1. Programming with Microsoft Visual Basic 2008 Fourth Edition
Chapter Eleven
Classes and Objects

2. Previewing the ABC Company Application
Open the ABC.exe file
ABC Company application calculates and displays gross pay for salaried and hourly employees
Salaried employees are paid twice per month
Hourly employees are paid weekly
Programming with Microsoft Visual Basic 2008, Fourth Edition

3. Previewing the ABC Company Application (continued)
Figure 11-1: Interface showing the gross pay for an hourly employee
Programming with Microsoft Visual Basic 2008, Fourth Edition

4. Lesson A Objectives After studying Lesson A, you should be able to:
Define a class
Instantiate an object from a class that you define
Add Property procedures to a class
Include data validation in a class
Create a default constructor
Include methods in a class
Programming with Microsoft Visual Basic 2008, Fourth Edition

5. Object-Oriented Programming Terminology
Class: Pattern or blueprint for object
OOP: Object-oriented programming
Instance: Object created from class
Instantiation: Process of creating an object from a class
Attributes: Characteristics that describe the object
Behaviors: Methods and events that define how the object will act or react
Programming with Microsoft Visual Basic 2008, Fourth Edition

6. Object-Oriented Programming Terminology (continued)
Methods: Operations (actions) that object is capable of performing
Events: Actions to which an object can respond
Encapsulate: To contain
A class encapsulates all attributes and behaviors of object it instantiates
Programming with Microsoft Visual Basic 2008, Fourth Edition

7. Creating a Class Class statement: Used to define class
Includes attributes and behaviors of objects created from the class
Two types of classes used in VB applications:
Built-in classes, such as Button class
Programmer-defined classes
After class has been defined, it can be used to instantiate objects
Programming with Microsoft Visual Basic 2008, Fourth Edition

8. Creating a Class (continued)
Figure 11-2: Syntax and an example of the Class statement
Programming with Microsoft Visual Basic 2008, Fourth Edition

9. Creating a Class (continued)
Figure 11-6: Syntax and examples of instantiating an object from a class
Programming with Microsoft Visual Basic 2008, Fourth Edition

10. Example 1—Using a Class that Contains Public Variables Only
Willow Pools application from Chapter 10
Input: Length, width, and depth of pool
Calculates volume of water required
This program was coded with structure in Chapter 10
The structure will be replaced with a class
Programming with Microsoft Visual Basic 2008, Fourth Edition

11. Figure 11-4: Code for the Willow Pools application (with a structure)
Example 1—Using a Class that Contains Public Variables Only (continued)
Figure 11-4: Code for the Willow Pools application (with a structure)
Programming with Microsoft Visual Basic 2008, Fourth Edition

12. Figure 11-5: Add New Item dialog box
Example 1—Using a Class that Contains Public Variables Only (continued)
Figure 11-5: Add New Item dialog box
Programming with Microsoft Visual Basic 2008, Fourth Edition

13. Example 1—Using a Class that Contains Public Variables Only (continued)
Figure 11-6: Comments and Option statements entered in the RectangularPool.vb file
Programming with Microsoft Visual Basic 2008, Fourth Edition

14. Example 1—Using a Class that Contains Public Variables Only (continued)
Figure 11-8: Class definition, GetGallons function,
and btnCalc Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

15. Example 2—Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods
Disadvantages of Public variables in class:
Class cannot control values assigned to Public variables
Violates concept of encapsulation, in which class behaviors control class’s data
Programming with Microsoft Visual Basic 2008, Fourth Edition

16. Private Variables and Property Procedures
Private class variables: Can only be used within class
Not visible to rest of the application
Public property: Used to refer to (expose) Private variable for use by other parts of application
Property procedure: Creates a Public property
Programming with Microsoft Visual Basic 2008, Fourth Edition

17. Private Variables and Property Procedures (continued)
Used to create public property
Data type of Property must match Private variable
Can be set to ReadOnly or WriteOnly if necessary
Property procedure contains blocks of code:
Get block: Retrieves contents of Private variable
Set block: Used to assign value to Private variable
Blocks may be used individually or together
Programming with Microsoft Visual Basic 2008, Fourth Edition

