Abstract Class vs Interface

Well, I can say that interface is for "PURE ABSTRACTION". Pure abstraction means you can not have concrete methods. But an abstract class can have concrete methods. Interfaces should not have any concrete methods, it should only have method declarations.

I will explain the differences between the abstract class and Interface in an old-fashioned tabular format –

Abstract Class	Interface
It can have concrete methods	It can only have abstract methods
It can have variables of any access specifier	It can have only public static final data members(i.e. only constant)
Concrete methods can have any access specifier	For interfaces, all methods are public and abstract by default
A class can extend only one abstract class	A class can implement any no. of interfaces

Abstract classes are meant to be inherited from, and when one class inherits from another it means that there is a strong relationship between the 2 classes.

An abstract class would be better to use or I can say it would be more appropriate when there is a strong relationship between the abstract class and the classes that will derive from it. This is because an abstract class is very closely linked to inheritance, which signifies a strong relationship. But, with interfaces there need not be a strong relationship between the interface and the classes that implement the interface.

In Java, a class can only derive from one class, whether it’s abstract or not. However, a class can implement multiple interfaces – which could be considered as an alternative to for multiple inheritance. So, one major difference between the two is that a Java class can inherit from only one abstract class, but can implement multiple interfaces.

At times there is a lot of confusion among the developers regarding which should one use, abstract classes or interfaces?

Below are some of the scenarios where one can consider using either the abstract class or an interface -

·         Consider using abstract classes if any of these statements applies:

o You want to share code among several closely related classes.

o You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).

o You want to declare non-static or non-final fields. This enables you to define methods that can access and modify the state of the object to which they belong.

·         Consider using interfaces if any of these statements applies:

o You expect that unrelated classes would implement your interface.

o You want to specify the behavior of a particular data type, but not concerned about who implements its behavior.

o You want to take advantage of multiple inheritance of type.

Use abstract classes when you want common behavior for same category classes.

Use interfaces when you want common behavior for different category classes.

In simple words,

When you want to develop some defined methods and some undefined methods as a common for multiple developers then use abstract class, otherwise if you want give only definition of methods for multiple developers as a common methods in such a way that according to their need they will implement those methods then use interface... In simple terms Abstract class contains some implemented methods and some unimplemented methods, where as interface contains only unimplemented methods....

Here are some guidelines on when to use an abstract class and when to use interfaces in Java programming:

·   An abstract class is good if you think you will plan on using inheritance since it provides a common base class implementation to derived classes.

·   An abstract class is also good if you want to be able to declare non-public members. In an interface, all methods must be public.

·   If you think you will need to add methods in the future, then an abstract class is a better choice. Because if you add new method headings to an interface, then all of the classes that already implement that interface will have to be changed to implement the new methods. That can be quite a hassle.

·  Interfaces are a good choice when you think that the API will not change for a while.

· Interfaces are also good when you want to have something similar to multiple inheritance, since you can implement multiple interfaces.

I think that interfaces should be used when we need to define a common behaviour which can have different variations of implementation, but abstract classes are used when we have some algorithmic steps/template for a behaviour that should be followed by all subclasses irrespective of each step implementation in the algorithm.
Interfaces are just the blueprints whereas abstract class include partial implementation. Obviously we can do our job without using interfaces and only using abstract classes but this will be bad practice because the code will NOT be modular.

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Abstraction

Hey Guys, I am going to explain you a very basic and an important oops concept - Abstraction.

Abstraction in simple way or Abstraction in easy way can be said as Hiding . I have explained the whole abstraction concept with the help of abstract classes and abstraction examples.

Abstract classes -

What are the Abstract classes meant for?

The answer to my question is abstract classes are meant for 'abstracting' means if some classes are having common behavior, instead of writing every time the same thing in each class, write that in one class and ask the other classes to use it (by making the classes as sub classes to the abstract class). this is nothing but inheritance.

In simple words I would say - A class must be declared abstract when it has one or more abstract methods. So a class having no method can be declared as abstract or not ?? Think Think about it

or simply try in eclipse.

A method is declared abstract when it has a method heading, but no body – which means that an abstract method has no implementation code inside curly braces like normal methods do.

A non-abstract class is called a concrete class.

Now, the question arises - When to use abstract methods in java?

I would like to explain this by illustrating an example here -

/* the GeometricalFigure class must be declared as abstract

   because it contains an abstract method  */

public abstract class GeometricalFigure

{

        /* because this is an abstract method so the

       body will be blank  */

        public abstract float getArea();

}

public class Circle extends GeometricalFigure

{

        private float radius;

        public float getArea()

        {

                return (3.14 * (radius * 2));

        }

}

public class Rectangle extends GeometricalFigure

{

        private float length, width;

public float getArea(GeometricalFigure other)

        {

                return length * width;

        }

}

But one question you all would be thinking is as to why did I declare the getArea method to be abstract in the GeometricalFigure class? Well, what does the getArea method do? It returns the area of a specific shape. But, because the GeometricalFigure class isn’t a specific shape (like a Circle or a Rectangle), there’s really no definition we can give the getArea method inside the GeometricalFigure class. That’s why we declare the method and the GeometricalFigure class to be abstract. Any classes that derive from the GeometricalFigure class basically has 2 options: 1. The derived class must provide a definition for the getArea method OR 2. The derived class must be declared abstract itself.

In a nutshell, An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.

When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract.

With abstract classes, you can declare fields that are not static and final, and define public, protected, and private concrete methods.

Important points you all need to make a note of -

• An abstract class may have static fields and static methods.

• An abstract class can also have constructors and instance variables as well. It can have static or final instance variables as well.

• An abstract class may have static fields and static methods.

• An abstract class can also have constructors and instance variables as well. It can have static or final instance variables as well.

I would want like to give  an illustration on the concept of abstract classes and methods with the help of a basic self explanatory example ---->

You want to use an AbstractClass when although your sub-types have to implement some specific logic, you still have logic common to all sub-types like in this case:

public abstract class GeometricalShape

{

     private String name;

     private int  noOfEdges;

     protected GeometricalShape(name,noOfEdges)

**abstract methods

     public abstract GetArea();

      public abstract GetPerimeter();

**Concrete methods

      public GetName()

                                          {

return name;

}

     

public int GetNoOfEdges()

{

return noOfEdges;

}

}

In this example each geometric shape will have to implement specific logic regarding Area and Perimeter like before, but now all sub-types will share common methods for retrieving the name and no. of edges of the geometrical, which would be redundant to be defined in each and every sub-class.

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