Real World Example of Abstract Class and Interface

An abstract class is a class that you cannot create an instance of. It can provide basic functionality, but in order for that functionality to be used, one or more other classes must derive from the abstract class. One of the major benefits of abstract classes is that you can reuse code without having to retype it. 

That has a plethora of benefits, such as reducing bugs and making coding faster. A concrete example of an abstract class would be a class called Animal. You see many animals in real life, but there are only kinds of animals. That is, you never look at something purple and furry and say "that is an animal and there is no more specific way of defining it". Instead, you see a dog or a cat or a pig... all animals. The point is, that you can never see an animal walking around that isn't more specifically something else (duck, pig, etc.). 

The Animal is the abstract class and Duck/Pig/Cat are all classes that derive from that base class. Animals might provide a function called "Age" that adds 1 year of life to the animals. It might also provide an abstract method called "IsDead" that, when called, will tell you if the animal has died. Since IsDead is abstract, each animal must implement it. So, a Cat might decide it is dead after it reaches 14 years of age, but a Duck might decide it dies after 5 years of age. The abstract class Animal provides the Age function to all classes that derive from it, but each of those classes has to implement IsDead on their own.

Now, an interface is like an abstract class, except it does not contain any logic. Rather, it specifies an interface. So, there might be an interface called IFly. This might have the methods GoForward and GoDown. Those methods would not actually contain any logic... each class that implements interface IFly would have to implement those GoForward and GoDown methods. You could have classes Duck and Finch implement interface IFly. Then, if you want to keep a list of instances that can fly, you just create a list that contains items of type IFly. That way, you can add Ducks and Finches and any other instance of a class the implements IFly to the list.

So, abstract classes can be used to consolidate and share functionality, while interfaces can be used to specify what the common functionality that will be shared between different instances will be, without actually building that functionality for them. Both can help you make your code smaller, just in different ways. There are other differences between interfaces and abstract classes, but those depend on the programming language, so I won't go into those other differences here.

Do you mean real-world as in "A live software system which includes  Abstract classes or interfaces" or do you mean "

"A contrived example which demonstrates their usefullness"

If you mean the latter think of Vehicle as an abstract class. You can't yet do anything with it because you have no idea what it does, or how to drive it.

abstract class Vehicle{}




Vehicles could be split into morotized and pedal-powered, but still this is abstract, we still dont know what to do with it.

abstract class MotorVehicle : Vehicle {} 

abstract class PedaledVehicle : Vehicle {}




You could now define a concrete (non-abstract) class, like car.

class MotorCar : MotorVehicle {}




 Intefaces come in handy you can only inherit from one base class. So imagine some vehicles are drivable, others are remote controlled, some vehicles use a stearing wheel, others dont

interface IDrivable{}

interface IHasStearingWheel{}




Now you could derive a DrivableMotorCar from its base clas, and also implement other behaviours.

class DrivableMotorCar : MotorVehicle, IDrivable, IHasStearingWheel {}







Recommendations for Abstract Classes vs. Interfaces

The choice of whether to design your functionality as an interface or an abstract class (a MustInherit class in Visual Basic) can sometimes be a difficult one. An abstract class is a class that cannot be instantiated, but must be inherited from. An abstract class may be fully implemented, but is more usually partially implemented or not implemented at all, thereby encapsulating common functionality for inherited classes. For details, see Abstract Classes.

An interface, by contrast, is a totally abstract set of members that can be thought of as defining a contract for conduct. The implementation of an interface is left completely to the developer.

Here are some recommendations to help you to decide whether to use an interface or an abstract class to provide polymorphism for your components.

• If you anticipate creating multiple versions of your component, create an abstract class. Abstract classes provide a simple and easy way to version your components. By updating the base class, all inheriting classes are automatically updated with the change. Interfaces, on the other hand, cannot be changed once created. If a new version of an interface is required, you must create a whole new interface.
• If the functionality you are creating will be useful across a wide range of disparate objects, use an interface. Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes.
• If you are designing small, concise bits of functionality, use interfaces. If you are designing large functional units, use an abstract class.
• If you want to provide common, implemented functionality among all implementations of your component, use an abstract class. Abstract classes allow you to partially implement your class, whereas interfaces contain no implementation for any members.

Reference:-

MSDN LINK