Introduction to Classes & Objects

Introduction to Classes & Objects

Chapter 4

04/19/2023 VBN2008-04 1

Quotes for Today

You will see something new. Two things. And I call them Thing One and Thing Two.

Dr. Theordor Seuss Geisel

Nothing can have value without being an object of utility.

Karl Marx

04/19/2023 VBN2008-04 2

Classes

Properties, Methods, & Constructors

04/19/2023 VBN2008-04 3

Methods & Attributes

• Method represents task in a program– Describes the mechanisms that actually

perform its tasks– Hides from its user the complex tasks

that it performs– Method same as a Procedure

• Classes contain one or more attributes– Specified by instance variables– Carried with the object as it is used

04/19/2023 VBN2008-04 4

Class

• Is a blueprint or template for one or more objects

• Defines what the object can do• Plenty of pre-defined classes

available in Visual Basic– Ex. System.Math

System.Windows.Forms.Form

04/19/2023 VBN2008-04 5

Object

• Is a user interface element• Can be created on VB form using

control in Toolbox• Forms are objects• Inherit functionality from their class

04/19/2023 VBN2008-04 6

Method

• Is a special statement that performs an action or a service for a particular object– Syntax Object.Method(Value)– Example Math.Min(3,5)

Math.Min(Math.Min(3,5),4)

04/19/2023 VBN2008-04 7

Methods cont

• Any class or module that contains a Main method can be used to execute an application

• Visual Basic is extensible– Programmers can create new classes

• Class instance creation expression– Keyword New– Then name of class to create and add parentheses

• Calling a method (Class.Method)– Object’s name, then dot separator (.)– Then method’s name and parentheses

04/19/2023 VBN2008-04 8

Example of Method

' Fig. 4.1: GradeBook.vb

' Class declaration with one method.Public Class GradeBook ‘ Method Header ' display a welcome message to the GradeBook user Public Sub DisplayMessage()

Console.WriteLine("Welcome to the Grade Book!") End Sub ' DisplayMessageEnd Class ' GradeBook

04/19/2023 VBN2008-04 9

When Method is called, it is output to

screen

UML Diagramming

• Unified Modeling Language– Developed by the “three amigos”:

• Grady Booch – Booch Method• James Rumbaugh – OMT (Object Modeling

Technique) • Ivar Jacobson – OOSE (Object-Oriented

Software Engineering)

04/19/2023 VBN2008-04 10

UML Two Goals

• Provide consistency in giving feedback to project sponsor that the problem domain is well understood.

• Provide a consistent model for proper software implementation.

• Based on Synergy Model

04/19/2023 VBN2008-04 11

UML 9 Diagrams

04/19/2023 VBN2008-04 12

Activity Class Collaboration

Component

Deployment

Use Case

Sequence State

Some also Include

Class

Use Case

Sequence

UML Class Diagrams

• Top compartment – name of class

• Middle compartment – class’s attributes or instance variables

• Bottom compartment– Class’s operations or methods– Plus sign indicates Public modifier

04/19/2023 VBN2008-04 13

Example of UML Diagram

• Indicates that class GradeBook has a public DisplayMessage operation.

04/19/2023 VBN2008-04 14

Declaring Methods

• Method parameters– Information passed to method– Called arguments– Supplied in the method call

• Input method– Reads line of input - Console.ReadLine– Dim nameOfCourse As String = _

Console.ReadLine()

04/19/2023 VBN2008-04 15

Method with Parameter

• Parameters specified in method’s parameter list– Part of method header– Uses a comma-separated list– Keyword ByVal

• The argument is passed by value

04/19/2023 VBN2008-04 16

More about ByVal &

ByRef later

UML Example

• indicates that class GradeBook has a DisplayMessage operation with a courseName parameter of type String.

04/19/2023 VBN2008-04 17

Instance Variables & Properties

• Variables declared in the body of method– Called local variables– Can only be used within that method

• Variables declared in a class declaration– Called fields or instance variables– Each object of the class has a separate

instance of the variable04/19/2023 VBN2008-04 18

' Fig. 4.7: GradeBook.vb' GradeBook class that contains instance variable courseNameValue' and a property to get and set its value.Public Class GradeBook Private courseNameValue As String ' course name for this GradeBook ' property CourseName Public Property CourseName() As String Get ' retrieve courseNameValue Return courseNameValue End Get Set(ByVal value As String) ' set courseNameValue courseNameValue = value ' store the course name in the object End Set End Property ' CourseName ' display a welcome message to the GradeBook user Public Sub DisplayMessage() ' use property CourseName to display the ' name of the course this GradeBook represents Console.WriteLine("Welcome to the grade book for " _ & vbCrLf & CourseName & "!") End Sub ' DisplayMessageEnd Class ' GradeBook

Example

04/19/2023 VBN2008-04 19

Instance variable courseNameValue

Get accessor for property courseNameValue

Set accessor for property courseNameValue

Calls the Get accessor of property CourseName

Helpful Console Hints

04/19/2023 VBN2008-04 20

Predefined Constant Identifiers

• vbCrLf– Represents a combination of carriage

return and linefeed character– Outputting this constant’s value causes

subsequent text to display at the beginning of the next line

• vbTab– A constant that represents a Tab

character

04/19/2023 VBN2008-04 21

Private keyword

• Used for most instance variables• Private variables and methods are

accessible only to methods of the class in which they are declared

• Declaring instance variables Private is known as information hiding

04/19/2023 VBN2008-04 22

Instance Variables & Properties continued

• Property Declaration– Declaration consist of an access modifier,

keyword Property, name with parentheses, and type

– Get and Set allows you to access and modify private variables outside of the class, respectively

