)
ccolLastName.Add "Undefined", CStr(sNewKey)
ccolAnnualIncome.Add CDbl(0), CStr(sNewKey)
End Sub
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