Mark Dixon 1 22 – Object Oriented Programming

Mark Dixon 2 Questions: Databases How many primary keys? How many foreign keys? 3 2

Mark Dixon 3 Questions: HTML in VB Are these correct (assume variables and fields exist)? s = s + + r("Model") s = s r("Length") h = " " + h + " "  

Mark Dixon 4 Questions: SQL in VB Are these correct (assume variables and fields exist)? id = 4 sql = SELECT * FROM Customer sql = sql " WHERE [CustID] = " + id + ";" cmd = New OldDbCommand(sql, cn)  

Mark Dixon 5 Questions: Writing to Databases What SQL command is used to add a new record to a database table. What SQL command is used to remove a record from a database table. Write an SQL command to put "Hello" into the Description field of all records in the Message table. INSERT DELETE UPDATE Message SET Description = ‘Hello’;

Mark Dixon 6 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

Mark Dixon 7 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

Mark Dixon 8 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

Mark Dixon 9 Object Concepts - Implementation Properties – implemented as –data structures (variables and arrays) 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

Mark Dixon 10 Example: Animals

Mark Dixon 11 Example: Student

Mark Dixon 12 Public and Private Control access to properties and methods Class a Public x As Single Private y As Single Public Sub ResetY() y = 0 End Sub End Class Dim b As New a b.x = 5 b.ResetY() b.y = 10 this works (x is public) this works (ResetY is public)  this will fail (y is private)

Mark Dixon 13 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

Mark Dixon 14 Example: Counter (html) Counter

Mark Dixon 15 Example: Counter (code) Dim c As Counter Sub Page_Load() If Session("c") Is Nothing Then Session("c") = New Counter End If c = Session("c") End Sub Sub btnReset_Click(s As Object, e As EventArgs) Handles btnReset.ServerClick c.Reset() End Sub Sub btnUp_Click(s As Object, e As EventArgs) Handles btnUp.ServerClick c.Up() End Sub Sub Page_LoadComplete(s As Object, e As EventArgs) parMsg.innerText = c.GetCount() End Sub Public 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.vb Class file must be in App_Code folder

Mark Dixon 16.NET Folders Right click project –App_Code – used for classes (put all classes here) –App_Data – used for databases

Mark Dixon 17 Questions: OOP How many –classes –properties –methods –functions –procedures Public 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 Function Twice(x As Long) As Long Return x * 2 End Function

Mark Dixon 18 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

Mark Dixon 19 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

Mark Dixon 20 Tutorial Exercise: Counter Task 1: Get the Counter example from the lecture working. Task 2: Modify your code – so that the value cannot go below 0 or above 10. hint: you can't actually stop it going outside the range, but you can detect if it does and then change it.