24 – Datatypes and Object Association

Mark Dixon, SoCCE SOFT 131 Page 1

24 – Datatypes and Object Association


Session Aims & Objectives• Aims

– To introduce• the idea of datatypes• some of the more subtle aspects of object oriented

design (such as object associations)

• Objectives,by end of this week’s sessions, you should be able to:

– use datatypes to make you programs more memory efficient and faster

– create a project with several associated objects


Data Types• Variant – all types of data

– slow, memory hungry

• Boolean – true or false

• Integer – whole numbers (-32768 to 32768)

• Long – whole numbers (large)

• Single – decimal numbers

• Double – decimal numbers (more precise)

• String – text

• Object – object instances


Data Type Selection• Number of e.g. 4 Integer/Long

Rooms

• Height e.g. 1.87m Single/Double

• Surname e.g. Smith String

• Car Reg e.g. XY55 ABC String


Using data types• Variable declaration

Dim x As Long

• Parameters Sub Thing(boo As String, y As Long)

• Functions Function IsTall() As Boolean


client-side vs. server-side code• client-side code – only variant

• server-side code – all datatypes

<script language="vbscript">Dim xDim yDim sDim b x = 23 y = 18.5 s = "Hello there" b = false</script>

<script language="vbscript" runat="server">Dim x As LongDim y As DoubleDim s As StringDim b As Boolean x = 23 y = 18.5 s = "Hello there" b = false</script>


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


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


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


Class Diagrams• Used to describe structure of object classes:

Module

Code: stringTitle: string

GetTitle(): stringSetTitle(t: string)Count(): integer

Class Attributes/Properties

Class Operations/Methods

Class Name


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


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 IntegerEnd Class

Implementing Class Diagrams

Module

Code: StringTitle: String

GetTitle(): stringSetTitle(t: string)Count(): integer


Public and Private• Control access to properties and methods

Class a Public x As Single Private y As SingleEnd Class

Dim b As New a b.x = 5 b.y = 10

this works (x is public) this will fail (y is private)


Example: Counter (html)<html> <head><title>Counter page</title></head> <body> <%Main()%> <form action="Main.aspx" method="post"> <input name="btnReset" type="submit" value="Reset" /> <input name="btnUp" type="submit" value="Up" /> <input name="btnDown" type="submit" value="Down" /> </form> <p><%=msg%></p> </body></html>


Example: Counter (code)<script language="vbscript" runat="server" src="Counter.vbs"></script><script language="vbscript" runat="server">Dim c As ObjectDim 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 IfEnd Sub</script>

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 SubEnd Class

Counter.vbs


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


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.


Example: Bar (Design v1)• Object Classes, properties, and methods

– Class diagram:

Order

mDrinks(): Drink

Add(string, long)Remove(long)Display (): StringCost(): double

Drink

mType: StringmQty: Long


Example: Bar (Drink module)• Drink (class module):

Class Drink Public mType As String Public mQty As LongEnd Class

Drink

mType: StringmQty: Long


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 SubEnd Class

Order

mDrinks(): Drink



Example: Bar (Order module)• method (procedure) to display order's drinks

in a list boxPublic Function Display() As StringDim d As LongDim 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 = tmpStrEnd Function

Order

mDrinks(): Drink



Example: Bar (Order module)• method (procedure) to remove drink from

orderPublic Sub Remove(d As Long) mDrinks(d) = NothingEnd Sub

Order

mDrinks(): Drink

Add(string, long)Remove(long)Display (ListBox)Cost(): double


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.

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24 – Datatypes and Object Association