18. Private Variables and Property Procedures (continued)
Figure 11-13: Completed Length Property procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

19. Constructors
Constructor: Class method whose purpose is to initialize class’s Private variables
Processed each time an object is created
Must be coded as Sub procedure named New
Class can have more than one constructor
Names are same, but parameter lists must differ
Default constructor: Constructor without parameters
Programming with Microsoft Visual Basic 2008, Fourth Edition

20. Constructors (continued)
Figure 11-14: Syntax and an example of a constructor
Programming with Microsoft Visual Basic 2008, Fourth Edition

21. Methods Other Than Constructors
A class may contain methods other than constructors
May be either Sub or Function procedures
Functions return value; Sub procedures do not
Rules for naming methods:
Name should be entered using Pascal case
First word in name should be verb
Subsequent words should be nouns and adjectives
Programming with Microsoft Visual Basic 2008, Fourth Edition

22. Methods Other Than Constructors (continued)
Figure 11-15: Syntax and an example of a method that is not a constructor
Programming with Microsoft Visual Basic 2008, Fourth Edition

23. Coding the Carpet Haven Application
Figure 11-16: Pseudocode for the Calculate button’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

24. Coding the Carpet Haven Application (continued)
Figure 11-17: Rectangle class definition and btnCalc Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

25. Coding the Carpet Haven Application (continued)
Figure 11-17: Rectangle class definition and btnCalc Click event procedure (continued)
Programming with Microsoft Visual Basic 2008, Fourth Edition

26. Example 3—Reusing a Class
Rectangle class from Ex. 2 will be reused to represent square pizza for Pizza Roma application
Square is rectangle with 4 equal sides
Reusing class saves programming time and money
Reuse is one advantage of OOP
Programming with Microsoft Visual Basic 2008, Fourth Edition

27. Example 3—Reusing a Class (continued)
Figure 11-19: Pseudocode for the btnCalc control’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

28. Example 3—Reusing a Class (continued)
Figure 11-20: Code entered in the btnCalc control’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

29. Lesson A Summary Class statement is used to define class
Defined classes are saved in files with a .vb extension
Objects are instantiated from defined class
Access to Private class variables should be done with Public properties
Public properties are created using Property procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

30. Lesson A Summary (continued)
Constructor initializes variables of a class
Constructor method must be named New
Default constructor has no parameters
Class can have methods other than constructors
Programming with Microsoft Visual Basic 2008, Fourth Edition

31. Lesson B Objectives After studying Lesson B, you should be able to:
Include a ReadOnly property in a class
Include a parameterized method in a class
Overload a method in a class
Programming with Microsoft Visual Basic 2008, Fourth Edition

32. Example 4—Using a Class that Contains a ReadOnly Property
ReadOnly keyword: Indicates that property’s value can be retrieved (read) but not set (written)
ReadOnly property gets its value from class instead of from application
Grade Solution application: Will return letter grade based on numeric grade
Programming with Microsoft Visual Basic 2008, Fourth Edition

33. Example 4—Using a Class that Contains a ReadOnly Property (continued)
Figure 11-28: Property procedure for the ReadOnly LetterGrade property
Programming with Microsoft Visual Basic 2008, Fourth Edition

34. Example 4—Using a Class that Contains a ReadOnly Property (continued)
Figure 11-29: DetermineGrade method entered in the class definition
Programming with Microsoft Visual Basic 2008, Fourth Edition

35. Example 4—Using a Class that Contains a ReadOnly Property (continued)
Figure 11-30: The message indicates that the property is ReadOnly
Programming with Microsoft Visual Basic 2008, Fourth Edition

36. Example 5—Using a Class that Contains Two Constructors
A class can have more than one constructor
Parameterized constructor: Constructor that requires parameters
FormattedDate constructor with parameters:
Requires values for two attributes in order to create the object: Month number and day number
Method’s signature = method name + optional parameter list
Programming with Microsoft Visual Basic 2008, Fourth Edition

