Interface vs Abstract Class (general OO)

How about an analogy: when I was in the Air Force, I went to pilot training and became a USAF (US Air Force) pilot. At that point I wasn't qualified to fly anything, and had to attend aircraft type training. Once I qualified, I was a pilot (Abstract class) and a C-141 pilot (concrete class). At one of my assignments, I was given an additional duty: Safety Officer. Now I was still a pilot and a C-141 pilot, but I also performed Safety Officer duties (I implemented ISafetyOfficer, so to speak). A pilot wasn't required to be a safety officer, other people could have done it as well.

All USAF pilots have to follow certain Air Force-wide regulations, and all C-141 (or F-16, or T-38) pilots 'are' USAF pilots. Anyone can be a safety officer. So, to summarize:

Pilot: abstract class

C-141 Pilot: concrete class

ISafety Officer: interface

added note: this was meant to be an analogy to help explain the concept, not a coding recommendation. See the various comments below, the discussion is interesting.

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While your question indicates it's for "general OO", it really seems to be focusing on .NET use of these terms.

In .NET (similar for Java):

interfaces can have no state or implementation

a class that implements an interface must provide an implementation of all the methods of that interface

abstract classes may contain state (data members) and/or implementation (methods)

abstract classes can be inherited without implementing the abstract methods (though such a derived class is abstract itself)

interfaces may be multiple-inherited, abstract classes may not (this is probably the key concrete reason for interfaces to exist separately from abtract classes - they permit an implementation of multiple inheritance that removes many of the problems of general MI).

As general OO terms, the differences are not necessarily well-defined. For example, there are C++ programmers who may hold similar rigid definitions (interfaces are a strict subset of abstract classes that cannot contain implementation), while some may say that an abstract class with some default implementations is still an interface or that a non-abstract class can still define an interface.

Indeed, there is a C++ idiom called the Non-Virtual Interface (NVI) where the public methods are non-virtual methods that 'thunk' to private virtual methods:

http://www.gotw.ca/publications/mill18.htm

http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Non-Virtual_Interface

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I think the answer they are looking for is the fundamental or OPPS philosophical difference.

The abstract class inheritance is used when the derived class shares the core properties and behaviour of the abstract class. The kind of behaviour that actually defines the class.

On the other hand interface inheritance is used when the classes share peripheral behaviour, ones which do not necessarily define the derived class.

For eg. A Car and a Truck share a lot of core properties and behaviour of an Automobile abstract class, but they also share some peripheral behaviour like Generate exhaust which even non automobile classes like Drillers or PowerGenerators share and doesn't necessarily defines a Car or a Truck, so Car, Truck, Driller and PowerGenerator can all share the same interface IExhaust.

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Short: Abstract classes are used for Modelling a class hierarchy of similar looking classes (For example Animal can be abstract class and Human , Lion, Tiger can be concrete derived classes)

AND

Interface is used for Communication between 2 similar / non similar classes which does not care about type of the class implementing Interface(e.g. Height can be interface property and it can be implemented by Human , Building , Tree. It does not matter if you can eat , you can swim you can die or anything.. it matters only a thing that you need to have Height (implementation in you class) ).

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There are a couple of other differences -

Interfaces can't have any concrete implementations. Abstract base classes can. This allows you to provide concrete implementations there. This can allow an abstract base class to actually provide a more rigorous contract, wheras an interface really only describes how a class is used. (The abstract base class can have non-virtual members defining the behavior, which gives more control to the base class author.)

More than one interface can be implemented on a class. A class can only derive from a single abstract base class. This allows for polymorphic hierarchy using interfaces, but not abstract base classes. This also allows for a pseudo-multi-inheritance using interfaces.

Abstract base classes can be modified in v2+ without breaking the API. Changes to interfaces are breaking changes.

[C#/.NET Specific] Interfaces, unlike abstract base classes, can be applied to value types (structs). Structs cannot inherit from abstract base classes. This allows behavioral contracts/usage guidelines to be applied on value types.

