Object Based Programming

1
Object Based Programming
2
Summary Slide

• Instantiating An Object
• Encapsulation
• Inheritance
• Polymorphism
• Overriding Methods
• Overloading vs. Overriding
• Implementing a Time Abstract Data type with a
  Class
• Controlling Access to Members
• Initializing Class Objects Constructors
• Properties
• Composition
• Shared Class Members
• Const and ReadOnly Members
• Garbage Collection

3
Object Terminology Review

• Object - like a noun, a thing
• Buttons, Text Boxes, Labels
• Properties - like an adjective, characteristics
  of object
• Text, ForeColor, Checked, Visible, Enabled
• Methods - like a verb, an action or behavior,
  something the object can do or have done to it
• ShowDialog, Focus, Clear, ToUpper, ToLower
• Events - object response to user action or other
  events
• Click, Enter, Activate

4
Instantiating An Object

• Creating a new object based on a class
• Create an instance of the class by using the New
  keyword and specify the class

5
Encapsulation

• Sometimes referred to as data hidingan object
  can expose only those data elements and
  procedures that it wishes

6
Inheritance

• Inheritance is a form of reusability in which
  classes are created by absorbing an existing
  classs data and behaviors and improving by
  adding new capabilities.
• Inheritance allows a software developer to derive
  a new class from an existing one
• The existing class is called the parent class, or
  superclass, or base class
• The derived class is called the child class or
  subclass.
• As the name implies, the child inherits
  characteristics of the parent

7
Inheritance

• Inheritance relationships are often shown
  graphically in a class diagram, with the arrow
  pointing to the parent class

8
Deriving Subclasses

• In VB.NET, we use the reserved word Inherits to
  establish an inheritance relationship

9
Controlling Inheritance

• Visibility modifiers determine which class
  members get inherited and which do not
• Variables and methods declared with public
  visibility are inherited, and those with private
  visibility are not
• But public variables violate our goal of
  encapsulation
• There are two more visibility modifiers that
  helps in inheritance situations Protected and
  Friend

10
The Protected Modifier

• The Protected visibility modifier allows a member
  of a base class to be inherited into the child
• But Protected visibility provides more
  encapsulation than public does
• However, Protected visibility is not as tightly
  encapsulated as Private visibility

11
The Friend Modifier

• The Friend visibility modifier allows a member of
  a base class to be inherited into the child only
  if the the derived class is in the same assembly

12
The MyBase Reference

• Constructors are not inherited, even though they
  have public visibility
• Yet we often want to use the parent's constructor
  to set up the "parent's part" of the object
• The MyBase reference can be used to refer to the
  parent class, and is often used to invoke the
  parent's constructor
• Also, when a derived-class method overrides a
  base-class member, the base-class member can be
  accessed from the derived class b using the
  MyBase reference.

13
Single vs. Multiple Inheritance

• VB.NET supports single inheritance, meaning that
  a derived class can have only one parent class
• Multiple inheritance allows a class to be derived
  from two or more classes, inheriting the members
  of all parents
• Collisions, such as the same variable name in two
  parents, have to be resolved
• In most cases, the use of interfaces gives us the
  best aspects of multiple inheritance without the
  overhead

14
Polymorphism

• Different classes of objects may have behaviors
  that are named the same but are implemented
  differently
• Programmers can request an action without knowing
  exactly what kind of object they have or exactly
  how it will carry out the action

15
Polymorphism Implemented

• Overloading
• Argument type determines which version of a
  method is used
• Example MessageBox.Show method
• Overriding
• Refers to a class that has the same method name
  as its base class
• Method in subclass takes precedence

16
Overriding Methods

• A child class can override the definition of an
  inherited method in favor of its own
• That is, a child can redefine a method that it
  inherits from its parent
• The new method must have the same signature as
  the parent's method, but can have different code
  in the body
• The type of the object executing the method
  determines which version of the method is invoked
• In VB.NET, a base-class method must be declared
  Overridable if that method is to overriden in a
  derived class.

