chapter 11: classes and objects

Programming with Microsoft Visual Basic 2012

Chapter 11: Classes and Objects

Previewing the Woods Manufacturing Application

Programming with Microsoft Visual Basic 2012 2

Figure 11-1 Interface showing Charika’s gross pay and information Figure 11-2 Interface showing Chris’s gross pay and information

• Calculates a salary for either an hourly worker or a salaried worker

Lesson A Objectives

After studying Lesson A, you should be able to:• Explain the terminology used in object-oriented

programming• Create a class• Instantiate an object• Add Property procedures to a class• Include data validation in a class• Create a default constructor• Create a parameterized constructor• Include methods other than constructors in a classProgramming with Microsoft Visual Basic 2012 3

• Object-oriented programming language– Uses objects to accomplish a program’s goal

• Class– A pattern or blueprint for an object

• Instance– An object created from a class

• Instantiated– The process of creating an object from a class

• Attributes– Characteristics that describe an object


Object-Oriented Programming Terminology

• Behaviors – Methods and events that define how the object will act or

react• Methods

– Operations (actions) that an object is capable of performing• Events

– Actions to which an object can respond• Encapsulates

– To enclose in a capsule– A class encapsulates all attributes and behaviors of an

object it instantiatesProgramming with Microsoft Visual Basic 2012 5

Object-Oriented Programming Terminology (cont.)


Creating a Class

• Two types of classes used in VB applications:– Built-in classes, such as the TextBox class – Programmer-defined classes

• Class statement– Used to define a class– Defines attributes and behaviors of objects created from

the class• After a class has been defined, it can be used to

instantiate objects


Figure 11-4 Class statement entered in the TimeCard.vb class file

Figure 11-3 Syntax of the Class statement

Creating a Class (cont.)


Creating a Class (cont.)

Figure 11-5 Syntax and examples of instantiating an object

• Norbert Pool & Spa Depot application from Chapter 10– Input: Length, width, and depth of a pool– Calculates the volume of water required

• This program was coded with a structure in Chapter 10– The structure will be replaced with a class


Example 1—A Class that Contains Public Variables Only


Figure 11-6 Code for the Norbert Pool & Spa Depot application (with a structure)

Example 1—A Class that Contains Public Variables Only (cont.)


Figure 11-7 Comments and Option statements entered in the class file

Figure 11-8 Public variables included in the IntelliSense list



Figure 11-9 Class statement, GetGallons function, and btnCalc_Click procedure



Figure 11-10 Interface showing the number of gallons


• Disadvantages of using Public variables in a class:– A class cannot control the values assigned to its Public

variables– This violates the concept of encapsulation, in which class

behaviors control a class’s data


Example 2—A Class that Contains Private Variables, Public Properties, and Methods

Private Variables and Property Procedures• Private class variables

– Can only be used within the class– Not visible to the rest of the application

• Public property– Used to refer to (expose) the Private variable for use by

other parts of the application• Property procedure

– Creates a Public property


Example 2—A Class that Contains Private Variables, Public Properties, and Methods (cont.)

Private Variables and Property Procedures (cont.)• ReadOnly keyword

– Indicates an application can read the property’s value– Cannot set the value

• WriteOnly keyword– Indicates an application can set the property’s value– But it cannot retrieve the value

• Property procedures contain blocks of code:– Get block: Retrieves the contents of the Private variable – Set block: Used to assign a value to the Private variable – Blocks may be used individually or togetherProgramming with Microsoft Visual Basic 2012 16



Figure 11-13 Syntax and examples of a Property procedure


Private Variables and Property Procedures (cont.)

Private Variables and Property Procedures (cont.)


Figure 11-14 Length Property procedure entered in the class


Constructors• Constructor

– A class method whose purpose is to initialize the class’s Private variables

– Processed each time an object is created – Must be coded as a Sub procedure named New

• A class can have more than one constructor– The names are the same, but the parameterLists must

differ • Default constructor

– A constructor without parameters Programming with Microsoft Visual Basic 2012 19


Constructors (cont.)• Parameterized constructor

– A constructor containing one or more parameters• Method’s signature

– A method name combined with an optional parameterList




Figure 11-16 Statements that invoke the constructors shown in Figure 11-15

Figure 11-15 Syntax and examples of a constructor


Constructors (cont.)



Methods Other than Constructors• May be either Sub or Function procedures

– Functions return a value; Sub procedures do not• Rules for naming methods:

– The name should be entered using Pascal case– The first word in a name should be a verb– Subsequent words should be nouns and adjectives


Figure 11-17 Syntax and examples of a method that is not a constructor


Methods Other than Constructors (cont.)


Figure 11-18 Pseudocode for the Calculate button’s Click event procedure

Coding the Carpets Galore Application

Figure 11-19 TryParse methods entered in the procedure



Figure 11-20 Rectangle class definition and btnCalc_Click procedure (continues)


Coding the Carpets Galore Application (cont.)


Figure 11-20 Rectangle class definition and btnCalc_Click procedure




Figure 11-21 Interface showing the square yards and cost



• A parameterized constructor is simply a constructor that has parameters

• When a Rectangle object is created, a parameterized constructor allows an application to specify the object’s initial values


Example 3—A Class that Contains a Parameterized Constructor

Figure 11-22 Default and parameterized constructors


Figure 11-23 Modified Rectangle class definition and btnCalc_Click procedure (continues)

Example 3—A Class that Contains a Parameterized Constructor (cont.)