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Inheritance Consider a car and a bus. They are two different vehicles. But still, they share some common properties like they have a steering, brakes, gears, engine etc. So with the inheritance concept, this can be represented as following ...

public class Vehicle {
    private Driver driver;
    private Seat[] seatArray; //In java and most of the Object Oriented Programming(OOP) languages, square brackets are used to denote arrays(Collections).
    //You can define as many properties as you want here ...
}

Now a Bicycle ...

public class Bicycle extends Vehicle {
    //You define properties which are unique to bicycles here ...
    private Pedal pedal;
}

And a Car ...

public class Car extends Vehicle {
    private Engine engine;
    private Door[] doors;
}

That's all about Inheritance. We use them to classify objects into simpler Base forms and their children as we saw above.

Abstract Classes

Abstract classes are incomplete objects. To understand it further, let's consider the vehicle analogy once again. A vehicle can be driven. Right? But different vehicles are driven in different ways ... For example, You cannot drive a car just as you drive a Bicycle. So how to represent the drive function of a vehicle? It is harder to check what type of vehicle it is and drive it with its own function; you would have to change the Driver class again and again when adding a new type of vehicle. Here comes the role of abstract classes and methods. You can define the drive method as abstract to tell that every inheriting children must implement this function. So if you modify the vehicle class ...

//......Code of Vehicle Class
abstract public void drive();
//.....Code continues

The Bicycle and Car must also specify how to drive it. Otherwise, the code won't compile and an error is thrown. In short.. an abstract class is a partially incomplete class with some incomplete functions, which the inheriting children must specify their own.

Interfaces Interfaces are totally incomplete. They do not have any properties. They just indicate that the inheriting children are capable of doing something ... Suppose you have different types of mobile phones with you. Each of them has different ways to do different functions; Ex: call a person. The maker of the phone specifies how to do it. Here the mobile phones can dial a number - that is, it is dial-able. Let's represent this as an interface.

public interface Dialable {
    public void dial(Number n);
}

Here the maker of the Dialable defines how to dial a number. You just need to give it a number to dial.

// Makers define how exactly dialable work inside.

Dialable PHONE1 = new Dialable() {
    public void dial(Number n) {
        //Do the phone1's own way to dial a number
    }
}

Dialable PHONE2 = new Dialable() {
    public void dial(Number n) {
        //Do the phone2's own way to dial a number
    }
}


//Suppose there is a function written by someone else, which expects a Dialable
......
public static void main(String[] args) {
    Dialable myDialable = SomeLibrary.PHONE1;
    SomeOtherLibrary.doSomethingUsingADialable(myDialable);
}
.....

Hereby using interfaces instead of abstract classes, the writer of the function which uses a Dialable need not worry about its properties. Ex: Does it have a touch-screen or dial pad, Is it a fixed landline phone or mobile phone. You just need to know if it is dialable; does it inherit(or implement) the Dialable interface.

And more importantly, if someday you switch the Dialable with a different one

......
public static void main(String[] args) {
    Dialable myDialable = SomeLibrary.PHONE2; // <-- changed from PHONE1 to PHONE2
    SomeOtherLibrary.doSomethingUsingADialable(myDialable);
}
.....

You can be sure that the code still works perfectly because the function which uses the dialable does not (and cannot) depend on the details other than those specified in the Dialable interface. They both implement a Dialable interface and that's the only thing the function cares about.

Interfaces are commonly used by developers to ensure interoperability(use interchangeably) between objects, as far as they share a common function (just like you may change to a landline or mobile phone, as far as you just need to dial a number). In short, interfaces are a much simpler version of abstract classes, without any properties. Also, note that you may implement(inherit) as many interfaces as you want but you may only extend(inherit) a single parent class.

More Info Abstract classes vs Interfaces

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If you consider java as OOP language to answer this question, Java 8 release causes some of the content in above answers as obsolete. Now java interface can have default methods with concrete implementation.