17
Overloading vs. Overriding

• Don't confuse the concepts of overloading and
  overriding
• Overloading deals with multiple methods in the
  same class with the same name but different
  signatures
• Overriding deals with two methods, one in a
  parent class and one in a child class, that have
  the same signature

18
Reusability

• The main purpose behind OOOP and Inheritance in
  particular
• New classes created with Class Module can be used
  in multiple projects
• Each object created from the class can have its
  own properties

19
Multitier Applications

• Common use of classes is to create multitier
  applications
• Each of the functions of a multitier application
  can be coded in a separate component and stored
  and run on different machines
• Goal is to create components that can be combined
  and replaced

20
Three-tier Model

• Most common implementation of multitier

21
Implementing a Time Abstract Data type with a
Class

• VB programmers concentrate on creating their own
  user-defined types called classes (also referred
  as programmer defined types)
• Classes in VB facilitate the creation of special
  data types, called abstract data types (ADT)

22
Example of a New Class
23
Class Scope

• A classs instance variables and methods belong
  to the classs scope.
• Class members that are visible can be accessed
  only through a handle (ObjectReferenceName.membe
  rName)
• Variables within methods
• Only methods can access that variable
• Keyword Me is a hidden instance variable can be
  accessed in a method by preceding its name with
  the keyword Me and dot operator

24
Controlling Access to Members

• The member access modifiers Public, Private,
  Protected, and Friend control access to a classs
  instance variables and methods.
• Control access to a classs instance variables
  and methods
• Public Serves primarily to present interfaces of
  a class
• Private Holds clients private data safely
• Get and set functions have ability to access
  private data

25
Initializing Class Objects Constructors

• A constructor method initializes its classs
  members
• When appropriate, provide a default constructor
  to ensure that every object is initialized with
  meaningful values
• Parametized constructors have arguments
• If a class does not have a defined constructor,
  the compiler will create an empty constructor.
• If an instance variable is not initialized the
  compiler will assign a default value
• Overloaded Constructors must have different
  numbers and/or types and/or orders of parameters

26
Example of a Constructor
27
Properties

• Methods in a class can manipulate the classs
  Private instance variables. Public methods allow
  other object to change a classs properties.
• Get accessor
• In Visual Basic instance variables as private
  does not guarantee data integrity
• Set accessor
• Cannot return values indicating a failed attempt
  to assign invalid data to objects of the class
• Control the setting of instance variables to
  valid values
• Get and Set accessors are not required
• A property with only Get accessor is called
  ReadOnly
• A property with only Set accessor is called
  WriteOnly
• After we define a property, we can use it in the
  same way as we use a variable.

28
Example of a Property Definition
29
Composition

• Composition is the use of objects of preexisting
  classes as members of new objects.
• A form of composition is software reuse

30
Using the Me Reference

• Every object can access a reference to itself
  using a Me reference.
• Me explicitly
• Me implicitly
• The explicit use of the Me reference can increase
  program clarity where Me is optional

31
Shared Class Members

• Contains only one copy of this variable in memory
• When a single copy of the data will suffice, use
  Shared class variables to save storage.
• Shared class variables are not the same as global
  variables because Shared class variables have
  class scope
• A classs shared class members are available as
  soon as the class is loaded into memory at
  execution time
• Shared method has no Me reference
• A shared method cannot access non-shared class
  members.

32
Const and ReadOnly Members

• Const
• A data member must be initialized in its
  declaration
• Cannot be modified once initialized
• ReadOnly
• A data member can be initialized either in the
  class structure or in its declaration
• Cannot be modified once initialized

33
Garbage Collection

• Resource leaks
• Objects must have an efficient way to return
  memory and release resources when the program no
  longer uses those objects
• Memory leaks
• In Visual Basic memory is reclaimed
  automatically, hence it experiences rare memory
  leaks as compared to C and C
• Finalization
• Finalizer method performs termination
  housekeeping on that object just before the
  garbage collector reclaims the object's memory.
• Feature of .NET Common Language Runtime (CLR)
  that cleans up unused components
• Periodically checks for unreferenced objects and
  releases all memory and system resources used by
  the objects
• Microsoft recommends depending on Garbage
  Collection rather than Finalize procedures