In real time how the abstraction and interface uses when we go for those concept in real time.
Both abstract classes and interfaces are used when there is a difference in behaviour among the sub-types extending the abstract class or implementing the interface.
When the sub-types behaviour is totally different then you use an interface, when the sub-types behaviour is partially common and different with respect to the supertype an abstract class is used. In an abstract class the partially common behaviour is given a concrete implementation. Since there is no common behaviour between an interface and a sub-type an interface does not have an implementation for any of its behaviour.
Lets assume you are developing a framework for a 'killer' competition where the people who participate have to demonstrate their killing skills. Members participating in the competetion have their own ways of killing.
It could be as different as 'using a gun to kill', 'throwing the victim up in the air and kicking his balls when he victim comes down, 'slitting his throat or behaeding the victim'
The expected behaviour here is to 'KILL', though all the members do have that behaviour 'KILL" each member manifests the behaviour in a different way. Since the behaviour manifestation varies from member to member this scenario calls for an interface.
Interface KilingCompetition{
Corpse kill();
}
Each member will implement this interface and give an implementation in his/her own way.
Now, lets consider a karate competition(is it called sparring?). Here each member has to follow certain things as laid out by the organizers or the karate protocol. It may include things like bowing to the opponent before and after the fight starts, taking a stand etc. Now these behaviours are common and has to manifested in the same way by every paticipating member. But the behaviour 'fight' will differ from member to member. So now we have a set of common behaviour and also a behaviour which manifests differently from member to member. this calls for an abstract class where you give implementation to the common behaviour and make the differeing behaviour abstract and hence the class abstract.
public abstract class KarateFight{
public void bowOpponent(){
//implementation for bowing which is common
// for every participant
}
public void takeStand(){
//implementation which is common
// for every participant
}
public abstract boolean fight(Opponent op);
//this is abstract because it differs from
// person to person
}
Vinay.
What is the difference between interface and abstract class?
* interface contains methods that must be abstract; abstract class may contain concrete methods.
* interface contains variables that must be static and final; abstract class may contain non-final and final variables.
* members in an interface are public by default, abstract class may contain non-public members.
* interface is used to "implements"; whereas abstract class is used to "extends".
* interface can be used to achieve multiple inheritance; abstract class can be used as a single inheritance.
* interface can "extends" another interface, abstract class can "extends" another class and "implements" multiple interfaces.
* interface is absolutely abstract; abstract class can be invoked if a main() exists.
* interface is more flexible than abstract class because one class can only "extends" one super class, but "implements" multiple interfaces.
* If given a choice, use interface instead of abstract class.
When to use an Abstract Class and an Interface
For some odd reason, work allows me to handle phone screens and interviews. Each time I give an interview, I try to do three things. First I ask them about general programming questions. This might be OO questions. It might be methodology questions. It might be design pattern questions. Next I like to ask them more specific technologies questions, such as questions “how do you do ABC in Flex? Java?”. Lastly I want to know what they do in their spare time. What books they read? Do they code outside of work? How they go about researching new technologies? Etc.
However, the place I find people getting stuck are basic/general programming knowledge. Recently I conducted an interview, and this person just missed every question I asked. I don’t mind if people miss questions. Sometimes, it just takes some leading and they will get the correct answer. There are certain things though that if you miss entirely, then we have a problem. This has inspired me for this new section that I would like to call “Learn This”. These are topics that I find rather important for a potential candidate to know. There are a few past articles I could think about putting into this section, but I will start fresh.
Abstract Class vs an Interface.
I normally used this “What is the difference between an Abstract Class and an Interface” as a quick way to gauge someone. Lots of times, its the first or second question I will ask. I cannot tell you how many times people will mess this question up. 9 times out of 10, people read about it at some www.basicinterviewquestions.com (not a real site hehe), giving the canned response of “You can define default functionality in an abstract class and you can just define functions in an interface”. The curve ball is thrown when you ask “Why would you use one over the other?”. That will earn you the ‘deer in headlights’ look. The other 1 out of 10 you will get a “I never had to use that so I don’t know”.
