Getting Comfortable with ObjectOriented Programming

1
Chapter 9

• Getting Comfortable with Object-Oriented
  Programming

2
Introduction

• In this chapter, you will learn about Visual
  Basic object-oriented programming
  
• This will include learning about a number of
  new programming concepts.
  
• Abstraction
• Encapsulation
• Inheritance
• Polymorphism
• You will also learn more about working with
  objects
  
• Examples of objects provided by VB and the .NET
  Framework
  
• How to create your own objects by creating custom
  classes
  
• How to store and protect the data stored in your
  custom objects and to provide access to it
  through the implementation of properties and
  methods.
• Specifically we will learn
• About object-oriented programming concepts and
  terms
  
• How to define custom classes
• How to instantiate objects based on your custom
  classes
  
• How to set up method overloading within custom
  classes
  
• How to provide controlled access to the data
  stored within objects

3
Object-Oriented Programming

• OOP is a major feature of Visual Basic. In OOP,
  data and code are grouped together as objects.
  
• Objects are self-contained entities.
• For example
• the forms and controls each represent a different
  type of object
  
• Objects can store data and provide a carefully
  designed interface for accessing their data and
  exercising their functionality.
  
• This access is provided through object properties
  and methods
  
• Objects have the ability to validate any data
  that is passed to them and to reject it if it
  fails to meet certain predefined criteria.
  
• Properly defined objects make applications more
  reliable
  
• Because objects control access to themselves and
  to their data, you can think of them as being
  like a black box. This means that you may know
  what a given object does and to use it. But in
  fact you dont really know how it was put
  together or what makes it work.
• For example
• Think about the Form object. DO YOU KNOW HOW
  THEY WORK?
  
• Visual Basic makes forms available to you as
  classes. Forms have their own properties, such
  as background color and foreground color, such as
  maximize and minimize, which you do not have to
  define.
• All you know is how to add new controls to your
  forms in order to modify them.
  
• Forms also have events like the Load event, to
  which they automatically know how to respond
  

4
OOP Terms

• In visual Basic, objects are created from
  classes. A class provides a template that
  specifies data, properties, and methods that are
  available for working with data.
• A class is a template that can be used to
  instantiate objects
  
• Visual Basic provides access to a large
  collection of classes. The code behind a Form
  comes from the Form class, which is located in
  the System.Windows.Forms namespace.
• A namespace is an organized collection of
  classes.
  
• The .NET Framework provides access to numerous
  namespaces, which it makes available in its class
  Library.
  
• Examples of other classes include
  System.Windows.Forms.Button, where the Button
  class is defined
  
• System.Windows.Forms.CheckBox, where the CheckBox
  class is defined.
  
• We have been working with classes in all of the
  chapter game projects throughout the book.
  
• We should have realized this due to the syntax
• Public Class Form1
• End Class

5
OOP Terms

• The first statement
• Public Class Form1
• Publically defines a new Class called Form1, and
  the last statement
  
• End Class
• Defines the end of the class.
• Visual Basic makes things easy on the programmer
  by hiding much of the work that is truly does
  behind the scenes.
  
• If you wish to see everything that is happening
  you can simply click on the Show All File button
  in the Solution Explorer, expand the Form1.vb
  entry and click on the Form1.designer.VB entry.
• Now we wish to learn a littler about what is
  going on in this Form.
  
• First look at these statements
• Partial Public Class Form1
• Inherits System.Windows.Forms.Form
• As we can see the form that Visual Basic has
  automatically provided you with is actually
  inherited (or cloned) from the Form class stored
  in the System.Windows.Forms namespace

6
OOP Terms

• OOP simplifies the coding process by hiding many
  of the complexities of and application from the
  programmer. In addition, it promotes the reuse
  of code by making it possible to define a class
  once and then use it over and over again to
  create as many objects as required.
• Visual Basic also allows you to create you own
  custom classes, which is much of the focus of
  this chapter.
  
