chapter 11: classes and objects programming with microsoft visual basic.net, second edition

Chapter 11: Classes and Objects

Programming with Microsoft Visual Basic .NET, Second Edition

Programming with Microsoft Visual Basic .NET, Second Edition 2

Classes and ObjectsLesson A Objectives

• Define a class

• Add properties to a class

• Instantiate an object from a class that you define

Defining a Class

• Classes that you define must specify the properties and methods of the objects they create

– Properties describe the characteristics of the objects

– Methods specify the tasks that the objects can perform

Defining a Class (continued)

• Use the Class statement to define a class in Visual Basic .NET

• Enter the Class statement in a class file

• After defining a class, you can use it to create objects

Figure 11-3: Syntax and an example of the Class statement

Figure 11-4: Procedure for adding a class file to a project

Figure 11-7: Syntax and examples of creating an object from a class

Using a Class That Contains Properties Only

• The sales manager at Sweets Unlimited wants an application to allow him to save each salesperson’s name, quarterly sales amount, and quarterly bonus amount in a sequential access file

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

Using a Class That Contains Properties Only (continued)

Figure 11-8: Sample run of the Sweets Unlimited application

Using a Class That Contains Properties Only (continued)

• The Salesperson class contains three properties: Name, Sales, and Bonus

• Rules for naming properties

– A name should be composed of one or more words, with the first letter of each word being capitalized

– Use nouns and adjectives to name a property

More on Classes and Objects Lesson B Objectives

• Add Property procedures to a class

• Create constructors

• Add methods to a class

Using a Class That Contains Properties and Methods

• You will learn how to create a class named Square and use the class in the Area application

• Square class

– Contains one property and two methods

– Creates an object that calculates and returns the area of a square, using the side measurement provided by the application

Using a Class That Contains Properties and Methods (continued)

Figure 11-11: Sample run of the Area application

• An application cannot directly refer to a Private variable in a class; it must refer to the variable indirectly, through the use of a Public property

• You create a Public property using a Property procedure

Figure 11-13: Syntax and an example of creating a Property procedure

Figure 11-13: Syntax and an example of creating a Property procedure (continued)

• Within the Property procedure you define a Get block of code and a Set block of code

• Code in the Get block allows an application to retrieve the contents of the Private variable associated with the property

• Code in the Set block allows the application to assign a value to the Private variable associated with the property

• The Get block uses the Get statement, which begins with the keyword Get and ends with the keywords End Get

• The Set block uses the Set statement, which begins with the keyword Set and ends with the keywords End Set

Constructors

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

• The purpose of a constructor is to initialize the variables of the class

Constructors (continued)

Figure 11-14: Syntax and an example of creating a constructor

Constructors (continued)

• Every class should have at least one constructor

• A constructor that has no parameters is called the default constructor

• Each constructor included in a class has the same name, New, but its parameters (if any) must be different from any other constructor in the class

Methods Other Than Constructors

• Methods, other than constructors, included in a class can be either Sub procedures or Function procedures

• Rules for naming methods

– Names should be composed of one or more words, with the first letter of each word being capitalized

– The first word in a name should be a verb; subsequent words should be nouns and adjectives

Methods Other Than Constructors (continued)

Figure 11-15: Syntax and an example of creating a method that is not a constructor

Coding the Cornwall Calendars Application

Lesson C Objectives

• Create a class that contains more than one constructor

• Include data validation in a class

Using a Class That Contains Two Constructors and Data

Validation• MyDate class

– Contains more than one constructor

– Performs data validation

– Creates an object that returns a month number, followed by a slash, and a day number

• MyDate class is used in the Personnel application

Using a Class That Contains Two Constructors and DataValidation (continued)

Figure 11-18: Sample run of the Personnel application

Using a Class That Contains Two Constructors and DataValidation (continued)

• MyDate class contains:

– Two Private variables: monthNum and dayNum

– Two Property procedures: Month and Day

– Three methods: two named New and one named GetNewDate

Coding the Cornwall Calendars Application

• Cornwall Calendars application

– Created for Jesse Washington, the manager of the Accounts Payable department at Cornwall Calendars

– Should allow Jesse to record (in a sequential access file) the check number, date, payee, and amount of each check written by his department

Coding the Cornwall Calendars Application (continued)

Figure 11-21: Interface for the Cornwall Calendars application

Figure 11-22: TOE chart for the Cornwall Calendars application

Figure 11-22: TOE chart for the Cornwall Calendars application (continued)

Creating the Check Class

• The Check class will contain four properties and two methods

• The Cornwall Calendars application will use the Check class to create a Check object

– It will store the user input in the object’s properties

– It will use the object’s methods to initialize the Private variables and save the check information to a sequential access file

Creating the Check Class (continued)

Figure 11-23: Pseudocode for the Check class

Coding the uiSaveButton Click Event Procedure

Figure 11-31: Pseudocode for the uiSaveButton’s Click event procedure

Coding the uiSaveButton Click Event Procedure (continued)

• The selection structure in the pseudocode determines whether the user entered the check information—in this case, the check number, date, payee, and amount

• If the user neglected to enter one or more of the items, the selection structure’s false path should display an appropriate message

Coding the uiSaveButton Click Event Procedure (continued)

Figure 11-34: Message box that appears when the user does not enter all of the check information

Summary

• Two versions of the syntax used to create (instantiate) an object from a class:

– Version 1: {Dim | Private} objectVariable As class objectVariable = New class

– Version 2:{Dim | Private} objectVariable As New class

• To access the properties of an object, use the syntax: objectVariable.property

Summary (continued)

• To create a Public property, use a Property procedure

– The Get block allows an application to retrieve the contents of the Private variable associated with the property

– The Set block allows an application to assign a value to the Private variable associated with the property

Summary (continued)

• A constructor that has no parameters is called the default constructor

• To include more than one constructor in a class, each constructor’s parameters must be different from any other constructor in the class

• To include data validation in a class, place the data validation in the Set block of a Property procedure

chapter 11: classes and objects programming with microsoft visual basic.net, second edition

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