37. Example 5—Using a Class that Contains Two Constructors (continued)
Figure 11-32: FormattedDate class definition
Programming with Microsoft Visual Basic 2008, Fourth Edition

38. Example 5—Using a Class that Contains Two Constructors (continued)
Figure 11-32: FormattedDate class definition (continued)
Programming with Microsoft Visual Basic 2008, Fourth Edition

39. Example 5—Using a Class that Contains Two Constructors (continued)
Figure 11-33: Click event procedures for the btnDefault
and btnParameterized controls
Programming with Microsoft Visual Basic 2008, Fourth Edition

40. Example 6—Using a Class that Contains Overloaded Methods
Basic features of an Employee object:
Two attributes: Employee number and name
Public properties to get and set attributes
Default constructor and parameterized constructor
Method to return gross pay of salaried employee
Method to return gross pay of hourly employee
Overloaded methods:
Methods with same name, but having different parameters
Programming with Microsoft Visual Basic 2008, Fourth Edition

41. Example 6—Using a Class that Contains Overloaded Methods (continued)
Figure 11-36: Box displaying one of the signatures of the ToString method
Programming with Microsoft Visual Basic 2008, Fourth Edition

42. Example 6—Using a Class that Contains Overloaded Methods (continued)
Figure 11-37: Interface for the ABC Company application
Programming with Microsoft Visual Basic 2008, Fourth Edition

43. Example 6—Using a Class that Contains Overloaded Methods (continued)
Figure 11-38: Pseudocode for the Calculate button’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

44. Example 6—Using a Class that Contains Overloaded Methods (continued)
Figure 11-39: Code entered in the selection structure’s false path
Programming with Microsoft Visual Basic 2008, Fourth Edition

45. Lesson B Summary
Use ReadOnly keyword to create property that can be retrieved but not set by application
To create parameterized method, enter parameters between parentheses in method definition
Overloaded methods share one name, but have different parameter lists
Programming with Microsoft Visual Basic 2008, Fourth Edition

46. Lesson C Objectives After studying Lesson C, you should be able to:
Create a derived class using inheritance
Override a method in the base class
Refer to the base class using the MyBase keyword
Programming with Microsoft Visual Basic 2008, Fourth Edition

47. Example 7—Using a Base Class and a Derived Class
Inheritance: Using one class to create another
Base class: Original class providing behaviors and attributes
Derived class: New class that inherits attributes and behaviors from base class
Inherits clause: Enables derived class to inherit from base class
Included in derived class’s Class statement
Programming with Microsoft Visual Basic 2008, Fourth Edition

48. Example 7—Using a Base Class and a Derived Class (continued)
Overriding:
Allows derived class to replace method inherited from its base class
Base class method header must include Overridable keyword
Derived class method header must include Overrides keyword
MyBase keyword: Used to refer to base class
MyBase.New ([parameterlist]): Used in derived class to call its base class’s constructor
Programming with Microsoft Visual Basic 2008, Fourth Edition

49. Example 7—Using a Base Class and a Derived Class (continued)
Figure 11-44: Code for the Employee base class and Salaried derived class
Programming with Microsoft Visual Basic 2008, Fourth Edition

50. Example 7—Using a Base Class and a Derived Class (continued)
Figure 11-44: Code for the Employee base class and Salaried derived class (continued)
Programming with Microsoft Visual Basic 2008, Fourth Edition

51. Example 7—Using a Base Class and a Derived Class (continued)
Figure 11-46: Pseudocode for the Calculate button’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

52. Lesson C Summary
Use Inherits clause to allow derived class to inherit attributes and behaviors of base class
Use Overridable keyword in base class method to indicate that derived class can replace it
Use Overrides keyword in derived class method to indicate that method has been replaced
Programming with Microsoft Visual Basic 2008, Fourth Edition