Visual Basic.NET How to Program (PDF)

192 Procedures Chapter 6
Class FrmMaximum uses a GUI consisting of three TextBoxes (txtFirst, txt-
Second and txtThird) for user input, a Button (cmdMaximum) to invoke the calculation
and four Labels, including lblMaximum, which displays the results. We create
these components visually, using the Toolbox, and change their properties in the Properties
window. Lines 7–21 are declarations indicating the name of each component.
Although these lines of code are actually part of the Visual Studio .NET generated code,
we display them to indicate the objects that are part of the form (as always, the complete
code for this program is on the CD-ROM that accompanies this book and at
www.deitel.com).
Line 5 indicates that class FrmMaximum Inherits from System.Windows.Forms.Form.
Remember that all forms inherit from class System.Windows.Forms.Form.
A class can inherit attributes and behaviors (data and methods)
from another class if that class is specified to the right of the Inherits keyword. We discuss
inheritance in detail in Chapter 9, Object-Oriented Programming: Inheritance.
FrmMaximum contains two programmer-defined methods. Method Maximum (lines
39–43) takes three Double parameters and returns the value of the largest parameter. Note
that this method definition looks just like the definition of a Function procedure in a
module. The program also includes method cmdMaximum_Click (lines 26–36). When
the user double-clicks a component, such as a Button, in Design mode, the IDE generates
a method that Handles an event (i.e., an event handler). An event represents a user
action, such as clicking a Button or altering a value. An event handler is a method that is
executed (called) when a certain event is raised (occurs). In this case, method
cmdMaximum_Click handles the event in which Button cmdMaximum is clicked.
Programmers write code to perform certain tasks when such events occur. By employing
both events and objects, programmers can create applications that enable more sophisticated
user interactions than those we have seen previously. Event-handler names created
by the IDE begin with the object’s name, followed by an underscore and the name of the
event. We explain how to create our own event handlers, which can be given any name, in
Chapter 12, Graphical User Interface Concepts: Part 1.
When the user clicks cmdMaximum, procedure cmdMaximum_Click (lines 26–36)
executes. Lines 31–33 retrieve the values in the three TextBoxes, using the Text property.
The values are converted implicitly to type Double and stored in variables value1,
value2 and value3.
Line 35 calls method Maximum (lines 39–43) with the arguments value1, value2
and value3. The values of these arguments are then stored in parameters valueOne,
valueTwo and valueThree in method Maximum. Maximum returns the result of the
expression on line 42, which makes two calls to method Max of the Math class. Method
Max returns the largest of its two Double arguments, meaning the computation in line 42
first compares valueOne and valueTwo, then compares the value returned by the first
method call to valueThree. Calls to methods, such as Math.Max, that are defined in a
class in the FCL must include the class name and the dot (.) operator (also called the
member access operator). However, calls to methods defined in the class that contains the
method call need only specify the method name.
When control returns to method cmdMaximum_Click, line 35 assigns the value
returned by method Maximum to lblMaximum’s Text property, causing it to be displayed
for the user.