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12 – Object Oriented Programming
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Questions: Functions Consider the following code: Function Smallest(num1, num2) If num1 < num2 Then Smallest = num1 Else Smallest = num2 End If End Function name a function. what is left in small after the following is executed? Dim small small = Smallest(23, 15) Smallest 15
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Session Aims & Objectives
To introduce the fundamental ideas of object orientation Objectives, by end of this week’s sessions, you should be able to: create a class definition, which includes properties, and methods create an object instance, for the class use the object instance, by assigning values to its properties, and calling its methods
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Example: Ball Bounce v1 <html>
<head><title>Ball Bounce</title></head> <body style="margin: 0;"> <img id="imgBall" src="Ball.gif" style="position: absolute;" /> </body> </html> <script language="vbscript"> Option Explicit Dim x Dim y Dim xInc Dim yInc Sub window_onLoad() window.setinterval "Main", 20 xInc = 5 yInc = 3 End Sub Sub Main() x = imgBall.style.pixelLeft + xInc If x <= 0 Or x >= document.body.clientWidth - imgBall.width Then xInc = -xInc Else imgBall.style.pixelLeft = x End If y = imgBall.style.pixelTop + yInc If y <= 0 Or y >= document.body.clientHeight - imgBall.height Then yInc = -yInc imgBall.style.pixelTop = y </script>
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Structured Paradigm Program made up of
data structures, and routines (procedures and functions) that process the data within those structures. Each routine should perform a single, clearly identifiable operation. Each routine should be self-contained Abstract data type = structure + procedures
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Example: Ball Bounce v2 Sprite.vbs BallBounce.htm Option Explicit
Dim x Dim y Dim xInc Dim yInc Sub Init(tmpXInc, tmpYInc) xInc = tmpXInc yInc = tmpYInc End Sub Sub Move(img) x = img.style.pixelLeft + xInc If x <= 0 Or x >= document.body.clientWidth - img.width Then xInc = -xInc Else img.style.pixelLeft = x End If y = img.style.pixelTop + yInc If y <= 0 Or y >= document.body.clientHeight - img.height Then yInc = -yInc img.style.pixelTop = y BallBounce.htm <html> <head><title>Ball Bounce</title></head> <body style="margin: 0;"> <img id="imgBall" src="Ball.gif" style="position: absolute;" /> </body> </html> <script language="vbscript" src="Sprite.vbs"></script> <script language="vbscript"> Option Explicit Sub window_onLoad() window.setinterval "Main", 20 Init 5, 3 End Sub Sub Main() Move imgBall </script>
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Object-Oriented Paradigm
program made up of 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
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Classes and Instances Object Classes Object Instances
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
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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
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Example: Animals Class: Animal Properties: Name, Species, Gender
Instances: myPet, yourPet
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Question: Objects Consider the following code: Class FetalBloodSample Dim Time Dim Acidity Dim Reactivity End Class Dim fbs1 Set fbs1 = New FetalBloodSample Name all: classes properties instances FetalBloodSample Time, Acidity, Reactivity fbs1
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Example: Ball Bounce v3 Sprite.vbs BallBounce.htm Class Definition
Option Explicit Class Sprite Dim x Dim y Dim xInc Dim yInc End Class Sub Init(spr, tmpXInc, tmpYInc) spr.xInc = tmpXInc spr.yInc = tmpYInc End Sub Sub Move(spr, img) spr.x = img.style.pixelLeft + spr.xInc If spr.x <= 0 Or spr.x >= document.body.clientWidth - img.width Then spr.xInc = -spr.xInc Else img.style.pixelLeft = spr.x End If spr.y = img.style.pixelTop + spr.yInc If spr.y <= 0 Or spr.y >= document.body.clientHeight - img.height Then spr.yInc = -spr.yInc img.style.pixelTop = spr.y Class Definition BallBounce.htm <html> … </html> <script language="vbscript" src="Sprite.vbs"></script> <script language="vbscript"> Option Explicit Dim ball Sub window_onLoad() window.setinterval "Main", 20 Set ball = New Sprite Init ball, 5, 3 End Sub Sub Main() Move ball, imgBall </script>
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Example: Students Method: Clear
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Example: Ball Bounce v4 Sprite.vbs BallBounce.htm
Option Explicit Class Sprite Dim x Dim y Dim xInc Dim yInc Sub Init(tmpXInc, tmpYInc) xInc = tmpXInc yInc = tmpYInc End Sub Sub Move(img) x = img.style.pixelLeft + xInc If x <= 0 Or x >= document.body.clientWidth - img.width Then xInc = -xInc Else img.style.pixelLeft = x End If y = img.style.pixelTop + yInc If y <= 0 Or y >= document.body.clientHeight - img.height Then yInc = -yInc img.style.pixelTop = y End Class BallBounce.htm <html> … </html> <script language="vbscript" src="Sprite.vbs"></script> <script language="vbscript"> Option Explicit Dim ball Sub window_onLoad() window.setinterval "Main", 20 Set ball = New Sprite ball.Init 5, 3 End Sub Sub Main() ball.Move imgBall </script> procedures now methods inside the class
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Example: Ball Bounce v5 Multiple instances:
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Example: Ball Bounce v5 Now have easy way of:
<html> <head><title>Ball Bounce</title></head> <body style="margin: 0;"> <img id="imgBall" src="Ball.gif" style="position: absolute;" /> <img id="imgFace" src="BallChar2.GIF" style="position: absolute;" /> </body> </html> <script language="vbscript" src="Sprite.vbs"></script> <script language="vbscript"> Option Explicit Dim ball Dim face Sub window_onLoad() window.setinterval "Main", 20 Set ball = New Sprite ball.Init 5, 3 Set face = New Sprite face.Init 1, 12 End Sub Sub Main() ball.Move imgBall face.Move imgFace </script> Now have easy way of: creating & using multiple sprites each with own identity (separate characteristics) only a few (4) lines of code
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Questions: OOP Name a House number, road, district ChangeDist h class
Class House Dim number Dim road Dim district Sub ChangeDist(newDist) district = newDist End Sub End Class Dim h Set h = New House Name a class property method instance House number, road, district ChangeDist h
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OOP: Errors Object doesn't support this property or method:
Class Animal Dim name Dim species End Class Dim a Set a = New Animal a.name = "Skippy" a.specys = "Kangaroo"
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Private make properties and methods invisible outside class:
Class Counter Private value Sub Reset() value = 0 End Sub Sub Increase() value = value + 1 Function GetValue() GetValue = value End Function End Class Dim c Set c = New Counter c.Reset c.Increase parRes.innerText = c.GetValue() c.value = -6
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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 Reduces chance of accidental interference Less prone to error
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Example: Balloon Shoot
Question: what objects? what properties? what classes? what methods
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Tutorial Exercise: Ball Bounce
Learning Objective: To create and use your own class. Task 1: Get the Ball Bounce examples (1, 2, and 5) from the lecture working. Task 2: Add a hit method to the sprite class, which detects the collision with another sprite. Task 3: Modify your page to count the number of hits between the two sprites. Task 4: Modify your page to make sprites bounce off each other. Task 5: Add another sprite.
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Tutorial Exercise: Balloon Shoot
Learning Objective: To create and use your own classes. Task 1: Create the Balloon Shoot example (from the lecture) using object oriented concepts (classes, properties, methods, and instances) hint: use some of the code from your Interceptor example (from last week)
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