Oracle website provides key differences between interface and abstract class.

Consider using abstract classes if :

You want to share code among several closely related classes. 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). You want to declare non-static or non-final fields.

Consider using interfaces if :

You expect that unrelated classes would implement your interface. For example,many unrelated objects can implement Serializable interface. You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour. You want to take advantage of multiple inheritance of type.

In simple terms, I would like to use

interface: To implement a contract by multiple unrelated objects

abstract class: To implement the same or different behaviour among multiple related objects

Have a look at code example to understand things in clear way : How should I have explained the difference between an Interface and an Abstract class?

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The interviewers are barking up an odd tree. For languages like C# and Java, there is a difference, but in other languages like C++ there is not. OO theory doesn't differentiate the two, merely the syntax of language.

An abstract class is a class with both implementation and interface (pure virtual methods) that will be inherited. Interfaces generally do not have any implementation but only pure virtual functions.

In C# or Java an abstract class without any implementation differs from an interface only in the syntax used to inherit from it and the fact you can only inherit from one.

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By implementing interfaces you are achieving composition ("has-a" relationships) instead of inheritance ("is-a" relationships). That is an important principle to remember when it comes to things like design patterns where you need to use interfaces to achieve a composition of behaviors instead of an inheritance.

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Conceptually speaking, keeping the language specific implementation, rules, benefits and achieving any programming goal by using anyone or both, can or cant have code/data/property, blah blah, single or multiple inheritances, all aside

1- Abstract (or pure abstract) Class is meant to implement hierarchy. If your business objects look somewhat structurally similar, representing a parent-child (hierarchy) kind of relationship only then inheritance/Abstract classes will be used. If your business model does not have a hierarchy then inheritance should not be used (here I am not talking about programming logic e.g. some design patterns require inheritance). Conceptually, abstract class is a method to implement hierarchy of a business model in OOP, it has nothing to do with Interfaces, actually comparing Abstract class with Interface is meaningless because both are conceptually totally different things, it is asked in interviews just to check the concepts because it looks both provide somewhat same functionality when implementation is concerned and we programmers usually emphasize more on coding. [Keep this in mind as well that Abstraction is different than Abstract Class].

2- an Interface is a contract, a complete business functionality represented by one or more set of functions. That is why it is implemented and not inherited. A business object (part of a hierarchy or not) can have any number of complete business functionality. It has nothing to do with abstract classes means inheritance in general. For example, a human can RUN, an elephant can RUN, a bird can RUN, and so on, all these objects of different hierarchy would implement the RUN interface or EAT or SPEAK interface. Don't go into implementation as you might implement it as having abstract classes for each type implementing these interfaces. An object of any hierarchy can have a functionality(interface) which has nothing to do with its hierarchy.

I believe, Interfaces were not invented to achieve multiple inheritances or to expose public behavior, and similarly, pure abstract classes are not to overrule interfaces but Interface is a functionality that an object can do (via functions of that interface) and Abstract Class represents a parent of a hierarchy to produce children having core structure (property+functionality) of the parent

When you are asked about the difference, it is actually conceptual difference not the difference in language-specific implementation unless asked explicitly.

I believe, both interviewers were expecting one line straightforward difference between these two and when you failed they tried to drove you towards this difference by implementing ONE as the OTHER

What if you had an Abstract class with only abstract methods?

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i will explain Depth Details of interface and Abstract class.if you know overview about interface and abstract class, then first question arrive in your mind when we should use Interface and when we should use Abstract class. So please check below explanation of Interface and Abstract class.