• Now we look at key OOP terms
• Abstraction, encapsulation, inheritance, and
  polymorphism
  
• Abstraction
• This is the process by which you define in a
  program code a logical represenstation of a
  class. This includes the specification of the
  base functionality belonging to the class and the
  definitions of properties and methods that are
  required for the proper operation of the class.
• Returning to the Form class as an example, base
  properties include the Text, Size, BackColor, and
  ForeColor properties

7
OOP Terms

• Now we look at key OOP terms
• Abstraction, encapsulation, inheritance, and
  polymorphism
  
• Encapsulation
• Is the process whereby you package the base
  functionality of a class and provide access to
  the features of the class through a collection of
  properties and methods. When an object is
  created from a particular class, the inner
  workings of the object are hidden from the
  programmer.
• Again, consider the Form class. You dont know
  what the code behind it looks like or how it is
  written. You cannot directly access its internal
  code or data. You only know that to work with
  the form, you modify its properties, execute its
  methods, and provide code for its events. How
  the Form class was designed to make all these
  things work is hidden from you. Thus,
  encapsulation helps simply program development
  and hide complexity.
• It also includes data protection, meaning that
  code can be added to process and verify the
  validity of any data passed to an object to
  ensure that it meets certain criteria
• Inheritance
• Is the process whereby one class is derived from
  another. When you define a new class in your
  program code, such as when you define a new class
  for Visual Basic form, the class that you create
  inherits its features from a base class, which in
  the case of this example is System.Windows.Froms.F
  orm

8
OOP Terms
9
Classes and Objects

• A class is a template that defines all of the
  attributes, properties, methods, and events
  required to create an object. Using abstraction,
  programmers develop a design for a class. Using
  encapsulation, they implement a class structure.
  One class can be established by inheriting it
  from another class. Finally, using overloading,
  programmers can extend procedures within classes
  to handle different sets of arguments
• For example, you might analogize the relationship
  between a class and an object as being akin to
  the relationship between a mold and Jell-O. The
  mold defines an overall form and shape of the
  Jell-O. Once poured into that mold and
  solidified, the new instance of Jell-O takes on
  the basic qualities of the mold. As another,
  more specific, example, the Form object is an
  instance of the Form class and has access to all
  the features provided by that class.
• In order to use a class, you must first
  instantiate it. The instantiation process varies
  depending on the class your are working with.
  
• For example, to instantiate a Button or CheckBox
  class (control), all that you have to do is place
  a copy of it on a form. However, if you create
  your own custom class as part of a Visual Basoc
  application, you will create a new class instance
  using the New keyword.

10
Creating a New Class

• There are a number of ways that you can create a
  new class. First, as you have already seen,
  anytime you create a new Visual Basic
  application, a new class is created for you based
  on the System.Windows.Forms.Form class. In the
  case of the Dice Poker game, the new class was
  named frmMain.
• You can also add a new class to an application by
  simply inserting its declaration under the
  existing class definition in the code editor
  
• Public Class frmMain
• what ever is happening within the form
• End Class
• Public Class Greeting
• whatever is happening within the NEW CLASS
• End Class
• A data member is a variable that is defined
  within a class.
  
• Ordinarily, data members are hidden from the
  outside world and used only within the class.
  However, data members can e made directly
  accessible from outside the class using the
  Public keyword.

11
Creating a New Class

• Public Class Form2
• Private Sub Form2_Load(ByVal sender As
  System.Object, _
  
• ByVal e As System.EventArgs) Handles
  MyBase.Load
  
• Dim SampleGreeting As New Greeting
• MessageBox.Show(SampleGreeting.Message)
• End Sub
• End Class
• Public Class Greeting
• Public Message As String Hello World!
• End Class
• The statement that you added to the Greeting
  class sets up a data member. Data members are by
  default private, meaning they cannon be accessed
  from outside of the class itself.
• However, in this example, I made the data member
  public just so that we could have a working
  example that does something
  
• By itself, the Greeting class does not do
  anything. You need to instantiate an object
  based on the class in order to be able to do
  something. The first statement in the Form2_Load
  event procedure instantiates a new object named
  SampleGreeting using the New keyword.
• Then the MessageBox.Show method is used to access
  and display the SampleGreeting objects data
  member.
  

12
Creating a New Class

• Another way to add a new class to your
  application is to create it in its own code
  module, separate and distinct from the forms
  class code, as demonstrated in the following
  example.
• Click on the Add Class option located on the
  Project menu.
  
• Select the Class icon and type a name for the
  class that you are about to define in the Name
  field, and then click on the Add button.
  
• A new tab will appear in the code editor, and the
  opening and closing class statements for the new
  class will be displayed.
  
• Modify the program code
• Type Public Message As String Hello World!
  between the existing class scope

13
Understand Data Memebers

• In the previous two examples that demonstrated
  how to create a new class, you learned how to
  define a new data member. By default, class data
  member are private, which means that they cannot
  be seen or accessed from outside the class.
  However, as the examples showed, you can make
  data member publicly available. You can also
  declare them as protected, in which case they can
  be accessed only by derived classes of the base
  class.
• Although it is possible for data member to be
  accessed from outside the class, they are not
  properties of the class. However, when made
  publicly available, you lose the ability to
  restrict control over data members. Instead of
  making data member public and set up properties
  to provide controlled access to any data stored
  in your classes.

