The
concept of Abstract classes and Interfaces is a bit confusing for beginners of
Object Oriented programming. Therefore, discuss the theoretical aspects of both
the concepts and compare their usage. And how to use them with C#.
An Abstract class without any implementation just looks like an Interface; however there are lot of differences than similarities between an Abstract class and an Interface.
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About Author:
An Abstract class without any implementation just looks like an Interface; however there are lot of differences than similarities between an Abstract class and an Interface.
What is an Abstract Class?
An
abstract class is a special kind of class that cannot be instantiated. So the
question is why we need a class that cannot be instantiated? An abstract class
is only to be sub-classed (inherited from). In other words, it only allows
other classes to inherit from it but cannot be instantiated. The advantage is
that it enforces certain hierarchies for all the subclasses. In simple words,
it is a kind of contract that forces all the subclasses to carry on the same
hierarchies or standards.
What is an Interface?
An
interface is not a class. It is an entity that is defined by the word
Interface. An interface has no implementation; it only has the signature or in
other words, just the definition of the methods without the body. As one of the
similarities to Abstract class, it is a contract that is used to define
hierarchies for all subclasses or it defines specific set of methods and their
arguments. The main difference between them is that a class can implement more
than one interface but can only inherit from one abstract class. Since C#
doesn’t support multiple inheritance, interfaces are used to implement multiple
inheritance.
Both Together
When
we create an interface, we are basically creating a set of methods without any
implementation that must be overridden by the implemented classes. The advantage
is that it provides a way for a class to be a part of two classes: one from
inheritance hierarchy and one from the interface.
When
we create an abstract class, we are creating a base class that might have one
or more completed methods but at least one or more methods are left uncompleted
and declared abstract. If all the methods of an abstract class are
uncompleted then it is same as an interface. The purpose of an abstract class
is to provide a base class definition for how a set of derived classes will
work and then allow the programmers to fill the implementation in the derived
classes.
There
are some similarities and differences between an interface and an abstract
class that I have arranged in a table for easier comparison:
Feature
|
Interface
|
Abstract class
|
Multiple inheritance
|
A class may inherit
several interfaces.
|
A class may inherit
only one abstract class.
|
Default
implementation
|
An interface cannot
provide any code, just the signature.
|
An abstract class
can provide complete, default code and/or just the details that have to be
overridden.
|
Access Modifiers
|
An interface cannot
have access modifiers for the subs, functions, properties etc everything is
assumed as public
|
An abstract class can
contain access modifiers for the subs, functions, properties
|
Core VS Peripheral
|
Interfaces are used
to define the peripheral abilities of a class. In other words both Human and
Vehicle can inherit from a IMovable interface.
|
An abstract class
defines the core identity of a class and there it is used for objects of the
same type.
|
Homogeneity
|
If various
implementations only share method signatures then it is better to use
Interfaces.
|
If various
implementations are of the same kind and use common behavior or status then
abstract class is better to use.
|
Speed
|
Requires more time
to find the actual method in the corresponding classes.
|
Fast
|
Adding functionality
(Versioning)
|
If we add a new
method to an Interface then we have to track down all the implementations of
the interface and define implementation for the new method.
|
If we add a new
method to an abstract class then we have the option of providing default
implementation and therefore all the existing code might work properly.
|
Fields and Constants
|
No fields can be
defined in interfaces
|
An abstract class
can have fields and constants defined
|
Using the Code
Let
me explain the code to make it a bit easier. There is an Employee abstract
class and an IEmployee interface.
Within
the Abstract class and the Interface entity I am commenting on the differences
between the artifacts.
I
am testing both the Abstract class and the Interface by implementing objects
from them. From the Employee abstract class, we have inherited one object: Emp_Fulltime.
Similarly from IEmployee we have inherited one object:Emp_Fulltime2.
In the test code under GUI, I am creating instances of both Emp_Fulltime and Emp_Fulltime2 and then setting their attributes and finally calling calculateWage method of the objects.
Abstract Class Employee
using System;
namespace AbstractsANDInterfaces
{
///
///
Summary description for Employee.
///
public abstract class Employee
{
//we
can have fields and properties
//in
the Abstract class
protected String id;
protected String lname;
protected String fname;
//properties
public abstract String ID
{
get;
set;
}
public abstract String FirstName
{
get;
set;
}
public abstract String LastName
{
get;
set;
}
//completed
methods
public String Update()
{
return "Employee
" +
id + " " +
lname + " " + fname +
" updated";
}
//completed
methods
public String Add()
{
return "Employee
" +
id + " " +
lname + " " + fname +
" added";
}
//completed
methods
public String Delete()
{
return "Employee
" +
id + " " +
lname + " " + fname +
" deleted";
}
//completed
methods
public String Search()
{
return "Employee
" +
id + " " +
lname + " " + fname +
" found";
}
//abstract
method that is different
//from
Fulltime and Contractor
//therefore
i keep it uncompleted and
//let
each implementation
//complete
it the way they calculate the wage.
public abstract String CalculateWage();
}
}
Interface Employee
using System;
namespace AbstractsANDInterfaces
{
/// <span class="code-SummaryComment"><summary></span>
///
Summary description for IEmployee.