When we should use Interface? if you don't know about implementation just we have requirement specification then we go with Interface When we should use Abstract Class? if you know implementation but not completely (partially implementation) then we go with Abstract class. Interface every method by default public abstract means interface is 100% pure abstract. Abstract can have Concrete method and Abstract method, what is Concrete method, which have implementation in Abstract class, An abstract class is a class that is declared abstract—it may or may not include abstract methods. Interface We cannot declared interface as a private, protected Q. Why we are not declaring Interface a private and protected? Because by default interface method is public abstract so and so that reason that we are not declaring the interface as private and protected. Interface method also we cannot declared interface as private,protected,final,static,synchronized,native..... i will give the reason: why we are not declaring synchronized method because we cannot create object of interface and synchronize are work on object so and son reason that we are not declaring the synchronized method Transient concept are also not applicable because transient work with synchronized. Abstract we are happily use with public,private final static.... means no restriction are applicable in abstract. Interface Variables are declared in Interface as a by default public static final so we are also not declared variable as a private, protected. Volatile modifier is also not applicable in interface because interface variable is by default public static final and final variable you cannot change the value once it assign the value into variable and once you declared variable into interface you must to assign the variable. And volatile variable is keep on changes so it is opp. to final that is reason we are not use volatile variable in interface. Abstract Abstract variable no need to declared public static final.

i hope this article is useful.

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These answers are all too long.

Interfaces are for defining behaviors.

Abstract classes are for defining a thing itself, including its behaviors. That's why we sometimes create an abstract class with some extra properties inheriting an interface.

This also explains why Java only supports single inheritance for classes but puts no restriction on interfaces. Because a concrete object can not be different things, but it can have different behaviors.

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For .Net,

Your answer to The second interviewer is also the answer to the first one... Abstract classes can have implementation, AND state, interfaces cannot...

EDIT: On another note, I wouldn't even use the phrase 'subclass' (or the 'inheritance' phrase) to describe classes that are 'defined to implement' an interface. To me, an interface is a definition of a contract that a class must conform to if it has been defined to 'implement' that interface. It does not inherit anything... You have to add everything yourself, explicitly.

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Interface : should be used if you want to imply a rule on the components which may or may not be related to each other

Pros:

Allows multiple inheritance Provides abstraction by not exposing what exact kind of object is being used in the context provides consistency by a specific signature of the contract

Cons:

Must implement all the contracts defined Cannot have variables or delegates Once defined cannot be changed without breaking all the classes

Abstract Class : should be used where you want to have some basic or default behaviour or implementation for components related to each other

Pros:

Faster than interface Has flexibility in the implementation (you can implement it fully or partially) Can be easily changed without breaking the derived classes

Cons:

Cannot be instantiated Does not support multiple inheritance

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I think they didn't like your response because you gave the technical differences instead of design ones. The question is like a troll question for me. In fact, interfaces and abstract classes have a completely different nature so you cannot really compare them. I will give you my vision of what is the role of an interface and what is the role of an abstract class.

interface: is used to ensure a contract and make a low coupling between classes in order to have a more maintainable, scalable and testable application.

abstract class: is only used to factorize some code between classes of the same responsability. Note that this is the main reason why multiple-inheritance is a bad thing in OOP, because a class shouldn't handle many responsabilities (use composition instead).

So interfaces have a real architectural role whereas abstract classes are almost only a detail of implementation (if you use it correctly of course).

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Interface: We do not implement (or define) methods, we do that in derived classes. We do not declare member variables in interfaces. Interfaces express the HAS-A relationship. That means they are a mask of objects. Abstract class: We can declare and define methods in abstract class. We hide constructors of it. That means there is no object created from it directly. Abstract class can hold member variables. Derived classes inherit to abstract class that mean objects from derived classes are not masked, it inherit to abstract class. The relationship in this case is IS-A.

This is my opinion.

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After all that, the interviewer came up with the question "What if you had an 
Abstract class with only abstract methods? How would that be different
from an interface?" 

Docs clearly say that if an abstract class contains only abstract method declarations, it should be declared as an interface instead.

An another interviewer asked me what if you had a Public variable inside
the interface, how would that be different than in Abstract Class?

