Advanced Interactive Programming Object Oriented Programming Cont'

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Advanced Interactive ProgrammingObject Oriented
Programming (Cont.)
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Review

• Concepts and terms in OOP
• Object
• Class
• Message
• Information hiding
• Data abstraction
• Encapsulation
• Object oriented programming in VB IDE
• Create and manipulate classes, objects, methods
  and properties
• Use classes and object in applications

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Outline

• Concepts and terms in OOP
• Inheritance
• Interface
• Collection
• Polymorphism
• Delegation
• Object oriented programming in VB IDE
• Create and implement interfaces
• Use interface inheritance and polymorphism in
  applications

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Inheritance

• A mechanism for creating new classes using
  classes that have already been defined.
• The new class is referred to as a derived class
  or a child class or a subclass.
• The existing class is referred to as a base
  class, a parent class or a superclass.

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Relationship between a Subclass and a Superclass

• A subclass inherits all instance attributes and
  behaviors of the superclass from which it is
  derived.
• A subclass can override the definition of
  existing methods by providing its own
  implementation method overriding
• Overridden members must accept the same data type
  and number of arguments
• The subclass can be subclassed further
• Derived classes inherit overridden members

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Method Overriding Example
In the superclass
Public Function Addition(num1 As Integer, num2 As
Integer) As Integer Addition num1 num2 End
Function
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Comparison 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.

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
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Relationship between a Subclass and a Superclass

• A subclass can define its own attributes and
  behaviors in terms of application requirements -
  extension.
• A subclass consists only of the code for the
  changes and additions to the superclass.

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Generalization

• Generalization refers to the relationship between
  a class and one or more refined versions of it.
• Example a fruit is a generalization of apple,
  orange, mango and many others.
• Generalization implies the process of abstracting
  common properties and behaviors and creating
  shared control code for objects with sufficiently
  similar interface.
• Generalization usually leads to a superclass in
  OOP.

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Specialization

• Specialization also refers to the relationship
  between a class and one or more refined versions
  of it.
• Example an apple is a specialization of fruit.
• If class A is a generalization of class B, then
  class B is a specialization of class A
• Specialization implies the process of defining a
  new object based on a (typically) more narrow
  definition of an existing object.
• Specialization, also known as subtyping, leads to
  a subclass through extension and method overriding

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Class Hierarchy

• Inheritance relationships form a treelike
  structure called a class hierarchy.
• A class can become (or have) a superclass and a
  subclass when it is used to form a hierarchy.
• There is a is a relationship between a subclass
  and a superclass.
• Every object of a subclass is an object of that
  subclass superclass but this does not hold
  conversely.

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Class Hierarchy Example
CShape
CTwoDimensionalShape
CThreeDimensionalShape
CSphere
CCube
CCylinder
CTriangle
CSquare
CCircle
A portion of a CShape class hierarchy
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Class Hierarchy in C
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Class Hierarchy in Java
Class Hierarchy in Java
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Classes in VB
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The Power of Inheritance

• No need to write new classes from scratch but
  reuse ones from libraries through inheritance.
• No need to know the implementation details, just
  the interfaces.
• Meet your requirements by overriding and
  extension.
• Substantial and useful class libraries are
  available for reuse.
• Programming is subclassing and composition

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References

• Encapsulation and Inheritance in Object-Orlented
  Programming Languages by Alan Snyder, Software
  Technology Laboratory, Hewlett-Packard
  Laboratories at http//delivery.acm.org/10.1145/30
  000/28702/p38-snyder.pdf?key128702key2531245041
  1collGUIDEdlGUIDECFID65582496CFTOKEN317688
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• The online free encyclopedia
• http//en.wikipedia.org/wiki/Object-oriented_progr
  amming
• Microsoft online MSDN library
• http//msdn2.microsoft.com/en-us/library/527aztek
  .aspx
• Sun MicroSystem online documentation
• http//java.sun.com/docs/books/tutorial/java/conc
  epts/object.html

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Any Questions?
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Interface
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Interface

• An interface, like a class, defines a set of
  properties, methods, and events. But unlike a
  class, an interface does not provide
  implementation.
• Interfaces are usually not used to create
  objects.
• interfaces are implemented by classes.
• An interface is a contract.