Figure 11-23 Modified Rectangle class definition and btnCalc_Click procedure

Figure 11-24 Square yards and cost shown in the interface

Example 3—A Class that Contains a Parameterized Constructor (cont.)

• Rectangle class from Examples 2 and 3:– Reused here to represent a square pizza– A square is a rectangle with four equal sides

• Using an object for more than one purpose saves programming time and money– An advantage of object-oriented programming


Example 4—Reusing a Class

Figure 11-25 Interface for the Pete’s Pizzeria application



Example 4—Reusing a Class (cont.)


Figure 11-27 btnCalc_Click procedure

Example 4—Reusing a Class (cont.)

Figure 11-28 Number of pizza slices shown in the interface

Lesson A Summary

• The Class statement is used to define a class• Defined classes are added to a PROJECT with a .vb

extension• Objects are instantiated from a defined class• The Get block allows an application to retrieve the

contents of the Private variable associated with the Property procedure

• The Set block allows an application to assign a value to the Private variable associated with the Property procedure


Lesson A Summary (cont.)

• A constructor initializes the variables of a class– The constructor method must be named New– The default constructor has no parameters– A class may have many parameterized constructors

• A class can have methods other than constructors


Lesson B Objectives

After studying Lesson B, you should be able to:• Include a ReadOnly property in a class• Create an auto-implemented property• Overload a method in a class



Example 5—A Class that Contains a ReadOnly Property

• ReadOnly keyword– Indicates that the property’s value can be retrieved (read)

but not set (written)• The ReadOnly property gets

its value from the class instead of from the application

• Grade Calculator application– Returns a letter grade

based on a numeric gradeFigure 11-33 Interface for the Grade Calculator application


Figure 11-34 ReadOnly property message

Figure 11-35 CourseGrade class definition and btnDisplay_Click procedure (continues)

Example 5—A Class that Contains a ReadOnly Property (cont.)


Figure 11-35 CourseGrade class definition and btnDisplay_Click procedure

Figure 11-36 Grade shown in the interface

(continued)

Example 5—A Class that Contains a ReadOnly Property (cont.)


Example 6—A Class that Contains Auto-Implemented Properties

• Auto-implemented properties feature – Enables you to specify the property of a class in one line of

code• Visual Basic automatically creates a hidden Private

variable that it associates with the property• It also automatically creates hidden Get and Set blocks• It provides a shorter syntax to use when creating a class

– You don’t need to create the Private variable associated with a property, nor do you need to enter the property’s Get and Set blocks of code


Figure 11-37 Syntax and examples of creating an auto-implemented property

Example 6—A Class that Contains Auto-Implemented Properties (cont.)


Figure 11-38 Modified CourseGrade class definition

Example 6—A Class that Contains Auto-Implemented Properties (cont.)

• Overloaded methods– Methods with the same name but different parameters


Example 7—A Class that Contains Overloaded Methods

Figure 11-39 Attributes and behaviors of an Employee object


Figure 11-40 Employee class definition

Example 7—A Class that Contains Overloaded Methods (cont.)


Figure 11-42 Interface for the Woods Manufacturing application



Figure 11-45 btnCalc_Click procedure



Figure 11-46 Jake’s gross pay and information shown in the interface Figure 11-47 Sherri’s gross pay and information shown in the interface



Lesson B Summary

• Use the ReadOnly keyword to create a property whose value an application can only retrieve

• To specify the property of a class in one line:– Create an auto-implemented property using the syntax Public

Property propertyName As dataType• To include a parameterized method in a class:

– Enter the parameters between the parentheses that follow the method’s name

• To create two or more methods that perform the same task but require different parameters:– Overload the methods by giving them the same name but

different parameterLists

Lesson C Objectives

After studying Lesson C, you should be able to:• Create a derived class• Refer to the base class using the MyBase keyword• Override a method in the base class



Example 8—Using a Base Class and a Derived Class

• Inheritance– Using one class to create another

• Base class– The original class providing behaviors and attributes

• Derived class– The new class that inherits attributes and behaviors from

the base class• Inherits clause

– Enables a derived class to inherit from a base class– Included in the derived class’s Class statement


Example 8—Using a Base Class and a Derived Class (cont.)

• Overriding– Allows a derived class to replace a method inherited from its

base class– The base class method header must include the Overridable

keyword– The derived class method header must include the Overrides

keyword• MyBase keyword

– Used to refer to the base class• MyBase.New ([parameterList])

– Used in a derived class to call its base class’s constructor


Figure 11-51 Contents of the Shapes.vb file



Figure 11-52 Modified Square and Cube class definitions



Figure 11-54 btnSquare_Click and btnCube_Click procedures

Figure 11-53 Interface showing the square’s area



Lesson C Summary

• To allow a derived class to inherit the attributes and behaviors of a base class:– Enter the Inherits clause immediately below the Public Class

clause in the derived class– The Inherits clause is the keyword Inherits followed by

the name of the base class• Use the MyBase keyword to refer to the base class• To indicate that a method in the base class can be

overridden (replaced) in the derived class:– Use the Overridable keyword in the method’s header in

the base class


Lesson C Summary (cont.)

• To indicate that a method in the derived class overrides (replaces) a method in the base class:– Use the Overrides keyword in the method’s header in

the derived class

chapter 11: classes and objects

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