Variables in Interfaces are by default public static and final. Question could be framed like what if all variables in abstract class are public? Well they can still be non static and non final unlike the variables in interfaces.

Finally I would add one more point to those mentioned above - abstract classes are still classes and fall in a single inheritance tree whereas interfaces can be present in multiple inheritance.

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Copied from CLR via C# by Jeffrey Richter...

I often hear the question, “Should I design a base type or an interface?” The answer isn’t always clearcut.

Here are some guidelines that might help you:

■■ IS-A vs. CAN-DO relationship A type can inherit only one implementation. If the derived type can’t claim an IS-A relationship with the base type, don’t use a base type; use an interface. Interfaces imply a CAN-DO relationship. If the CAN-DO functionality appears to belong with various object types, use an interface. For example, a type can convert instances of itself to another type (IConvertible), a type can serialize an instance of itself (ISerializable), etc. Note that value types must be derived from System.ValueType, and therefore, they cannot be derived from an arbitrary base class. In this case, you must use a CAN-DO relationship and define an interface.

■■ Ease of use It’s generally easier for you as a developer to define a new type derived from a base type than to implement all of the methods of an interface. The base type can provide a lot of functionality, so the derived type probably needs only relatively small modifications to its behavior. If you supply an interface, the new type must implement all of the members.

■■ Consistent implementation No matter how well an interface contract is documented, it’s very unlikely that everyone will implement the contract 100 percent correctly. In fact, COM suffers from this very problem, which is why some COM objects work correctly only with Microsoft Word or with Windows Internet Explorer. By providing a base type with a good default implementation, you start off using a type that works and is well tested; you can then modify parts that need modification.

■■ Versioning If you add a method to the base type, the derived type inherits the new method, you start off using a type that works, and the user’s source code doesn’t even have to be recompiled. Adding a new member to an interface forces the inheritor of the interface to change its source code and recompile.

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An interface defines a contract for a service or set of services. They provide polymorphism in a horizontal manner in that two completely unrelated classes can implement the same interface but be used interchangeably as a parameter of the type of interface they implement, as both classes have promised to satisfy the set of services defined by the interface. Interfaces provide no implementation details.

An abstract class defines a base structure for its sublcasses, and optionally partial implementation. Abstract classes provide polymorphism in a vertical, but directional manner, in that any class that inherits the abstract class can be treated as an instance of that abstract class but not the other way around. Abstract classes can and often do contain implementation details, but cannot be instantiated on their own- only their subclasses can be "newed up".

C# does allow for interface inheritance as well, mind you.

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tl;dr; When you see “Is A” relationship use inheritance/abstract class. when you see “has a” relationship create member variables. When you see “relies on external provider” implement (not inherit) an interface.

Interview Question: What is the difference between an interface and an abstract class? And how do you decide when to use what? I mostly get one or all of the below answers: Answer 1: You cannot create an object of abstract class and interfaces.

ZK (That’s my initials): You cannot create an object of either. So this is not a difference. This is a similarity between an interface and an abstract class. Counter Question: Why can’t you create an object of abstract class or interface?

Answer 2: Abstract classes can have a function body as partial/default implementation.

ZK: Counter Question: So if I change it to a pure abstract class, marking all the virtual functions as abstract and provide no default implementation for any virtual function. Would that make abstract classes and interfaces the same? And could they be used interchangeably after that?

Answer 3: Interfaces allow multi-inheritance and abstract classes don’t.

ZK: Counter Question: Do you really inherit from an interface? or do you just implement an interface and, inherit from an abstract class? What’s the difference between implementing and inheriting? These counter questions throw candidates off and make most scratch their heads or just pass to the next question. That makes me think people need help with these basic building blocks of Object-Oriented Programming. The answer to the original question and all the counter questions is found in the English language and the UML. You must know at least below to understand these two constructs better.

Common Noun: A common noun is a name given “in common” to things of the same class or kind. For e.g. fruits, animals, city, car etc.

