1. Mark Dixon Page 1 22 – Object Oriented Programming in ASP

2. Mark Dixon Page 2 Session Aims & Objectives Aims –To highlight that the object oriented techniques covered earlier can be used in ASP Objectives, by end of this week’s sessions, you should be able to: –create a class definition in server-side code –create an instance of a class –create a class definition from a class diagram

3. Mark Dixon Page 3 Object-Oriented Paradigm A program is made up of a number of objects that communicate with each other by passing messages Each object contains –attributes/properties that represent its state, and –operations/methods that represent its behaviour Objects often mirror the real world –Customers –Students –Patients

4. Mark Dixon Page 4 Classes and Instances Object Classes –general descriptions of types of objects, e.g. student, product, customer, lecturer, and room. Object Instances –specific items of a given class, e.g. each of you could be an instance of the student class Room 214 could be an instance of the room class I could be an instance of the lecturer class Bolt could be an instance of the part class

5. Mark Dixon Page 5 Object Concepts - Implementation Properties – implemented as –data structures (variables, arrays, and types). Methods – implemented as either –a procedure (to perform some processing), or –a function (to return a value). Object oriented paradigm builds on (rather than replaces) the structured paradigm

6. Mark Dixon Page 6 Class Diagrams Used to describe structure of object classes: Module Code: string Title: string GetTitle(): string SetTitle(t: string) Count(): integer Class Attributes/Properties Class Operations/Methods Class Name

7. Mark Dixon Page 7 Benefits of OOP in code Procedures and Functions are part of object –encapsulation Related Data and Operations together Private keyword – restrict access to data Clearer code Less prone to error

8. Mark Dixon Page 8 Class Module Public Code As String Public Title As String Public Function GetTitle() As string Public Sub SetTitle(t As String) Public Function Count() As Integer End Class Implementing Class Diagrams Module Code: String Title: String GetTitle(): string SetTitle(t: string) Count(): integer

9. Mark Dixon Page 9 Public and Private Control access to properties and methods Class a Public x As Single Private y As Single End Class Dim b As New a b.x = 5 b.y = 10 this works (x is public)  this will fail (y is private)

10. Mark Dixon Page 10 Example: Counter (html) Counter page

11. Mark Dixon Page 11 Example: Counter (code) Dim c As Object Dim msg As String Sub Main() If Session("c") Is Nothing Then Session("c") = New Counter Else c = Session("c") If Request.Form("btnReset") <> "" Then c.Reset() ElseIf Request.Form("btnUp") <> "" Then c.Up() ElseIf Request.Form("btnDown") <> "" Then c.Down() End If msg = c.GetCount() End If End Sub Class Counter Private mCount As Long Public Function GetCount() As Long GetCount = mCount End Function Public Sub Reset() mCount = 0 End Sub Public Sub Up() mCount = mCount + 1 End Sub Public Sub Down() mCount = mCount - 1 End Sub End Class Counter.vbs

12. Mark Dixon Page 12 Object Associations In practice projects will be made of –many object classes –that interact with each other (are associated) There are several types of association One of the most often used is the ‘part of’ association, –where one object class forms part of another object class A common example of this occurs where it is necessary to store multiple instances of a class

13. Mark Dixon Page 13 Example: Bar (Analysis) Scenario 1: small project, limited automation –Nouns: drinks, order, cost –Verbs: describe, calculate cost The students' Union bar needs a computer system for recording the purchase of drinks. Typically, a student will stagger to the bar and describe their order, consisting of one or (usually) more drinks. The bar staff will then prepare the drinks and calculate the cost of the order.

14. Mark Dixon Page 14 Example: Bar (Design v1) Object Classes, properties, and methods –Class diagram: Order mDrinks(): Drink Add(string, long) Remove(long) Display (): String Cost(): double Drink mType: String mQty: Long

15. Mark Dixon Page 15 Example: Bar (Drink module) Drink (class module): Class Drink Public mType As String Public mQty As Long End Class Drink mType: String mQty: Long

16. Mark Dixon Page 16 Example: Bar (Order module) Class Order Private mDrinks(9) As Drink Public Sub Add(tmpType As String, tmpQty As Long) Dim d As Long ' Find free slot. For d = 0 To 9 If mDrinks(d) Is Nothing Then Exit For End If Next ' Create object and store data. mDrinks(d) = New Drink mDrinks(d).mType = tmpType mDrinks(d).mQty = tmpQty End Sub End Class Order mDrinks(): Drink Add(string, long) Remove(long) Display (): String Cost(): double

17. Mark Dixon Page 17 Example: Bar (Order module) method (procedure) to display order's drinks in a list box Public Function Display() As String Dim d As Long Dim tmpStr As String tmpStr = "" For d = 0 To 9 If Not (mDrinks(d) Is Nothing) Then tmpStr = tmpStr & mDrinks(d).mQty & " " tmpStr = tmpStr & mDrinks(d).mType End If Next Display = tmpStr End Function Order mDrinks(): Drink Add(string, long) Remove(long) Display (): String Cost(): double

18. Mark Dixon Page 18 Example: Bar (Order module) method (procedure) to remove drink from order Public Sub Remove(d As Long) mDrinks(d) = Nothing End Sub Order mDrinks(): Drink Add(string, long) Remove(long) Display (ListBox) Cost(): double

19. Mark Dixon Page 19 Tutorial Exercise: Bar Task 1: Get the Bar example from the lecture working. Task 2: Modify your code – add code to calculate the cost of the order. This object method is not in the lecture notes – you need to create it (not necessarily on your own – discuss it with others, feel free to ask me for help). Task 3: What happens if the user tries to add more than 10 drinks? Modify your code to cope with this (you decide how it should respond). Task 4: What happens if the user tries to add a drink when none is selected (in the drinks list box)? Modify your code to cope with this. Task 5: What happens if the user tries to remove a drink when none is selected (in the order list box)? Modify your code to cope with this. Task 6: Modify your code – so that the cost is continuously calculated and there is no need for the cost button.