/// <span class="code-SummaryComment"></summary></span>
public interface IEmployee
{
//cannot
have fields. uncommenting
//will
raise error!
// protected String id;
// protected String lname;
// protected String fname;
//just
signature of the properties
//and
methods.
//setting
a rule or contract to be
//followed
by implementations.
String ID
{
get;
set;
}
String FirstName
{
get;
set;
}
String LastName
{
get;
set;
}
//
cannot have implementation
//
cannot have modifiers public
//
etc all are assumed public
//
cannot have virtual
String Update();
String Add();
String Delete();
String Search();
String CalculateWage();
}
}
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Inherited Objects
Emp_Fulltime:
using System;
namespace AbstractsANDInterfaces
{
///
///
Summary description for Emp_Fulltime.
///
//Inheriting
from the Abstract class
public class Emp_Fulltime : Employee
{
//uses
all the properties of the
//Abstract
class therefore no
//properties
or fields here!
public Emp_Fulltime()
{
}
public override String ID
{
get
{
return id;
}
set
{
id
= value;
}
}
public override String FirstName
{
get
{
return fname;
}
set
{
fname = value;
}
}
public override String LastName
{
get
{
return lname;
}
set
{
lname = value;
}
}
//common
methods that are
//implemented
in the abstract class
public new String Add()
{
return base.Add();
}
//common
methods that are implemented
//in
the abstract class
public new String Delete()
{
return base.Delete();
}
//common
methods that are implemented
//in
the abstract class
public new String Search()
{
return base.Search();
}
//common
methods that are implemented
//in
the abstract class
public new String Update()
{
return base.Update();
}
//abstract
method that is different
//from
Fulltime and Contractor
//therefore
I override it here.
public override String CalculateWage()
{
return "Full
time employee " +
base.fname + "
is calculated " +
"using
the Abstract class...";
}
}
}
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Emp_Fulltime2
using System;
namespace AbstractsANDInterfaces
{
///
///
Summary description for Emp_fulltime2.
///
//Implementing
the interface
public class Emp_fulltime2 : IEmployee
{
//All
the properties and
//fields
are defined here!
protected String id;
protected String lname;
protected String fname;
public Emp_fulltime2()
{
//
//
TODO: Add constructor logic here
//
}
public String ID
{
get
{
return id;
}
set
{
id
= value;
}
}
public String FirstName
{
get
{
return fname;
}
set
{
fname = value;
}
}
public String LastName
{
get
{
return lname;
}
set
{
lname = value;
}
}
//all
the manipulations including Add,Delete,
//Search,
Update, Calculate are done
//within
the object as there are not
//implementation
in the Interface entity.
public String Add()
{
return "Fulltime
Employee " +
fname + " added.";
}
public String Delete()
{
return "Fulltime
Employee " +
fname + " deleted.";
}
public String Search()
{
return "Fulltime
Employee " +
fname + " searched.";
}
public String Update()
{
return "Fulltime
Employee " +
fname + " updated.";
}
//if
you change to Calculatewage().
//Just
small 'w' it will raise
//error
as in interface
//it
is CalculateWage() with capital 'W'.
public String CalculateWage()
{
return "Full
time employee " +
fname + " caluculated using " +
"Interface.";
}
}
}
Code for Testing
//This is the sub that tests both
//implementations using
Interface and Abstract
private void InterfaceExample_Click(object sender,
System.EventArgs e)
{
try
{
IEmployee
emp;
Emp_fulltime2 emp1 = new Emp_fulltime2();
emp = emp1;
emp.ID = "2234";
emp.FirstName= "Rahman" ;
emp.LastName = "Mahmoodi" ;
//call
add method od the object
MessageBox.Show(emp.Add().ToString());
//call
the CalculateWage method
MessageBox.Show(emp.CalculateWage().ToString());
}
catch(Exception ex)
{
MessageBox.Show(ex.Message);
}
}
private void cmdAbstractExample_Click(object sender,
System.EventArgs e)
{
Employee emp;
emp = new Emp_Fulltime();
emp.ID = "2244";
emp.FirstName= "Maria" ;
emp.LastName = "Robinlius" ;
MessageBox.Show(emp.Add().ToString());
//call
the CalculateWage method
MessageBox.Show(emp.CalculateWage().ToString());
}
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About Author:
Nirmal Doshi is budding technology geek, who helps Systems Plus Pvt. Ltd. with his creativity and research on technology. He can be contacted at: nirmal.d@spluspl.com
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