Abstraction and the Object Model

Introduction

Abstraction is a process in which some characteristics of an entity are removed to reduce it to a set of essential characteristics that can effectively define that entity.It is basically done to reduce complexity.For example : the abstraction of a plastic container to a container will retain only the general information on the behavior and attributes of the container. The principle of abstraction is an essential element of Object-Oriented programming. One of the most powerful ways of managing abstraction is by using hierarchical classifications. This helps in layering the semantics of complex systems thus breaking them into chunks of manageable pieces. This method of hierarchical abstractions can also be applied to computer programs.The data from a process-oriented program can be transformed into its component objects using the principle of abstraction. In programming languages,abstraction is a mechanism that emphasizes the general properties of some segment of code and hides details. It involves separating a program into parts that contains certain details and parts where these details are hidden. In Object-Oriented terminology,data is considered as attributes and the functions are referred to as methods. The main advantage of Object-Oriented programming is that the data as well as the operations that manipulate the data known as code, are both encapsulated inside the object. For example : a Java Applet is an object.The browser executing that particular objet has no idea about its functionalities. When the object is loaded , the code inside it is executed by the browser using the data contained within that particular object. Objects are the building blocks of Object Oriented programming. The state of an object is the data contained inside that object also referred to as attributes. These attributes help in differentiating between the various objects. In object-oriented programming,the methods define the behavior of an object. The concept of getters and setters sometimes referred to as accessor methods provide controlled access to an object's data. A class is a kind of template from which objects are made.For example: if we are creating two employess,it is said that we have created to instances of the employee class with each instance or object having its own attributes and methods. An object can be instantiated or built only by using a class. For Example : To instantiate an object in java ,

                                     myClass myObject;

Here myClass is a class and myObject is an object.

Overview

Abstraction refers to the elimination of the irrelevant details and the amplification of the essential aspects in programming. It provides a basic platform for the creation of user defined data types which we call as objects. Abstract classes are declared with the keyword abstract and they cannot be instantiated.They can only be used as a superclass for classes that extend the abstract class. A class can inherit from only one abstract class and has to override all its methods and ,if it wants , can override its virtual methods too. There are two common terms associated with the term abstraction:

Client - that part of the program that uses the program component.

Implementation - That part of the program that defines the program component.

The interaction between these two entities is usually restricted to a specific interface. Most of the modern object-oriented languages provide access to only a set of public operations on an object. This restriction is provided by the designer and the implementer of the object. For Example : there is a program that manipulates geometric shapes.In such a program,each shape can be represented as an object. An object representing a circle can be implemented by storing the center and radius of the circle. The designer of cicle object can choose if the function that changes the center of the circle can be made part of the interface or not. Abstraction that is based on objects is quite similar to the abstraction based on abstract data types both combining functions and data as well as distinguishing between a public and a private interface.Abstraction and encapsulation are two aspects of Object-Oriented programming that go together.The type defined by a class is partially abstract as it can have both public and hidden components. When the hidden part is empty , the class resembles an object-oriented interface. This kind of classes is sometimes called as abstract class.

Types of Abstraction

There are three Kinds of abstraction:

Procedural Abstraction - This is one of the oldest abstraction mechanisms.The program making a function call is considered as a client and the function body consisting of instructions executed each time the function is called is considered as the implementation.

For Example : a code that stores the square root of a variable x in the variable y can be encapsulated into a function. By doing this, an interface which consists of a function name, the input parameters and the type of output are defined. It provides a kind of information hiding since the information about the underlying functionalities is contained in the function declaration but hidden from the program using that function. The function can be called on different arguments. In short, we can say that the code can be made generic as well as reusable by enclosing it inside a function.

Data Abstraction - The concept of data abstraction refers to hiding the information about the way data is represented. The mechanisms used for data abstraction are abstract data type declarations and modules. By data abstraction , an interface of the data structure can be identified. It also helps in information hiding by the separation of implementation decisions from those parts of the program that are using the data structure. It provides the reusability of data structure by many different programs. An abstract data type provides a specific interface to be used by other parts of a program. It also makes sure that it can only be used through its interface by restricting access to it.

Pure Abstract data type programming is value oriented rather than object-oriented. Object-Oriented programming uses procedural abstraction to achieve data abstraction. Objects are centered around the constructors of a data abstraction. Abstract data types use type abstraction centered around the operations.

Implementation of Abstraction in Object-Oriented Languages

Simula - Traces of data abstraction appeared in the class construct of Simula 67. Simula is considered as the first object oriented programming language with features like classes,objects,dynamic lookup,subtyping and inheritance. Simula does not have the concept of abstract classes since classes with pure virtual methods can be instantiated. The variables declared in a Simula67 class are not hidden from the clients that are creators of the objects of that class.Although Simula-67 cannot distinguish between public and private members of the classes,a later version provided this facility and allowed the attributes to be made protected.