At the top level, the are a few basic difference. Abstract classes allow for default default function definition. This means that whatever class extends the abstract class will have access to this. If we have a base class where all the classes will perform the same function, then we can define that in our Abstract class. An interface is a list of functions or properties that if a class implements it, it will have to have those functions defined within it. It is a situation of “Is-A” vs “Can-Do-this”. Objects that extends an Abstract class “Is-A” base class. Objects that implement “Can-Do-This”. Now if I asked this question and got the answer, yes, that would be the correct answer. However, I want to know why one would want to use an interface over an abstract class, and vice versa.
When to prefer an interface
Back when I wrote about the importance of composition, I mentioned that it is extremely useful when you don’t want a massive hierarchical type framework. The same applies to interfaces. This isn’t my example, but its the best one Ive come across. Lets say you have an interface for a Director and another interface for a Actor.
In reality, there are Actors who are also Directors. If we are using interfaces rather than abstract classes, we can implement both Actor and Director. We could even define an ActorDirector interface that extends both like this:
We could achieve the same thing using abstract classes. Unfortunately the alternative would require up to 2^n (where n is the number of attributes) possible combinations in order to support all possibilities.
When to prefer an Abstract class
Abstract classes allow you to provide default functionality for the subclasses. Common knowledge at this point. Why is this extremely important though? If you plan on updating this base class throughout the life of your program, it is best to allow that base class to be an abstract class. Why? Because you can make a change to it and all of the inheriting classes will now have this new functionality. If the base class will be changing often and an interface was used instead of an abstract class, we are going to run into problems. Once an interface is changed, any class that implements that will be broken. Now if its just you working on the project, that’s no big deal. However, once your interface is published to the client, that interface needs to be locked down. At that point, you will be breaking the clients code.
Speaking from personal experiences, frameworks is a good place to show when and where to use both an abstract class and an interface. Another general rule is if you are creating something that provides common functionality to unrelated classes, use an interface. If you are creating something for objects that are closely related in a hierarchy, use an abstract class. An example of this would be something like a business rules engine. This engine would take in multiple BusinessRules as classes perhaps? Each one of these classes will have an analyze function on it.
This can be used ANYWHERE. It can be used to verify the state of your application. Verify data is correct. Verify that the user is logged in. Each one of these classes just needs to implement the analyze function, which will be different for each rule.
Where as if we were creating a generic List object, the use of abstract classes would be better. Every single List object is going to display the data in a list in some form or another. The base functionality would be to have it go through its dataprovider and build that list. If we want to change that List object, we just extend it, override our build list function, change what we want and call super.buildList();
Almost everyone knows that interfaces means you are just defining a list of functions and that abstract classes has the option of providing default functionality. The snags come when you drop the ‘why would I use one over the other?’. Abstract classes and interfaces are some of the most important fundamentals of object oriented programming. Just knowing the differences between the two is not enough. When you can look at a situation and make a strong recommendation, you will known you have a much stronger knowledge of object oriented programming. Also it helps during interviews. 
Choosing between Abstract and Interface
Abstract classes and interfaces offer similar functionality, but both have unique pros and cons. Because abstract classes can offer implementations in addition to just an interface, they can make versioning much simpler. Thus, they are the default recommendation, although there are some scenarios in which interfaces make sense too.
As an example of the versioning difficulties they can introduce, imagine that you have released an abstract class and interface with two methods, void A() and void B(). You are basically stuck with them. That is, you cannot remove them without breaking classes that had derived from your class or implemented your interface. With abstract classes, however, you can extend your class over time. If you wanted to add a newvoid C() method, for example, you could add this on the abstract class with some default implementation. Similarly, if you want to add convenience overloads, you are free to do so with abstract classes. With interfaces, you simply cannot.
Conversely, abstract classes take over derived classes’ type hierarchy. A class can implement an interface yet still maintain some type hierarchy that makes sense. With abstract classes, this is not so. Furthermore, with interfaces you achieve multiple interface inheritance, whereas with abstract classes you cannot.
Source:- http://www.sap-img.com/java/when-we-go-for-abstract-and-interface.htm
http://en.csharp-online.net/Common_Type_System%E2%80%94Choosing_between_Abstract_and_Interface
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