1. Microsoft Visual Basic 2005: Reloaded Second Edition
Chapter 10
Creating Classes and Objects

2. Objectives After studying this chapter, 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 default and parameterized constructors
Microsoft Visual Basic 2005: Reloaded, Second Edition

3. Objectives (continued)
Include methods in a class
Overload the methods in a class
Create a derived class using inheritance
Microsoft Visual Basic 2005: Reloaded, Second Edition

4. Classes and Objects
Object-oriented programs are based on objects that are instantiated (created) from classes
Properties: attributes that describe the object
Methods: behaviors that allow the object to perform tasks
A class encapsulates (contains) properties and methods
VB.Net has many built-in classes
E.g. textbox, form, labels
You can define your own classes
E.g. Student, Book, DVD
An Object is a specific instance of a class
E.g. a specific student
Microsoft Visual Basic 2005: Reloaded, Second Edition

5. Defining a Class (continued)
Do with SimpleSquare – tryclass3
Microsoft Visual Basic 2005: Reloaded, Second Edition

6. Defining a Class (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

7. Defining a Class (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

8. Defining a Class Class statement: defines a class
Use Pascal casing for the class name
Define attributes and behaviors of the class within the class
Code editor automatically adds the Class statement
Microsoft Visual Basic 2005: Reloaded, Second Edition

9. Defining a Class (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

10. Defining a Class (continued)
In the file using the class
Microsoft Visual Basic 2005: Reloaded, Second Edition

11. Example 1 – Using a Class that Contains Public Variables Only
Microsoft Visual Basic 2005: Reloaded, Second Edition

12. Using a Class that Contains Public Variables Only (continued)
Any class variable declared with Public keyword can be accessed by any application that contains an instance of the class
Use Pascal case for Public variables in a class
Microsoft Visual Basic 2005: Reloaded, Second Edition

13. Using a Class that Contains Public Variables Only (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

14. Using a Class that Contains Public Variables Only (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

15. Using a Class that Contains Public Variables Only (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

16. Using a Class that Contains Public Variables Only (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

17. Using a Class that Contains Public Variables Only (continued)
Access an object’s attributes using the dot operator:
objectVariable.attribute
Disadvantages of Public variables in a class:
The class cannot control the values assigned to the variables
Violates the concept of OOP encapsulation
Microsoft Visual Basic 2005: Reloaded, Second Edition

18. Example 2 – Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods
Microsoft Visual Basic 2005: Reloaded, Second Edition

19. Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods (continued)
Class variables declared with Private keyword:
Can only be used by the class
Are hidden from the rest of the application
Names should start with underscore _
Application can change class variable values only by using the class’s methods
Microsoft Visual Basic 2005: Reloaded, Second Edition

20. Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods (continued)
A Public method in the class for manipulating a class variable
Exposes a Private class variable as a Property for use by the application
Microsoft Visual Basic 2005: Reloaded, Second Edition

21. Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

22. Using a Class that Contains a Private Variable, a Property Procedure, and Two Methods (continued)
Get block: contains code to allow an application to retrieve the contents of the variable exposed as a Property
Set block: allows an application to assign a value to the variable exposed as a Property
ReadOnly keyword: makes a property readable but not settable (no Set block)
WriteOnly keyword: makes a property settable but not readable (no Get block)
Microsoft Visual Basic 2005: Reloaded, Second Edition

23. Constructors Constructor:
A method whose instructions are processed automatically when an object is instantiated from a class
Purpose is to initialize the class’s Private variables
Method name must be New
May or may not have parameters
Default constructor: a constructor with no parameters
Microsoft Visual Basic 2005: Reloaded, Second Edition

24. Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

25. Methods Other than Constructors
Methods in a class can be Sub or Function procedures
Use Pascal casing for method names
Microsoft Visual Basic 2005: Reloaded, Second Edition

26. Methods Other than Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

27. Methods Other than Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

28. Methods Other than Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

29. Methods Other than Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

30. Example 3 – Using a Class that Contains Two Constructors
Microsoft Visual Basic 2005: Reloaded, Second Edition

31. Using a Class that Contains Two Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

32. Using a Class that Contains Two Constructors (continued)
Parameterized constructor: a constructor method that contains parameters
Method signature: method name and parameter list
Best practices:
Parameterized constructor should set the values of the class variables using the class’s Property procedures to take advantage of any validation code in the Property procedures
Microsoft Visual Basic 2005: Reloaded, Second Edition

33. Using a Class that Contains Two Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

34. Using a Class that Contains Two Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

35. Using a Class that Contains Two Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

36. Using a Class that Contains Two Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

37. Using a Class that Contains Two Constructors (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

38. Example 4 – Using a Class that Contains Overloaded Methods
Microsoft Visual Basic 2005: Reloaded, Second Edition

39. Using a Class that Contains Overloaded Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

40. Using a Class that Contains Overloaded Methods (continued)
Overloaded methods: two or more methods that have the same name but different parameters
Constructors can be overloaded
To overload a non-constructor method, use the Overloads keyword in the method declaration
Many of VB’s built-in methods are overloaded, as shown in the Intellisense feature:
Microsoft Visual Basic 2005: Reloaded, Second Edition

41. Using a Class that Contains Overloaded Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

42. Using a Class that Contains Overloaded Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

43. Using a Class that Contains Overloaded Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

44. Using a Class that Contains Overloaded Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

45. Using a Class that Contains Overloaded Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

46. Example 5 – Using a Base Class and a Derived Class
Inheritance: one class can be created from another class
Base class: the original class
Derived class: the new class created from the base class
Inherits keyword: specifies the base class
Microsoft Visual Basic 2005: Reloaded, Second Edition

47. Using a Base Class and a Derived Class (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

48. Using a Base Class and a Derived Class (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

49. Using a Base Class and a Derived Class (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

50. Using a Base Class and a Derived Class (continued)
Derived class contains all of the attributes and behaviors of the base class
Derived class may also contain its own attributes and behaviors
Overridable keyword: indicates the base class method can be overridden by the derived class
Overrides keyword: indicates that a method in the derived class overrides the method with the same name in the base class
Microsoft Visual Basic 2005: Reloaded, Second Edition

51. Using a Base Class and a Derived Class (continued)
MyBase keyword: refers to the base class
MyBase.New: tells the computer to process the code in the base class’s constructor
Microsoft Visual Basic 2005: Reloaded, Second Edition

52. Using a Base Class and a Derived Class (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

53. Programming Tutorial
Microsoft Visual Basic 2005: Reloaded, Second Edition

54. Programming Example
Microsoft Visual Basic 2005: Reloaded, Second Edition

55. Summary Objects are instantiated from classes
Classes encapsulate their attributes and behaviors
Class Public data members and methods are exposed to any application that creates an object from the class; class Private members are not
Class Public properties allow an application to manipulate private class data members
ReadOnly keyword allows a property to be retrieved but not changed
Microsoft Visual Basic 2005: Reloaded, Second Edition

56. Summary (continued)
WriteOnly allows a property to be changed but not retrieved
Property Get block implements retrieving the property value
Property Set block implements changing the property value
Constructor: a sub procedure named New that is processed when an object is created from the class
Default constructor: has no parameters
Microsoft Visual Basic 2005: Reloaded, Second Edition

57. Summary (continued)
Derived class: a new class created from another class (the base class)
Overridable keyword: indicates that a method in the base class can be overridden in the derived class
Overrides keyword: indicates that a method in the derived class overrides a method of the same name in the base class
Inherits clause: creates a derived class
MyBase keyword: refers to the base class
Microsoft Visual Basic 2005: Reloaded, Second Edition