CSC/ECE 517 Fall 2009/wiki2 15 ms

Introduction

"Object-oriented programming is a method of implementation in which programs are organized as cooperative collections of objects, each of which repressents an instance of some class, and whose classes are all members of a hierarchy of classes united via inheritance relationships."taken from Object-Oriented Analysis and Design by Grady Booch
For all the things that we define as object oriented, the main framework is the object model. An object model can be defined as the collection of principles that form the foundation of object oriented design. The main elements of an object model are -:

• Abstraction
• Encapsulation
• Modularity
• Hierarchy

The goal of this article is to describe how abstraction relates to object oriented languages.

Definition of Abstraction

Abstraction (from the Latin abs, meaning away from and trahere, meaning to draw) is the process of taking away or removing characteristics from something in order to reduce it to a set of essential characteristics.[taken from what is abstraction?]

An alternate definition given in the book object oriented analysis and design,states that-:

"An abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provide crisply defined conceptual boundaries,relative to the perceptive of the viewer."

The gist is that abstraction is a concept wherein all but the relevant information about an object is hidden in order to reduce complexity and increase the efficiency. This concept even applies in the field of arts wherein an object or a piece of art created by an artist is a representation of the original model with all the unwanted details removed. In other words it is nothing but recognizing the similarities between objects and ignoring for some time their differences. It focuses on the outside view of the object and separate it's behavior from it's implementation.

Abstraction focuses on the essential characteristics of some object, relative to perspective of the viewer.

Types of abstractions

There are different types of abstractions as suggested by Seidewitz and Stark of which some closely model problem domain entities These are as following (mentioned in the order of their usefulness) -:

• Entity abstraction - Object represents a useful model of a problem domain or solution domain entity.

• Action abstraction - Object provides a generalized set of operations, all of which perform the same kind of function.

• Virtual Machine abstraction - This is where an object groups together operations that are all used by some superior level of control, or operations that all use some junior-level set of operations.

• Conincidental abstraction - An object that packages a set of operations that have no relation to each other.

Entity abstractions is what is used widely because it directly parallels a given problem domain.

Definitions taken from object oriented analysis and design by Grady Booch

Key Abstractions

A abstraction which describes the system is referred as a key abstraction. They give boundaries to our problem.They highlight things that are more relevant to our system and hence to the design and suppress the things that are outside the system. There are two important steps in identifying the key abstractions. These are -:

• Discovery : Discovery is the process of recognizing the abstraction used in the design of the system.

For example, a customer doing a bank transaction speaks in terms of accounts,deposits and withdrawals; 
these works are part of the vocabulary of the problem domain.

• Invention : Through invention, we can create new classes and objects that are not necessarily part of the problem domain, but are useful in design and implementation.

For example A developer of such a system uses these same abstractions, but must also introduce new ones, such as databases, screen  
mangers, lists, queues and so on. These key abstractions are artifacts of the particular design, not of the problem domain.

Abstraction in Different O-O languages

Java

Java implements abstraction using Abstract classes and methods. Any class which cannot be instantiated and contains one or more abstract methods in referred as an Abstract class in JAVA. Such class can only be inherited.

 Example :  Java code for the above figure.


/* The abstract class car is extended by both the classes Mechanic and LayMan.         */
/* They implement the abstract method thingsILookFor                                   */
/* according to what they consider important in a car                                  */


public abstract class Car {

abstract void thingsILookFor();

}

public class Mechanic extends Car {

private int enginePower;
 
private int carMileage;

 void thingsILookFor() {
 
 enginePower = 1000;
 
 carMileage = 32; 
 }
 
}

public class LayMan extends Car {
 
private string color;

private int seats;

 void thingsILookFor() {

 color = "yellow";

 seats = 4; 
 }
 

C++

Abstract classes: These classes can be used as a framework to build new classes that provide new functionality. These class have one or more virtual function. Functions of the base class whose functionality can be overridden by functions of the derived class at run time are called virtual functions.

class debugger
{

public:
      virtual void dump()
      {
          cout << " No method implementation \n";
      }
}


class X:public debugger
{

public:
      virtual void dump()
      {
          cout << " dump method implementation for class X\n";
      }
  
}
class Y:public debugger
{

public:
      virtual void dump()
      {
          cout << " dump method implementation for class Y\n";
      }
  
}

Ruby

Ruby has no Abstract classes like in Java or Virtual function like in C++, but the same functionality can be implemented as follows

class Car
 def thingsILike()
  raise NotImplementedError.new("no implementation found \n")
 end
end

class Mechanic < Car
 def thingsILike()
  puts " I like the horse power of the engine and the mileage \n"
 end
end

class LayMan < Car
 def thingsILike()
  puts " I like the color \n"
 end
end

References

• Object oriented Analysis and design by Grady Booch

• Definition of Abstraction
• Key Abstraction
• Object Oriented Programming - Abstraction
• What is meant by abstraction?
• What is abstraction?
• A discussion on object oriented programming
• An example on Anstraction
• A paper on "Toward a general object-oriented software development methodology"
• An article on Abstraction by Bjarne Stroustrup

Further Reading

• iphone reference library - The object model
• Lesson: Object-Oriented Programming Concepts
• A textbook on Object oriented programming using Java