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Interface

• A class can implement many interfaces as it
  needs.
• A subclass will automatically inherit the
  implemented interfaces of the class from which it
  is derived.
• In VB, interfaces are treated the same as
  classes. Some other OOP languages treats
  interfaces as separated entities from classes.

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Examplesin Java
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Interface Example Scenario

• Consider a shape, point and circle class
  hierarchy
• Develop an interface class called IShape
• Develop class CPoint and CCircle that implement
  IShape interface
• Use a form to experience the use of classes and
  the interface

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Creating an Interface

• Any class can serve as an interface.
• An interface is created in the same way as any
  other class.
• IShape defines three public methods - Area, Name
  and ToString, which are all empty.

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Using an Interface

• Use keyword Implements to implement an interface
  in a class.
• Every concrete class implementing an interface
  must implement all methods within the class.
• Implemented methods could still be empty, but you
  need to put the shell method there.
• Naming conventions
• Classes can define its own instance variable and
  methods.

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The CCircle Class
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Accessing an Interfaces Methods

• Interfaces define methods as Public
• Classes implementing interfaces code methods, but
  as Private
• Objects of classes cannot access interface
  methods directly

CCircle Class
IShape Interface
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Accessing an Interfaces Methods

• Use an interface object to access its implemented
  methods
• Interface objects refer to the objects of a class
  that implements the interface.
• Create an interface reference and assign it an
  object of a class
• Classs own defined instance variable and methods
  can still be accessed in terms of its access
  specifier

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Using Interfaces and Classes
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Implementation Inheritance

• The ability that a subclass can inherit the
  public interfaces and their implementations from
  its superclass.

• For example, we develop a class CCircle with all
  methods as public. We create a new class
  CRoundPlate by inheriting CCircle. Then we create
  an object of CRoundPlate called myRoundPlat. Then
  I can use myRoundPlat .Area() to calculate the
  area directly. No need to rewrite the method
  implementation
• VB does not support this feature, only
  illustration

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Interface Inheritance

• The ability to inherit the public interface but
  not the implementation of a class.
• VB supports interface inheritance.
• Use keyword Implements.

For example, a class Implements an interface such
as IShape2 class will need to re-implement all
methods
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Any Questions?
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Collection
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Collection Concept

• A way of grouping a set of related items
• A collection usually consists of a group of
  objects of different types, known as its elements
• Example collections include set, array and list
• Some are ordered and others unordered
• Iteration and enumeration utility classes
• Provide mechanism to walk through all objects in
  a collection

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Forms and Controls Collections

• VB provides default form and control collection
  objects for managing and accessing several of
  them easier
• Collection members have index values
• These collections are 0-based, i.e. the index
  value starts from 0
• Collections have a Count property

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Example Forms Collections
4 - 1
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The Generic Collection Class

• Unlike the default Forms and Controls collections
• Used for grouping programmers own objects
• Generic collections are 1-based
• The generic collection has a single property
  Count
• It must first be declared and bound to a variable
  name

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Generic Collection Methods

• The Add method is used to add members to the
  collection
• The Remove method is used to remove members from
  the collection
• The Item method is used to return single members
  of the collection when furnished an index value

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A Generic Collection Example
Set Queue New Collection Set Account New
AccountObj Account.AssignOwner (Fred) Queue.Add
(Account) Set Account New AccountObj Account.Ass
ignOwner (Mary) Queue.Add (Account) Print
Queue.Count 2 Print Queue(1).Owner Fred Print
Queue(2).Owner Mary
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Collection in OOP

