Objectives

Creating Classes and Objects

Chapter Microsoft Visual Basic .NET: Reloaded

1

2Microsoft Visual Basic .NET: Reloaded

Objectives

• Define a class

• Add properties to a class

• Instantiate an object from a class that you define

• Add Property procedures to a class


Objectives (continued)

• Create constructors

• Add methods to a class

• Include data validation in a class


Classes and Objects

• Object-oriented programs are based on objects, which are instantiated from classes

• A class contains (encapsulates) the properties (attributes) that describe a person, place, or thing

• Examples: Student, School, Book

• An object is a specific instance of a class

• Examples: “You” are a student that goes to a specific school and are using this specific book

• VB has many built-in objects such as Textbox controls, Forms, and Labels


Defining a Class• Use the Class statement

• Begins with keyword Class and ends with End Class


HOW TO…


Defining a Class (continued)


Defining a Class (continued)


HOW TO…


Example 1 – Using a Class that Contains Properties Only

• Assume that the sales manager at Sweets Unlimited wants an application that allows him to save each salesperson’s

• name

• quarterly sales amount

• quarterly bonus amount in a sequential access file

• The bonus amount is calculated by multiplying the sales amount by 5%


Example 1 – Using a Class that Contains Properties Only (continued)

• Figure 11.6 shows a sample run of the Sweets Unlimited application



• Figure 11.7 shows the Salesperson class defined in the Salesperson.vb file






Example 2 – Using a Class that Contains Properties and Methods

• Create a class named Square and then use in the Area application

• Square class creates an object that can calculate and return the area of a square, using side measurement provided by application

• Figure 11.9 shows a sample run of the Area application

• Figure 11.10 shows the Square class defined in the Square.vb file



(continued)



(continued)



(continued)

• Creating a Public Property

• Declare a class-level private variable to hold the value to be stored by the property

• Data type of property and private variable must match each other

• Heading begins with keywords Public Property



(continued)

• Code Get block to retrieve contents of private variable

• Code Set block to change contents of private variable

• Ends with keywords End Property


HOW TO…


Constructors

• Constructor method whose instructions the computer processes, automatically, each time an object is created (instantiated) from the class

• Constructor begins with Public Sub New followed by a set of optional parameters

• Parameter list may be empty - New()

• No parameters is called “default constructor”

• Every class should have at least one constructor and may have several


HOW TO…


Methods Other Than Constructors

• Sub methods do not return a value• Function methods return a value to the

calling procedure


Methods Other Than Constructors (continued)


Example 3 – Using a Class that contains two Constructors and Data

Validation

• Create a class named MyDate and then use in the Personnel application

• MyDate class creates an object that returns a month number, followed by a slash, and a day number

• Figure 11.15 shows a sample run of the Personnel application

• Figure 11.16 shows the MyDate class defined in the MyDate.vb file



Validation (continued)











Programming Example – Kessler Landscaping Application

• Monica Kessler, the owner of Kessler Landscaping, wants an application that she can use to estimate the cost of laying sod

• Use a MyRectangle class in this application


TOE Chart


User Interface


Objects, Properties, and Settings


Tab Order


Pseudocode

btnExit Click event procedure1. close application

btnCalc Click event procedure1. declare a MyRectangle object2. assign the length and width to the MyRectangle object’s properties3. assign the sod price to a variable4. calculate the area of the rectangle5. calculate the total price of the sod6. display the total price of the sod in lblTotalPrice

txtLength, txtWidth, and txtPrice TextChanged event procedures1. clear the contents of the lblTotalPrice control


Code (MyRectangle.vb file)


Code (MyRectangle.vb file) (continued)


Code (Kessler Form.vb file)


Code (Kessler Form.vb file) (continued)


Summary

• The objects used in an object-oriented program are created, or instantiated, from classes

• A class contains (encapsulates) the properties (attributes) that describe the object it creates, and the methods (behaviors) that allow the object to perform tasks

• In Visual Basic .NET, you can create objects from classes that you define with the Class statement


Summary (continued)

• Good programming practice to enter Option Explicit On and Option Strict On statement in both the form file and class file

• The first letter in the class name, as well as the first letter in any subsequent words in the name, should be capitalized

• The properties in a class should be assigned a name composed of one or more words, with the first letter of each word being capitalized

• You should use nouns and adjectives to name a property


Summary (continued)

• Methods in a class should be assigned a name composed of one or more words, with the first letter of each word being capitalized

• You should use a verb for the first word in the name, and nouns and adjectives for any subsequent words in the name

• Variables declared using the Public keyword in a class definition can be accessed by any application that uses an object created from the class

• Most classes contain properties and methods


Summary (continued)

• When an application needs to assign data to or retrieve data from a Private variable in a class, it must use a Public property to do so• You create a Public property using a Property

procedure

• The Get block in a Property procedure allows an application to access the contents of the class’s Private variables

• The Set block in a Property procedure allows an application to assign values to the class’s Private variables


Summary (continued)

• A class can have one or more constructors

• All constructions are Sub procedures

• The default constructor is automatically processed when an object is created from the class

Documents

Objectives