14
Defining Class Properties

• Properties can be used to set and retrieve
  values. To set up a property within a class, you
  start declaring a private data member and then
  define a public procedure that provides
  controlled access to the value stored in the data
  member. The following types of property
  procedures are supported by Visual Basic.
• Get Retrieves the value assigned to a property
• Set Assigns a value to the property
• Public Class Greeting
• Private Message As String
• Public Property Text() As String
• Get
• Return Message
• End Get
• Set (ByVal Value as String)
• Message Value
• End set
• End property
• End Class
• Before you can use an object, you must
  instantiate it. You can then reference its
  properties by specifying the name of the object,
  followed by a period and the name of a particular
  property. When you assign a value to the Text
  property, the Set property procedure block
  executes. Note that the Set property procedure
  is automatically passed an argument named Value
  in which the data passed to the Set property
  procedure is stored. All you have to do to
  finish up the property assignment is to assign
  the data stored in Value to a private data member
  (Message).

15
Defining Class Properties

• The following statements demonstrate how to
  derive an object from the Greeting class and then
  set and retrieve the objects Text property. The
  first statement shown below instantiates a new
  object named SampleGreeting that is derived from
  the Greeting class. The next two statements
  assign a value to the objects Text property and
  then use the MessageBox.Show method to
  demonstrate that the property assignment was
  successful.
• Dim objSample As New Greeting
• objSample.Text Good Morning
• MessageBox.Show(objSample.Text)
• The advantage of setting up properties, compared
  to simply making data members public, is that you
  can add code to your property procedure to
  perform data validation before allowing access to
  the data associated with a particular property.
• For example
• Using an if statement can limit access to the
  value

16
Adding Class Methods

• In addition to defining data members and
  properties in your custom classes, you can add
  your own methods. To add a method to a class,
  all you have to do is add a Sub or Function
  procedure.
• REFER TO BOOK FOR THE CODE BEING DISCUSSED
• The Greeting class has been modified by the
  addition of a method named PostWelcome().
  
• When executed, the method uses the
  MessageBox.Show method to display a text message.
  Like all of the other object methods that you
  have worked with, you can call on this method by
  specifying the name of its object followed by a
  period and the name of the method.
• One of Visual Basics OOP features is
  polymorphism. Polymorphism is implemented in
  Visual Basic by providing the ability to overload
  class methods.
• When you overload a method Visual Basic makes
  sure that the correct method is executed based on
  the argument list that is passed in.
  
• Notice that the keyword OVERLOADS is used within
  a Sub procedure during its definition.
  
• NOTE For overloading to work, you must provide
  each overloaded procedure with a unique set of
  arguments. You can vary the argument list by
  changing the number of arguments that are passed
  or by changing the types of argument passed, or a
  combination of both.

17
Inheriting from Another Class

• Anytime Visual Basic provides you with a new
  form, behind the scenes the new instance of the
  form is created by specifying the Inherits
  keyword and the name of the class to is to be
  inherited from.
• The derived class inherits its features from the
  base class. Anytime you add a control to a form,
  Visual Basic automatically handles the setup of
  the new instance of the control (object). In
  addition to inheriting from classes provided to
  you by Visual Basic and the .NET Framwork, you
  can derive new classes from classes that you
  create. For Example
• Public Class GreetingTwo
• Inherits Greeting
• Private Warning As String Action not
  allowed.
  
• Public ReadOnly Property ErrMsg() As String
• Get
• Return Warning
• End Get
• End Property
• End Class
• Youll notice that the GreetingTwo class not only
  inherits all of the proplerties and methods
  belonging to the Greetings class, but also
  extends this functionality by adding a new
  property of its own. In addition, youll notice
  that the new ErrMsg property has been made
  read-only, meaning that it can be retrieved but
  not modified by calling statements.

18
Lab 9 Rock Paper Scissors

• 100 pts
• Cannot be downloaded from the books websites
• Any instance where I think this has happened a 0
  will result.
  
• Every class used Should be represented in the Tab
  bar. This means no class should be defined under
  the From Class
  
• Worth 75
• Adding own comments/programming logic/ user
  interface 10
  
• Each Successfully completed challenge 5 each

19
Summary

• In this chapter, you learned about Visual Basic
  classes and namespaces. You learned how to
  create your own custom classes. You then learned
  how to instantiate new objects using your custume
  classes. The custom objects that you created
  included their own properties and methods. You
  also learned how to overload object methods and
  how to provide controlled access to data stored
  within your objects.