– Contain a Get accessor, Set accessor, or both

– After defining a property, you can use it like a variable

( object_Name.Property_Name )04/19/2023 VBN2008-04 23


• Get and Set Accessors– Get accessor contains a Return

statement – Set accessor assigns a value to its

corresponding instance variable

04/19/2023 VBN2008-04 24


• Default initial value– Provided for all fields not initialized

• 0 for numeric/value type variables• Nothing for Strings and reference types

04/19/2023 VBN2008-04 25

04/19/2023 VBN2008-04 26

' display initial value of property CourseName (invokes Get)Console.WriteLine( "Initial course name is: " _

& gradeBook.CourseName & vbCrLf)‘ prompt for course name

Console.WriteLine("Please enter the course name:")‘ read course name

Dim theName As String = Console.ReadLine()gradeBook.CourseName = theName ' set the CourseName (invokes Set)Console.WriteLine() ' output a blank line

‘display welcome message including the course name (invokes Get)gradeBook.DisplayMessage()End Sub ' Main

End Module ' GradeBookTest



Calls the Set accessor of property

CourseName

UML Diagram

– Model properties in the UML as attributes:• Public is indicated by the “+” sign• <<Property>>• Property’s name “:” property’s type• If the property only contains a Get accessor,

then place “{ReadOnly}” after the property’s type

– Modeling Private instance variables that are not properties:• Attribute’s name “:” attribute’s type• Private is indicated by the “-” sign04/19/2023 VBN2008-04 27

UML Class Diagram

04/19/2023 VBN2008-04 28

Indicates:• class GradeBook has a courseNameValue

attribute of type String• one public property and • one method


• Public variable – can be read or written by any property

or method

04/19/2023 VBN2008-04 29


• Private variables – can only be accessed indirectly through

the class’s non-Private properties – Class able to control how the data is set

or returned– Allows for data validation– Properties of a class should use class’s

own methods to manipulate the class’s Private instance variables• Creates more robust class

04/19/2023 VBN2008-04 30

Value Types vs. Reference Types

Types in Visual Basic

04/19/2023 VBN2008-04 31

Types

– Value (primitive types except String)• Contains a value of that type• List of Primitive Types in Appendix L

– Reference (sometimes called non-primitive types)• Objects• Default value of Nothing• Used to invoke an object’s methods and

properties

04/19/2023 VBN2008-04 32

Value Type Variable

04/19/2023 VBN2008-04 33

Reference Type Variable

04/19/2023 VBN2008-04 34

Constructors

04/19/2023 VBN2008-04 35

Constructors

– Initialize an object of a class– Required for every class– Provides a default no-argument

constructor ONLY when none is provided– Called when keyword New is followed by

the class name and parentheses– Can also take arguments– Header similar to Sub method header

except name is replaced with keyword New

04/19/2023 VBN2008-04 36

Constructor Example

' Fig. 4.12: GradeBook.vb' GradeBook class with a constructor to initialize the course name.Public Class GradeBook Private courseNameValue As String ' course name for this GradeBook ' constructor initializes course name with String supplied as argument Public Sub New(ByVal name As String) CourseName = name

' initialize courseNameValue via property End Sub ' New

Sub Main() ' create GradeBook object Dim gradeBook1 As New GradeBook( _ "CS101 Introduction to Visual Basic Programming") Dim gradeBook2 As New GradeBook( _ "CS102 Data Structures in Visual Basic")

04/19/2023 VBN2008-04 37

Constructor to initialize courseNameValue variable

Create second grade book object

Call constructor to create first grade book object

UML Class Diagram

– Constructors go in third compartment– Place “<<constructor>>” before New

and its arguments– By convention, place constructors first in

their compartment

04/19/2023 VBN2008-04 38

UML class diagram

04/19/2023 VBN2008-04 39

Indicates that class GradeBook has a constructor that has a name parameter of type String.

Validating Data with Set Accessors in Properties

04/19/2023 VBN2008-04 40

• Validations should be made in the Set accessor to check if the data is valid

• By default, the Get and Set accessor has the same access as the property, however they can vary.

• String– Length property returns the number of characters in the String

– Substring returns a new String object created by copying part of an existing String object

– To display a double quote, use two double quotes in a row

41

UML Class Diagrams

• Allows suppression of class attributes and operations– Called an elided diagram

• Solid line that connects two classes represents an association– numbers near end of each line are

multiplicity values

04/19/2023 VBN2008-04

Multiplicity Types

Symbol Meaning

0 None

1 One

m An integer value

0..1 Zero or one

m, n m or n

m..n At least m, but not more than n

* Any nonnegative integer (zero or more)

0..* Zero or more (identical to *)

1..* One or more

4204/19/2023 VBN2008-04

43

UML Class Diagrams

• Solid diamonds attached to association lines indicate a composition relationship

• Hollow diamonds indicate aggregation – a weaker form of composition

04/19/2023 VBN2008-04

Class Diagram showing Composition Relationships

4404/19/2023 VBN2008-04

Class Diagram for the ATM Model

4504/19/2023 VBN2008-04

Composition Relationships of a class Car.

46

Class Diagram

04/19/2023 VBN2008-04

ATM System Model including Class Deposit.

47

Class Diagram

04/19/2023 VBN2008-04

Next?

04/19/2023 VBN2008-04 48

Documents

Introduction to Classes & Objects