Advanced Interactive Programming Object Oriented Programming OOP Objects and Classes

1
Advanced Interactive ProgrammingObject Oriented
Programming (OOP)- Objects and Classes -
2
Outline

• Concepts and terminology
• Object properties, methods and events
• Class instance
• Message
• Information hiding
• Encapsulation
• Data abstraction
• Object oriented programming paradigm
• OOP in VB
• Create and manipulate classes, objects, methods
  and properties

3
Objects

• Originated from real world observation - an
  object is a dog, a car or a chair, a desk, etc.
• In theory, we should be able to describe all of
  our experience and surroundings as objects
• All objects have attributes and behaviors

4
Attributes

• Attributes are characteristics
• Use attributes to describe the states and
  appearance of an object
• Appearance may include size, shape and color
• States may include temperature, speed, on/off,
  etc.
• For example - a car is in Red with engine size
  1.1 made by Ford these characteristics represent
  values assigned to attributes

5
Behaviors

• Behaviors are actions
• Use behaviors to describe the active behavior
  of an object
• Example - a car, an automatic door, a mobile
  phone
• These behaviors become evident if a specific
  action of the object is activated

6
Software Objects

• Modelled after real-world objects in that they
  too have state and behavior
• In OOP, an object is a structure that contains
  both data and methods that manipulate that data
• The data represent the attributes of the object
• An object is responsible for its own data
• An object is active, not passive it does things
• The methods implement the behaviors
• An object can expose data and methods to other
  objects

7
Classes

• Originated from real world observation many
  objects are of the same kind
• A class is a blueprint, a type or say a pattern,
  which defines the implementation of a particular
  kind of object.
• You might think of a class as a cookie cutter and
  a set of objects as cookies created with that
  cutter (class)

Class
Object
8
A Conceptual View about Classes

• Classes describe the common characteristics of
  the same kind of objects. These include
• The attributes, represented by variables
• The behaviors, represented by methods
• The interface - how other objects access these
  variables and/or methods such as the front panel
  of a washing machine, a microwave, or the remote
  control of a TV
• The events

9
A Practical View about Classes

• In practice, a class is a unit or module of code
  that may be used to define the attributes and
  behaviors of a kind of object
• The commonly used functions have all been
  implemented in software and further packaged as
  classes. Classes pre-exist and are stored in
  special files called libraries
• Application programmers only need to write
  application specific classes also referred to
  as programmer-defined types

10
Objects versus Classes

• A class is a data type or blueprint or template,
  not an object itself. For example a blueprint of
  a car is not a car
• An object is a concrete, particular entity, which
  is generated by instantiating a class
• An object is an instance of a class
• An object has all the attributes, behaviours,
  interfaces and events defined by the class

11
Example A Time Class and Object

• You could create a class representing time
• It would have attribute data
• Hour, minute and second
• And methods
• Set time to a specific time
• Convert time to standard time
• Convert time to universal time
• Many time objects - a real world clock, watch,
  timer in microwave, ant real time control systems

12
Example Code
13
Information Hiding

• A principle for the hiding of design decisions
  (implementation details) in a computer program
  that are most likely to change
• A means to protect other parts of a program from
  change if the design decisions are changed
• A program needs to provide a stable interface
  which shields the implementation details from how
  the program is interacted by other programs
• Many ways encapsulation and polymorphism

14
Encapsulation

• A technique for wrapping up data and associated
  behaviors into a single unit or object, e.g. a
  class
• Provide controlled interface (namely, the class's
  public methods) to manipulate an objects data
• Support information hiding the interfaces
  specify how a program may use the object but
  hiding the implementation details.
• It is not necessary for a programmer who uses a
  pre-existing class to know or understand its code
  structure

15
Data Abstraction

• A mechanism or practice to describe and model
  data types, e.g. classes, in terms of behaviours
  without concern for how those behaviours are
  actually implemented
• It enforces a clear separation between the
  abstract properties of a data type and the
  concrete details of its implementation
• The abstract properties are those that are
  visible to client code, i.e. the interface, while
  the concrete implementation is kept entirely
  private, which has no impact on the interface
• Data abstraction leads to inheritance and
  polymorphism

16
Messages

• A single object will not do much, therefore not
  useful
• Objects interact with each other by sending
  messages
• A message is information which is sent from a
  source to a receiver
• Message content

The object to which the message is addressed
(YourCar) The name of the method to perform
(changeGears) Any parameters needed by the
method (secondGear)
17
CTime Class Revisited
18
A Class Interface CTime Class

• The implementation code of a class is
  encapsulated
• Attributes appear at the interface of the class
  as properties
• Actions appear at the interface of the class as
  methods

19
Access to Properties and Methods

• Only public variables and methods are accessible
• Programmers use dot notation to gain access to
  properties and methods
• To assign a value to a property of an object
  object.property value
• To run a method of an object object.method