Proper Noun: A proper noun is the name of an object, place or thing. Apple, Cat, New York, Honda Accord etc.

Car is a Common Noun. And Honda Accord is a Proper Noun, and probably a Composit Proper noun, a proper noun made using two nouns.

Coming to the UML Part. You should be familiar with below relationships:

Is A

Has A

Uses

Let’s consider the below two sentences. - HondaAccord Is A Car? - HondaAccord Has A Car?

Which one sounds correct? Plain English and comprehension. HondaAccord and Cars share an “Is A” relationship. Honda accord doesn’t have a car in it. It “is a” car. Honda Accord “has a” music player in it.

When two entities share the “Is A” relationship it’s a better candidate for inheritance. And Has a relationship is a better candidate for creating member variables. With this established our code looks like this:

abstract class Car
{
   string color;
   int speed;
}
class HondaAccord : Car
{
   MusicPlayer musicPlayer;
}

Now Honda doesn't manufacture music players. Or at least it’s not their main business.

So they reach out to other companies and sign a contract. If you receive power here and the output signal on these two wires it’ll play just fine on these speakers.

This makes Music Player a perfect candidate for an interface. You don’t care who provides support for it as long as the connections work just fine.

You can replace the MusicPlayer of LG with Sony or the other way. And it won’t change a thing in Honda Accord.

Why can’t you create an object of abstract classes?

Because you can’t walk into a showroom and say give me a car. You’ll have to provide a proper noun. What car? Probably a honda accord. And that’s when a sales agent could get you something.

Why can’t you create an object of an interface? Because you can’t walk into a showroom and say give me a contract of music player. It won’t help. Interfaces sit between consumers and providers just to facilitate an agreement. What will you do with a copy of the agreement? It won’t play music.

Why do interfaces allow multiple inheritance?

Interfaces are not inherited. Interfaces are implemented. The interface is a candidate for interaction with the external world. Honda Accord has an interface for refueling. It has interfaces for inflating tires. And the same hose that is used to inflate a football. So the new code will look like below:

abstract class Car
{
    string color;
    int speed;
}
class HondaAccord : Car, IInflateAir, IRefueling
{
    MusicPlayer musicPlayer;
}

And the English will read like this “Honda Accord is a Car that supports inflating tire and refueling”.

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Most answers focus on the technical difference between Abstract Class and Interface, but since technically, an interface is basically a kind of abstract class (one without any data or implementation), I think the conceptual difference is far more interesting, and that might be what the interviewers are after.

An Interface is an agreement. It specifies: "this is how we're going to talk to each other". It can't have any implementation because it's not supposed to have any implementation. It's a contract. It's like the .h header files in C.

An Abstract Class is an incomplete implementation. A class may or may not implement an interface, and an abstract class doesn't have to implement it completely. An abstract class without any implementation is kind of useless, but totally legal.

Basically any class, abstract or not, is about what it is, whereas an interface is about how you use it. For example: Animal might be an abstract class implementing some basic metabolic functions, and specifying abstract methods for breathing and locomotion without giving an implementation, because it has no idea whether it should breathe through gills or lungs, and whether it flies, swims, walks or crawls. Mount, on the other hand, might be an Interface, which specifies that you can ride the animal, without knowing what kind of animal it is (or whether it's an animal at all!).

The fact that behind the scenes, an interface is basically an abstract class with only abstract methods, doesn't matter. Conceptually, they fill totally different roles.

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As you might have got the theoretical knowledge from the experts, I am not spending much words in repeating all those here, rather let me explain with a simple example where we can use/cannot use Interface and Abstract class.

Consider you are designing an application to list all the features of Cars. In various points you need inheritance in common, as some of the properties like DigitalFuelMeter, Air Conditioning, Seat adjustment, etc are common for all the cars. Likewise, we need inheritance for some classes only as some of the properties like the Braking system (ABS,EBD) are applicable only for some cars.

