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On load of the form, the OSI Utilities
package is used to create the collections that will hold
our 3 properties; First Name, Last Name, and
Annual Income.
The Constructor uses a private number
“ciUniqueNumber” that will serve as each
new instance’s “key” in the collection.
When the Constructor function is called, it increments
ciUniquenumber, and calls CreateCollectionMember
passing in ciUniqueNumber. CreateCollectionMember
adds a new member to all the property collections using
standard values that will later be overwritten such as “Undefined”.
Finally the constructor returns a string that consists of
two parts: the object type, and the key of its properties
in the collections. So at this point MyEmployee is
equal to “;Employee::1;”. If I create another
Employee instance, it would hold the string “;Employee::2;”.
So now we have objNew equal to “;Employee::1;”.
The next line of code assigns a first name to our new object.
Code from Form1
Employee.setFirstName objNew, txtEmployeeFirstName.Text
Code from Employee
Public Function setFirstName(objEmployee,
sFirstName)
Dim iID
If (IsEmployee(objEmployee)) Then
iID = getID(objEmployee)
If Not (iID = -1) Then
ccolFirstName.Remove (CStr(iID))
ccolFirstName.Add sFirstName, CStr(iID)
Else
MsgBox "Employee::setFirstName was passed an object
with an invalid ID."
End If
Else
MsgBox "Employee::setFirstName was passed an invalid
object."
End If
End Function
As you can see, we can’t actually
call methods on our object, because our object is really
just a string. Instead we need to call methods of an objects
class. In this case we want to call the setFirstName
method of an Employee. Employee.setFirstName
takes as parameters an Employee object and the name
to assign to that Employee. The first thing the function
does is to check to make sure that objEmployee is really
an “Employee” by searching the string objEmployee
for the UNIQUE_EMPLOYEE_STRING. The next step is
to get the Key or ID of the specific Employee object
that was passed into the function. Once the Key is
known, the function removes and then adds the updated sFirstName
string with the same Key. The reason I had to remove the
member and then add a new one is because I couldn’t
find a way to edit a collection element using the OSI Utilities
package. All done. Our Employee now has a first name! We
continue, adding a LastName and AnnualIncome
in a similar fashion before finally adding our new object
to a form level collection.
ccolEmployees.Add (objNew)
Now remember that we aren’t actually
adding an object to ccolEmployees. Rather, we are
adding a string that is equal to “;Employee::1;”
and all its properties are stored in collections inside
our Employee class file at a key location of “1”.
Inheritance and Polymorphism
Now lets show off our inherited classes.
We will add a new Intern to our program using the following
code:
objNew = Intern.Constructor()
Intern.setFirstName objNew, txtInternFirstName.Text
Intern.setLastName objNew, txtInternLastName.Text
Intern.setHourlyWage objNew, txtInternHourlyWage.Text
ccolInterns.Add (objNew)
This implementation is pretty much identical
to the Employee class, but inside the class file
you can see how our inheritance works.
Private Const UNIQUE_INTERN_STRING = ";Intern::"
Private ccolHourlyWage As OSIUTIL.Collection
Private ciUniqueNumber As Integer
Private Sub Form_Load()
Set ccolHourlyWage = Factory.CreateObject("OSIUTIL.Collection")
End Sub
Public Function Constructor()
Dim sInternObject
ciUniqueNumber = ciUniqueNumber + 1
Call CreateCollectionMember(CStr(ciUniqueNumber))
sInternObject = UNIQUE_INTERN_STRING & CStr(ciUniqueNumber)
'Intern object inheritates from Employee
sInternObject = sInternObject & Employee.Constructor
'return the unique identifier to the caller
Constructor = sInternObject
End Function
If you look carefully you will notice
that the only property collection we have in our Intern
class is ccolHourlyWage. Where are the FirstName,
LastName, and AnnualIncome properties that
our Intern is supposed to be inheriting? Well the answer
is found in the line of code:
sInternObject = sInternObject & Employee.Constructor
All the properties that an Employee
has will remain stored in the Employee class, not only for
all instances of Employee, but also for all instances of
Employee’s descendents.
After Intern.Constructor has
finished running it will return a string “;Intern::1;Employee::2;”
indicating that our new object is an Intern, which inherited
from Employee. Just as we did with our Employee, lets assign
our new Intern some properties.
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