20
Differences between Objects and Classes
Properties and Methods

• Class variables vs. instance variables
• A class variable contains information that is
  shared by all instances of the class
• A instance variable contains information for the
  particular object
• Class methods vs. instance methods
• A class method is invoked without reference to a
  particular object. Class methods affect the class
  as a whole, not a particular instance of the
  class
• A instance method is invoked with reference to a
  particular object. It inspects or changes the
  state of a particular object
• VB does not distinguish this

21
Object-Oriented Programming

• OOP is a programming paradigm, i.e. a style of
  programming
• Building blocks (Unit of abstraction) are
  classes, objects
• Each object is capable of receiving messages,
  processing data, and sending messages to other
  objects
• A computer program is composed of a collection of
  structured objects

22
Object-Oriented Programming

• Programming in object-oriented languages is
  called object-oriented programming, C, C,
  JAVA, etc.
• VB is an object-oriented programming language
• Beware VB ?OOP, OOD ? OOP and VB is not a pure
  OOP language
• Important objects and classes (properties,
  methods, and events) are only one half of the
  object-oriented programming equation. True
  object-orient programming requires objects to
  support three qualities encapsulation,
  inheritance, and polymorphism.

23
Why Objects-oriented

• Object-oriented programming models the world as a
  collection of objects
• Intuitive, since this is how we tend to think in
  terms of the world (e.g., people, animals, cars,
  etc.)
• And how we construct real world artefacts, cars,
  houses, etc.
• Complex problems can be solved by Divide and
  conquer approach
• Enhance reusability, maintenance and flexibility
  you only write a very small portion of the code
  for your application, most code is from reusable
  objects in library

24
Approximate Terminology

• class user defined type an abstract data type
• object instance (of a class)
• state attribute property
• field variable
• method function
• behaviour operation action
• sending a message to an object calling a method
• These are all approximately true

25
Creating Classes in VB

• Properties, Methods and Events

26
Creating a Class

• Use class module similar to adding a form or a
  standard code module
• The name represents the class
• Only code, no GUI
• Code in class module contains the encapsulated
  description of the class i.e. properties,
  methods and events

27
Example A CTime Class

• We will construct a time class CTime2 that we
  mentioned before
• When completed, an object of CTime2 will be able
  to show various behavior of time
• Set the time
• Increase time by a second a time
• Increase a minute every 60 seconds and a hour
  every 60 minutes
• Display the time in universal format or standard
  format

28
Class Properties

• Properties refer to attributes that hold data
  that describe an object
• Attributes are declared as variables for the
  class module

29
Example
Class CEmployee
Attributes gt Variables

• They are called module-level variables before
• In OOP, variables within a class are called
  instance variables

30
Member Access Control

• Class members include variables and methods
• Member access specifiers include keywords
• Public accessible to all modules within the
  project
• Private accessible only to methods of the class

31
Example Code Access Control
Private instance variables can only be
manipulated by methods of the class
Public instance variables and Public methods are
accessible to every module within a project
Private methods can only be called by other
methods of the class utility or helper methods
32
Example Access Errors
33
Property Procedures

• Property Procedures refer to procedures that read
  and assign new values to private variables from
  outside the object
• These Property Procedures and the instance
  variables they manipulate collectively are called
  propeties
• A layer of indirection facilitates
• Validation checking
• Translation and transformation of interface data
  forms and implementation data

34
Manipulating a Class Property

• Property Get procedure (accessor or query method)
• Obtain the values of private instance variables
• Property Let/Set procedure (mutator method)
• Control the setting of instance variables to
  valid values
• Let procedure for normal instance variables
• Set procedure for object variables
• Get and Set or Let are not always required
• A property with only Get procedures is called
  ReadOnly
• A property with only Let/Set procedure is called
  WriteOnly

35
Example Code
36
Creating Class Methods

• Operations on class data are coded and placed in
  functions or procedures within the class
• We call these procedures methods
• Scope procedures in terms of access control into
  private and public
• Public methods form part interface of a class,
  through which data can be manipulated

37
Method Overloading

• A class method can have several signatures
• Each signature provides a different version of
  the method, which have the same name, but accept
  different number of arguments, or arguments with
  different data types.
• Not supported in VB 6

Public Function Addition(num1 As Integer, num2 As
Integer) As Integer Addition num1 num2 End
Function
Public Function Addition(num1 As Integer, s As
String, num2 As Integer) As Integer Addition
num18 (num2 3) CInt(s) End Function
38
Example Methods in Object Browser
39
Object Variables
40
Use of Objects and Classes

• Classes are seldom used directly except they have
  class variables and methods
• Use classes to create objects
• Use objects to build applications
• For example, all components in a form are objects

41
Object Variables

• Object variables are used to refer to objects
• Declare object variables similarly to numeric or
  string variables
• For example, int n does two things
• it declares that n is an integer variable
• it allocates space to hold a value for n
• Object variables are references or pointers to
  objects and do not contain the objects.

