Visual Basic.NET How to Program (PDF)

Chapter 8 Object-Based Programming 299
Before discussing classes in detail, we review how to add classes to a project in Visual
Studio. By now, you are familiar with adding a module to a project. The process of adding
a class to a project is almost identical to that of adding a module to a project. To add a class
to a project, select Project > Add Class. Enter the class name in the Name text field and
click the Open button. Note that the class name (ending with the .vb file extension)
appears in the Solution Explorer below the project name.
The following application consists of class CTime (Fig. 8.1) and module modTime-
Test (Fig. 8.2). Class CTime contains the information needed to represent a specific time;
module modTimeTest contains method Main, which uses an instance of class CTime to
run the application.
In Fig. 8.1, lines 4–5 begin the CTime class definition, indicating that class CTime
inherits from class Object (of namespace System). Visual Basic programmers use
inheritance to create classes from existing classes. The Inherits keyword (line 5) followed
by class name Object indicates that class CTime inherits existing pieces of class
Object. If the programmer does not include line 5, the Visual Basic compiler includes it
implicitly. Because this is the first chapter that exposes classes, we include these declarations
for the classes in this chapter; however, we remove them in Chapter 9. A complete
understanding of inheritance is not necessary to the understanding of the concepts and programs
in this chapter. We explore inheritance in detail in Chapter 9.
1 ' Fig. 8.1: CTime.vb
2 ' Represents time in 24-hour format.
3
4 Class CTime
5 Inherits Object
6
7 ' declare Integer instance values for hour, minute and second
8 Private mHour As Integer ' 0 - 23
9 Private mMinute As Integer ' 0 - 59
10 Private mSecond As Integer ' 0 - 59
11
12 ' Method New is the CTime constructor method, which initializes
13 ' instance variables to zero
14 Public Sub New()
15 SetTime(0, 0, 0)
16 End Sub ' New
17
18 ' set new time value using universal time;
19 ' perform validity checks on data;
20 ' set invalid values to zero
21 Public Sub SetTime(ByVal hourValue As Integer, _
22 ByVal minuteValue As Integer, ByVal secondValue As Integer)
23
24 ' check if hour is between 0 and 23, then set hour
25 If (hourValue >= 0 AndAlso hourValue < 24) Then
26 mHour = hourValue
27 Else
28 mHour = 0
29 End If
Fig. 8.1 Abstract data type representing time in 24-hour format (part 1 of 2).