The below class acts as a base class for all the cars:

public class Cars
{
    public string DigitalFuelMeter()
    {
        return "I have DigitalFuelMeter";
    }

    public string AirCondition()
    {
        return "I have AC";
    }

    public string SeatAdjust()
    {
        return "I can Adjust seat";
    }
}

Consider we have a separate class for each Cars.

public class Alto : Cars
{
    // Have all the features of Car class    
}

public class Verna : Cars
{
    // Have all the features of Car class + Car need to inherit ABS as the Braking technology feature which is not in Cars        
}

public class Cruze : Cars
{
    // Have all the features of Car class + Car need to inherit EBD as the Braking technology feature which is not in Cars        
}

Consider we need a method for inheriting the Braking technology for the cars Verna and Cruze (not applicable for Alto). Though both uses braking technology, the "technology" is different. So we are creating an abstract class in which the method will be declared as Abstract and it should be implemented in its child classes.

public abstract class Brake
{
    public abstract string GetBrakeTechnology();
}

Now we are trying to inherit from this abstract class and the type of braking system is implemented in Verna and Cruze:

public class Verna : Cars,Brake
{
    public override string GetBrakeTechnology()
    {
        return "I use ABS system for braking";
    }       
}

public class Cruze : Cars,Brake
{
    public override string GetBrakeTechnology()
    {
       return "I use EBD system for braking";
    }         
}

See the problem in the above two classes? They inherit from multiple classes which C#.Net doesn't allow even though the method is implemented in the children. Here it comes the need of Interface.

interface IBrakeTechnology
{
    string GetBrakeTechnology();
}

And the implementation is given below:

public class Verna : Cars, IBrakeTechnology
{
    public string GetBrakeTechnology()
    {
        return "I use ABS system for braking";
    }
}

public class Cruze : Cars, IBrakeTechnology
{
   public string GetBrakeTechnology()
   {
       return "I use EBD system for braking";
   }        
}

Now Verna and Cruze can achieve multiple inheritance with its own kind of braking technologies with the help of Interface.

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Interfaces are light weight way to enforce a particular behavior. That is one way to think of.

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1) An interface can be seen as a pure Abstract Class, is the same, but despite this, is not the same to implement an interface and inheriting from an abstract class. When you inherit from this pure abstract class you are defining a hierarchy -> inheritance, if you implement the interface you are not, and you can implement as many interfaces as you want, but you can only inherit from one class.

2) You can define a property in an interface, so the class that implements that interface must have that property.

For example:

  public interface IVariable
  {
      string name {get; set;}
  }

The class that implements that interface must have a property like that.

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Though this question is quite old, I would like to add one other point in favor of interfaces:

Interfaces can be injected using any Dependency Injection tools where as Abstract class injection supported by very few.

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From another answer of mine, mostly dealing with when to use one versus the other:

In my experience, interfaces are best used when you have several classes which each need to respond to the same method or methods so that they can be used interchangeably by other code which will be written against those classes' common interface. The best use of an interface is when the protocol is important but the underlying logic may be different for each class. If you would otherwise be duplicating logic, consider abstract classes or standard class inheritance instead.

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Interface Types vs. Abstract Base Classes

Adapted from the Pro C# 5.0 and the .NET 4.5 Framework book.

The interface type might seem very similar to an abstract base class. Recall that when a class is marked as abstract, it may define any number of abstract members to provide a polymorphic interface to all derived types. However, even when a class does define a set of abstract members, it is also free to define any number of constructors, field data, nonabstract members (with implementation), and so on. Interfaces, on the other hand, contain only abstract member definitions. The polymorphic interface established by an abstract parent class suffers from one major limitation in that only derived types support the members defined by the abstract parent. However, in larger software systems, it is very common to develop multiple class hierarchies that have no common parent beyond System.Object. Given that abstract members in an abstract base class apply only to derived types, we have no way to configure types in different hierarchies to support the same polymorphic interface. By way of example, assume you have defined the following abstract class:

public abstract class CloneableType
{
// Only derived types can support this
// "polymorphic interface." Classes in other
// hierarchies have no access to this abstract
// member.
   public abstract object Clone();
}