42
Declare and Create Objects

• Private mTime As CTime2 // does one thing, it
  declares that mTime is of type CTime2
• The Set statement is used to associate objects
  with variable names
• Set mTime New CTime2 // allocates space
  associating a object variable to an instance
• We say Set binds the object to the variable
  name
• Keyword New instantiates an object of a specified
  type

43
Equivalent Statements

• Either of these statement options may be used to
  instantiate a new object and associate it with a
  variable
• Dim mTime As New CTime2
• OR
• Dim mTime As CTime2
• Set mTime New CTime2
• But the mTime object is still "blank, only a
  placeholder in memory
• mTime.Hour txtHour.Text // dot notation
• mTime.Minute txtMinute.Text // assignment via
  Let property procedure
• In VB an object is not created until the first
  time a property or method of the object is called

44
CTime2 Class Demo
45
Classes and Events

• A class has only two built-in events, i.e.
  Initialize and Terminate
• A class can raise user-defined events declared in
  the class
• Purpose is to notify other objects that something
  happened

46
The Initialize Event

• Assign default values to class properties at
  instantiation
• Initialize event fires every time an instance of
  a class is created, including built-in classes
  such as Forms
• The Initialize event occurs before the Load event
  
• The event is raised once in an objects lifetime
• The event method is Class_Initialize()

47
The Terminate Event

• Used to clean up an object before the object is
  destroyed
• Fires when the last reference to the class
  instance is removed from memory
• Set to a different object
• Set to Nothing
• Largely related to memory management, releasing
  memory
• Occurs after the Unload event for all objects
• The event method is Class_Terminate()

48
Raising an Event in a Class

• An user-defined event must be declared in the
  classs general declaration
• Use Event keyword
• Must be declared as Public
• Event parameters are used to pass data to and
  receive data from the program
• Use RaiseEvent statement to raise an event

49
Example Code
50
React to Events in a Program

• A program (a form or separate class) that use the
  event class defines how to react to the events
• When an event is raised the object broadcasts a
  stimulus throughout the system
• Event handlers are procedures that execute when
  they detect a specific event stimulus

51
Demo
52
Reference a Field or Method

• Inside a class, no dots are necessary, use them
  directly
• Outside a class, you need to say which object you
  are talking to
• If mTime.Second 0 Then
• mTime.Minute (mTime.Minute 1) Mod 60
• If you don't have an object, you cannot use its
  fields or methods!

53
Changing Properties of Instances

• Properties of an object may be changed at run time

54
Objects Reuse

• Composition or aggregation building new classes
  from objects of existing classes
• Use references to refer to existing objects
• A class can have multiple instances
  simultaneously

55
Example a CDate1 Class
Declares three integers mMonth, mDay, and mYear
Assigns values to class variables mMonth, mDay,
mYear after error checking
56
Example an CEmployee Class
Declares two Private strings and two Private
object references
Class CEmployee references to objects of class
CDate1, mBirthDate and mHireDate
57
Example an Employee Class

• In the CEmployee class, two CDate1 objects are
  referenced (used)
• Objects can be used as arguments and passed among
  objects

CDate1 objects are passed to procedures to
create HireDate and BirthDate
58
Classes vs Objects in VB IDE

• The controls on the Toolbox in Visual Basic
  represent classes. When you drag a control from
  the Toolbox onto a form, you are creating an
  object an instance of a class
• The form you work with at design time is a class.
  At run time, Visual Basic creates an instance of
  the form's class that is, an object

59
Classes in VB IDE

• Visual Basic is totally object-oriented
• Object Browser
• Lists the Framework Class Library available in
  Visual Basic
• Or classes in specific packages

60
OOP in VB

• You have already used OOP in VB but only with
  built-in classes
• All the forms and controls we have used so far
  are objects arising from classes
• When you add a control to a form the control is
  already programmed for the events it will raise
• All VB objects (controls and forms) support the
  concept of encapsulation and information hiding
  implementation details are unseen to programmers

61
Objects as Control (class) Instances
62
Forms as Classes

• Designing a form is really the creation of a new
  class. This is different from other controls
• Individual forms are instantiated when you use
  the New keyword
• The form exists as an object, but it has no
  window. None of its controls exist yet before
  Load statement

63
Example Multiple Instances of a Form

• Properties of a new instance may be changed at
  run time

64
Creating OO Applications

• Design and write your own classes based on the
  business logic of your application
• Use as many pre-existing classes as possible
• Implement the application by aggregation and
  composition of your own classes and existing
  classes

65
Good Programming Practice

• Data members should almost always be scoped
  private to the class module
• Use a consistent naming convention for class data
  members
• Initialize all data members with default values
• Make code robust and reusable

66
Summary

• Object thinks concepts
• Object speaks terminology
• OOP in VB