• Very common because we need to manage a set of
  objects they might all have the same class or
  derived from different subclasses of the same
  superclass.
• More functionalities, for example, performing
  type or existence testing when adding an object
  or retrieving objects in terms of other
  attributes keys, the textual index.
• Extensive use of Iterator or Enumerator class to
  manipulate individual objects

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References

• See MSDN library
• Collections in Visual Basic
• The Visual Basic Collection Object

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Polymorphism
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Motivating Programming Problems

• CShape
• CTwoDimensionalShape
• CCircle
• CSquare
• CTriangle, etc.
• Common methods
• Calculate Area, perimeter, etc.
• Draw the shape
• Each has a different way to do these

CShape
CTwoDimensionalShape
CTriangle
CSquare
CCircle
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Traditional Solutions

• Type fields and Select Case statements
• Select Case Statements
• Use type fields to distinguish among object types
• Call upon an appropriate action of the object
• Problems
• Forget type test
• Forget all possible cases
• Every addition or deletion of a class require
  inserting new cases in all relevant Select Case
  statements

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Polymorphism

• Methods are defined in higher level classes but
  coded in various ways in subclasses as long as
  the class interface is unchanged.
• Method names, parameter names and types must stay
  the same.
• This is polymorphism, the many shapes of a
  method.

CShape
CTwoDimensionalShape
CTriangle
CSquare
CCircle
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Polymorphism

• Polymorphism refers to the ability to define
  multiple classes with functionally different, yet
  identically named methods or properties that can
  be used interchangeably by client code at run
  time.
• Inheritance-Based Polymorphism
• Defines methods in a superclass
• Overrides them with new implementations in
  subclasses
•  Interface-Based Polymorphism
• Defines methods in an interface
• Implements them differently in classes  

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Polymorphism Extending the Interface Example

• Consider a shape, point and circle hierarchy
• Extend it with a method Volume and a new class
  cylinder
• Develop an interface class called IShape
• Create a new interface IShape2
• Develop class CPoint and CCircle that implement
  IShape interface
• Add a CCylinder class
• Use a form to experience the use of classes and
  the interface
• Use an array as a collection

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Creating Interfaces and Classes

• The same way as we did before
• Add a new shell method in the IShape2
• Implement the Volume method in all classes as
  below in CCircle class
• Create the CCylinder class

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The CCylinder Class
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Using Polymorphism
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Supporting Polymorphism

• Name binding refers to the association of values
  with identifiers
• Early binding
• Associate an object to a specific reference type
• Late binding or dynamic binding
• Associate an object to a reference of Object
  class. The validity of object reference is
  resolved at run time
• In late binding, the programmer does not have to
  know the exact type of the object in advance

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Delegation

• Common sense understanding to hand over a task
  to another person, usually a subordinate.
• In OOP, it means the handing of a task over to an
  implemented object
• Delegation refers to the mechanism that allows a
  method to be invoked indirectly by way of a
  reference to the method

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Delegation in VB

• Interfaces could contain
• only shell methods, so-called abstract class
• a combination of shell methods and
  implemented methods
• Classes implementing an interface
• must implement all interface methods no matter
  whether they are implemented or not
• Could create an object for the partially
  implemented interface, usually called inner
  object
• Delegation
• Subclasses provides implementation by delegating
  responsibility to the inner object for
  implementation
• The object of a subclass is usually called outer
  object

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Example
shell methods
implemented methods
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Example (cont.)
Class definition
Implements interface
Create an interface object
Delegation
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Discussion

• Delegation and interface inheritance can be used
  to simulate implementation inheritance in VB, but
  does not make much sense
• Delegation could be selective
• In one method you might delegate directly to the
  inner object, passing the arguments unchanged
• while in another method you might execute some
  code of your own before calling the inner object
• in a third method you might execute only your own
  code, ignoring the inner object altogether

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Summary

• Inheritance
• Interface
• Collection
• Polymorphism
• Programming in VB