Given this definition, only members that extend CloneableType are able to support the Clone() method. If you create a new set of classes that do not extend this base class, you can’t gain this polymorphic interface. Also, you might recall that C# does not support multiple inheritance for classes. Therefore, if you wanted to create a MiniVan that is-a Car and is-a CloneableType, you are unable to do so:

// Nope! Multiple inheritance is not possible in C#
// for classes.
public class MiniVan : Car, CloneableType
{
}

As you would guess, interface types come to the rescue. After an interface has been defined, it can be implemented by any class or structure, in any hierarchy, within any namespace or any assembly (written in any .NET programming language). As you can see, interfaces are highly polymorphic. Consider the standard .NET interface named ICloneable, defined in the System namespace. This interface defines a single method named Clone():

public interface ICloneable
{
object Clone();
}

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Answer to the second question : public variable defined in interface is static final by default while the public variable in abstract class is an instance variable.

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For sure it is important to understand the behavior of interface and abstract class in OOP (and how languages handle them), but I think it is also important to understand what exactly each term means. Can you imagine the if command not working exactly as the meaning of the term? Also, actually some languages are reducing, even more, the differences between an interface and an abstract... if by chance one day the two terms operate almost identically, at least you can define yourself where (and why) should any of them be used for.

If you read through some dictionaries and other fonts you may find different meanings for the same term but having some common definitions. I think these two meanings I found in this site are really, really good and suitable.

Interface:

A thing or circumstance that enables separate and sometimes incompatible elements to coordinate effectively.

Abstract:

Something that concentrates in itself the essential qualities of anything more extensive or more general, or of several things; essence.

Example:

You bought a car and it needs fuel.

https://i.stack.imgur.com/2zvSS.png

Your car model is XYZ, which is of genre ABC, so it is a concrete car, a specific instance of a car. A car is not a real object. In fact, it is an abstract set of standards (qualities) to create a specific object. In short, Car is an abstract class, it is "something that concentrates in itself the essential qualities of anything more extensive or more general".

The only fuel that matches the car manual specification should be used to fill up the car tank. In reality, there is nothing to restrict you to put any fuel but the engine will work properly only with the specified fuel, so it is better to follow its requirements. The requirements say that it accepts, as other cars of the same genre ABC, a standard set of fuel.

In an Object Oriented view, fuel for genre ABC should not be declared as a class because there is no concrete fuel for a specific genre of car out there. Although your car could accept an abstract class Fuel or VehicularFuel, you must remember that your only some of the existing vehicular fuel meet the specification, those that implement the requirements in your car manual. In short, they should implement the interface ABCGenreFuel, which "... enables separate and sometimes incompatible elements to coordinate effectively".

Addendum

In addition, I think you should keep in mind the meaning of the term class, which is (from the same site previously mentioned):

Class:

A number of persons or things regarded as forming a group by reason of common attributes, characteristics, qualities, or traits; kind;

This way, a class (or abstract class) should not represent only common attributes (like an interface), but some kind of group with common attributes. An interface doesn't need to represent a kind. It must represent common attributes. This way, I think classes and abstract classes may be used to represent things that should not change its aspects often, like a human being a Mammal, because it represents some kinds. Kinds should not change themselves that often.

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From Coding Perspective

An Interface can replace an Abstract Class if the Abstract Class has only abstract methods. Otherwise changing Abstract class to interface means that you will be losing out on code re-usability which Inheritance provides.

From Design Perspective

Keep it as an Abstract Class if it's an "Is a" relationship and you need a subset or all of the functionality. Keep it as Interface if it's a "Should Do" relationship.

Decide what you need: just the policy enforcement, or code re-usability AND policy.

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