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Objectives

• Define a class
• Add properties to a class
• Instantiate an object from a class that you
  define
• Add Property procedures to a class

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Objectives (continued)

• Create constructors
• Add methods to a class
• Include data validation in a class

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Classes and Objects

• Object-oriented programs are based on objects,
  which are instantiated from classes
• A class contains (encapsulates) the properties
  (attributes) that describe a person, place, or
  thing
• Examples Student, School, Book
• An object is a specific instance of a class
• Examples You are a student that goes to a
  specific school and are using this specific book
• VB has many built-in objects such as Textbox
  controls, Forms, and Labels

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Defining a Class

• Use the Class statement
• Begins with keyword Class and ends with End Class

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HOW TO
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Defining a Class (continued)
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Defining a Class (continued)
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HOW TO
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Example 1 Using a Class that Contains
Properties Only

• Assume that the sales manager at Sweets Unlimited
  wants an application that allows him to save each
  salespersons
• name
• quarterly sales amount
• quarterly bonus amount in a sequential access
  file
• The bonus amount is calculated by multiplying the
  sales amount by 5

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Example 1 Using a Class that Contains
Properties Only (continued)

• Figure 11.6 shows a sample run of the Sweets
  Unlimited application

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Example 1 Using a Class that Contains
Properties Only (continued)

• Figure 11.7 shows the Salesperson class defined
  in the Salesperson.vb file

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Example 1 Using a Class that Contains
Properties Only (continued)
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Example 1 Using a Class that Contains
Properties Only (continued)
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Example 2 Using a Class that Contains
Properties and Methods

• Create a class named Square and then use in the
  Area application
• Square class creates an object that can calculate
  and return the area of a square, using side
  measurement provided by application
• Figure 11.9 shows a sample run of the Area
  application
• Figure 11.10 shows the Square class defined in
  the Square.vb file

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Example 2 Using a Class that Contains
Properties and Methods (continued)
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Example 2 Using a Class that Contains
Properties and Methods (continued)
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Example 2 Using a Class that Contains
Properties and Methods (continued)

• Creating a Public Property
• Declare a class-level private variable to hold
  the value to be stored by the property
• Data type of property and private variable must
  match each other
• Heading begins with keywords Public Property

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Example 2 Using a Class that Contains
Properties and Methods (continued)

• Code Get block to retrieve contents of private
  variable
• Code Set block to change contents of private
  variable
• Ends with keywords End Property

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HOW TO
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Constructors

• Constructor? method whose instructions the
  computer processes, automatically, each time an
  object is created (instantiated) from the class
• Constructor begins with Public Sub New followed
  by a set of optional parameters
• Parameter list may be empty - New()
• No parameters is called default constructor
• Every class should have at least one constructor
  and may have several

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HOW TO
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Methods Other Than Constructors

• Sub methods do not return a value
• Function methods return a value to the calling
  procedure

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Methods Other Than Constructors (continued)
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Example 3 Using a Class that contains two
Constructors and Data Validation

• Create a class named MyDate and then use in the
  Personnel application
• MyDate class creates an object that returns a
  month number, followed by a slash, and a day
  number
• Figure 11.15 shows a sample run of the Personnel
  application
• Figure 11.16 shows the MyDate class defined in
  the MyDate.vb file

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Example 3 Using a Class that contains two
Constructors and Data Validation (continued)
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Example 3 Using a Class that contains two
Constructors and Data Validation (continued)
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Example 3 Using a Class that contains two
Constructors and Data Validation (continued)
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Example 3 Using a Class that contains two
Constructors and Data Validation (continued)
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Programming Example Kessler Landscaping
Application

• Monica Kessler, the owner of Kessler Landscaping,
  wants an application that she can use to estimate
  the cost of laying sod
• Use a MyRectangle class in this application

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TOE Chart
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User Interface
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Objects, Properties, and Settings
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Tab Order
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Pseudocode

• btnExit Click event procedure
• 1. close application
• btnCalc Click event procedure
• 1. declare a MyRectangle object
• 2. assign the length and width to the
  MyRectangle objects properties
• 3. assign the sod price to a variable
• 4. calculate the area of the rectangle
• 5. calculate the total price of the sod
• 6. display the total price of the sod in
  lblTotalPrice
• txtLength, txtWidth, and txtPrice TextChanged
  event procedures
• 1. clear the contents of the lblTotalPrice
  control

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Code (MyRectangle.vb file)
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Code (MyRectangle.vb file) (continued)
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Code (Kessler Form.vb file)
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Code (Kessler Form.vb file) (continued)
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Summary

• The objects used in an object-oriented program
  are created, or instantiated, from classes
• A class contains (encapsulates) the properties
  (attributes) that describe the object it creates,
  and the methods (behaviors) that allow the object
  to perform tasks
• In Visual Basic .NET, you can create objects from
  classes that you define with the Class statement

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Summary (continued)

• Good programming practice to enter Option
  Explicit On and Option Strict On statement in
  both the form file and class file
• The first letter in the class name, as well as
  the first letter in any subsequent words in the
  name, should be capitalized
• The properties in a class should be assigned a
  name composed of one or more words, with the
  first letter of each word being capitalized
• You should use nouns and adjectives to name a
  property

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Summary (continued)

• Methods in a class should be assigned a name
  composed of one or more words, with the first
  letter of each word being capitalized
• You should use a verb for the first word in the
  name, and nouns and adjectives for any subsequent
  words in the name
• Variables declared using the Public keyword in a
  class definition can be accessed by any
  application that uses an object created from the
  class
• Most classes contain properties and methods

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Summary (continued)

• When an application needs to assign data to or
  retrieve data from a Private variable in a class,
  it must use a Public property to do so
• You create a Public property using a Property
  procedure
• The Get block in a Property procedure allows an
  application to access the contents of the classs
  Private variables
• The Set block in a Property procedure allows an
  application to assign values to the classs
  Private variables

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Summary (continued)

• A class can have one or more constructors
• All constructions are Sub procedures
• The default constructor is automatically
  processed when an object is created from the class