Advanced Object Oriented Concepts

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Advanced Object Oriented Concepts

• Tonga Institute of Higher Education

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Inheritance Similarities

• You look just like your father.
• People inherit features from their parents
• Eye color
• Hair color
• Height
• You are similar to your parents

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Inheritance in Visual Basic .Net

• Inheritance When a class inherits members from
  another class
• Objects are like people. They can have children!
• Children can inherit variables from a parent
• Children can inherit methods from a parent

Parent / Base Class
Child / Sub Class
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History of Inheritance

• In the sixties, a programmers created a program
  to simulate traffic
• They used objects for their vehicles
• Cars
• Trucks
• Vans
• They noticed that all vehicles did the same
  things
• Turn left
• Turn right
• Brake
• Go

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Plan 1 - Van, Car and Truck Objects

• Create one class for each vehicle
• Van
• Car
• Truck

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Plan 1 - Advantages

• Its quick and easy to understand

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Plan 1 - Disadvantages

• Code is repeated in each object
• Changing the code in Brake() requires 3 changes
  to 3 different objects
• Method names can be changed.
• After a while, the objects are not similar

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Plan 2 - Inheritance

• Make one object with common methods.
• The code in the parent object is used in the
  child objects.

Car TurnLeft() TurnRight() Brake() Go()
Truck TurnLeft() TurnRight() Brake() Go()
Methods in the parent come down to the children!
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Plan 2 Advantages

• A change in the method code in the parent
  automatically changes the children classes
• Method code is consistent and easy to maintain
• A change in the method name in the parent
  automatically changes the children.
• Names are consistent and easy to maintain
• We can change a class that someone else created
• It is difficult to write your own button class.
  But we can add changes to the button class using
  inheritance

Car TurnLeft() -gt Left() TurnRight() -gt
Right() Brake() Go()
Truck TurnLeft() -gt Left() TurnRight() -gt
Right() Brake() Go()
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Plan 2 Disadvantages

• Inheritance requires special code
• Inheritance requires more understanding

Car TurnLeft() TurnRight() Brake() Go()
Truck TurnLeft() TurnRight() Brake() Go()
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Inheritance Differences

• You look just like your father.
• People inherit features from their parents
• Eye color
• Hair color
• Height
• You are similar to your parents.
• But you are much taller
• You are also different from them.

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Inheritance Differences

• Each child object can have additional different
  members.

Car TurnLeft() TurnRight() Brake() Go() ConservePe
trol()
Truck TurnLeft() TurnRight() Brake() Go() CarryLoa
d()
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Example of Parent / Base Class
All non-private members are shared with child
classes
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Example of Child / Sub Class
Special keyword that brings members from parent
Child specific classes
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Example of Driver for Inheritance

• Inherited classes work exactly like a normal
  class
• There is no difference when using an inherited
  method and a non-inherited method

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Demonstration

• Inheritance Code

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Inheritance in VB.Net Classes

• Look in the class definitions.
• Almost every class inherits from another class!

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Demonstration

• Inheritance in VB .Net Classes

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Access Specifiers

• Public
• Can be used by everything
• Private
• Can only be used by code inside the same class
• Dim
• Same as Private
• Friend
• Can be used by code inside the same project

Access Specifier
Name
Dim FirstName as String
Type

• Protected
• Can be used by code that inherits from this class
• Protected Friend
• Combination of Protected and Friend

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Demonstration

• Access Specifiers

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Multi-Level Inheritance

• When a subclass is also a base class.

Vehicle TurnLeft() TurnRight() Brake() Go()
Car TurnLeft() TurnRight() Brake() Go() ConservePe
trol()
RentalCar TurnLeft() TurnRight() Brake() Go() Cons
ervePetrol() RentalPrice()
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Demonstration

• Multi-Level Inheritance

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Overriding Base Methods

• Overriding When a child class replaces the
  behavior of a method defined in a base class.
• To override (replace) a method
• Define a method in the parent class to be
  overridable
• Define a method in the child class to override

Parent Class
Child Class
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Demonstration

• Overriding Base Methods

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MyBase

• Use in a child class
• Use the MyBase keyword to call methods in a base
  class.

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Demonstration

• MyBase

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MyClass and Me 1

• If inheritance is not used, they work the same
  way.

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MyClass and Me 2

• If inheritance is used, use these in parent
  classes
• Use the MyClass keyword to call methods in the
  class this keyword is used.
• Use the Me keyword to call methods in the current
  instance.

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Demonstration

• MyClass and Me

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Abstract Classes and Methods

• Abstract class A class that must be inherited
  from
• An abstract class cannot be instantiated
• MustInherit Used to make a class abstract
• Abstract method A method that must be
  overridden
• Method code does not exist in the base class
  because it will be overridden
• MustOverride Used to make a method abstract

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Demonstration

• Abstract Class or Method

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Preventing Inheritance

• NotInheritable Used to make a class
  uninheritable.

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Demonstration

• Preventing Inheritance

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Introduction to Interfaces 1

• All DVD players have the same controls, even if
  they are made by different companies (Sony,
  Panasonic, Toshiba, etc.)
• The buttons are a contract for how to operate the
  DVD player
• An interface is a contract
• detailing how an object is used
• Using this interface, I know how to
• use the DVDPlayer Object

DVD Player Play() Pause() Stop() Rewind() FastForw
ard()
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Introduction to Interfaces 2

• All DVD players have the same controls, even if
  they are made by different companies (Sony,
  Panasonic, Toshiba, etc.)
• But these different companies are not related to
  each other
• This is the difference between interfaces and
  inheritance

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Defining an Interface

• Defining an interface tells the computer that any
  class that uses this interface is guaranteed to
  have the elements described in the interface
• Example Any class that implements the ISummary
  will have these functions and properties
• GetShortSummary
• GetFullSummary
• HasFullSummary

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Demonstration

• Defining an Interface

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Implementing an Interface
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Demonstration

• Implementing an Interface

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Primary/Native Interfaces vs. Secondary Interfaces

• Primary/Native Interface of a class is composed
  of all the members defined in a class
• Secondary Interfaces involves the implementation
  of other interfaces

Secondary Interface
Primary Interface
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Abstract Classes vs. Interfaces

• It is sometimes difficult to tell the difference
  between an abstract class and an interface
• An abstract class is a class that cannot be
  instantiated, but must be inherited from
• An interface, by contrast, is a set of members
  that defines a contract for conduct

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Abstract Classes vs. Interfaces 2

• It is sometimes also difficult to decide whether
  to use an abstract class or an interface
• A consideration is that a class may implement
  more than one interface. A class may only
  inherit from only 1 class.
• Abstract classes are best for objects that are
  closely related. Interfaces are best for
  providing common functionality to unrelated
  classes.
• Use an abstract class to provide members that use
  the same code among many objects. Interfaces
  only force classes to have the same member names.