Ada - is another language that does provide encapsulation and can be used to simulate abstract data types. These encapsulation constructs are known as packages. Packages can be written with a separate interface called package specification and the implementation is called package body.

Modula-2 -It uses modules instead of packages used in Ada to provide support for abstract data types. In Modula,a module interface is known as a definition module and an implementation,an implementation module. Modula-3 is based on implementing abstract data types using objects .

Beta - Another object oriented language that can be considered is Beta which is a modern object oriented language providing powerful abstraction mechanisms.These mechanisms include support for identification of objects,classification and composition. It is a strongly typed language with mechanisms such as class,procedure,function,process,exception all put together into a single abstraction mechanism called as pattern.

Eiffel - is another language whose design is closely based on OOP theory with a formal support for abstract data types.

CLU - It consists of three kinds of modules with each module for each kind of abstraction.Procedures support procedural abstraction, iterators support control abstraction, and clusters support data abstraction.

Scala - Scala supports two styles of abstraction : the functional style that uses parameterization and the abstract type representing the object oriented approach.

Smalltalk - It was the first object-oriented programming language to become popular. It is a dynamically typed ,reflective programming language developed with ideas taken from Simula67 and LISP. It was a completely new language with new terminology and an original syntax. The first version of Smalltalk to be made publicly available was Smalltalk-80. It uses procedural abstraction. Everything is an object,even a class and so all operations are referred to as messages to objects. In Smalltalk ,the methods are made public.Any code which has a pointer to an object can send any message to it.If that method is defined in the class or superclass,it will be invoked. Instance variables have been made protected.They are accessible only to the methods of the class of the object as well as to the methods of the subclasses.In the handling of primitive numbers and arrays ,it makes use of built in abstract data types.

C++ - In C++,classes can be used for describing both values and objects. It is a statically typed object-oriented language in which an abstract class is usually created to define an implementation and is intended to be inherited from the non-abstract classes. If we want to create a non-abstract class from an abstract class,we need to declare and define a matching member function for each abstract member function of the base class. A pure abstract class is a class that has all the functions as virtual and there is no data. The creation of an interface that can access only certain elements of data types is possible. The division of code into smaller fragments called functions helps in the reusability of code.

Java - while declaring an abstract class in Java, some of the methods in that particular class can be left unimplemented. Those methods are indicated with the keyword "abstract". These methods are sometimes referred to as subclasser responsibility since they have no implementation specified in the superclass.

The method is defined using the following syntax:

                        abstract type name(parameter-list);

For Example :

                        public abstract class AbstractClass
                        {
                        public String toString() {return "An AbstractClass object";}
                        public abstract void visit(Object o);
                        }

The above example has one implemented and one non-implemented abstract method.The declaration of an unimplemented method is similar to the way methods are declared in an interface.The difference is the addition of the "abstract" keyword. An abstract class is said to be totally abstract if it contains all the unimplemented methods.Such a class can extend at most one superclass of Java. There can be no objects of an abstract class which means that an abstract class cannot be instantiated directly with the new operator.Abstract static methods or abstract constructors cannot be declared. A subclass of an abstract class must either implement all of the abstract methods in the superclass,or should be declared abstract. Concrete methods are allowed in abstract classes with as much implementations that can fit. Abstract classes can be used to create object referances since Java's approach to run-time polymorphism is implemented using the superclass referances.

VB.NET - Visual Basic is an object based language but VB.NET is an object oriented language. To create an abstract class in VB.NET ,the class declaration is done as :

                                 MustInherit Class Classname

C# - It uses abstract modifier to support abstract class members and abstract classes. It just requires a property line with the abstract keyword with empty get and set methods. To create an abstract class in C#,the class declaration should be done as:

                                 abstract class Classname;
                                {
                                 }

The abstract class of C# contains abstract members which define what a subclass needs to have.

Uses

Flexibility - It provides flexibility since the programmer can now hide the details or data that are not required for presentation.It allows the specialization of inherited classes.

More security - It helps in hiding the implementation details and giving access only to the data.

Reducing Complexity - It helps a user to divide a large program into chunks of modules.This helps in making the debugging as well as testing a lot more easier.It provides a means to choose a variation from a common pattern.The complexity is reduced by ignoring unimportant details.

Easy replacement - It is easier to replace code without recompiling.New class could be easily added to an existing program without changing the main program.For example:we want to add a square,we just need to create a new class for square,add a square to the tool palette and modify the method in the tool palette to declare the new object square.

Reusability - It helps in reusing software components in other applications.For example:colour has the methods of darker() and brighter(). Once the idea of colour is abstracted into an object,it can be used for different programs thus reducing the time required in development as well as debugging.