C what is the difference between an abstract class and an interface

Abstract class vs interface An abstract class doesn't provide full abstraction but an interface does provide full abstraction; i. Using abstract we cannot achieve multiple inheritance but using an Interface we can achieve multiple inheritance. We can not declare a member field in an Interface. We can not use any access modifier i. An Interface member cannot be defined using the keyword static, virtual, abstract or sealed.

Abstract Class We can not create an object of an abstract class and can call the method of abstract class with the help of class name only.

Take a look at an Abstract class example, The code window looks like this, using System; using System. Active Oldest Votes. Interfaces An interface is a contract : The person writing the interface says, " hey, I accept things looking that way ", and the person using the interface says " OK, the class I write looks that way ". Abstract classes Abstract classes, unlike interfaces, are classes.

As usual with programming, there is theory, practice, and practice in another language Improve this answer. Anjana Silva 5, 3 3 gold badges 43 43 silver badges 48 48 bronze badges.

The key point about interfaces is not so much that they say what a class does, but allow objects that can Wizzle to make themselves useful to code that needs a Wizzler. Note that in many cases neither the person who writes the thing that can Wizzle, nor the person who needs a Wizzler, will be the person who writes the interface.

I don't think that CPU consumption is the highlight-worthy point on interfaces. Why does abstract class being a class increase CPU utilization? What kind of lookup are you referring to here? With this addition, I can no longer see the real difference between abstract classes and interface besides the fact that I should use interfaces because classes can implement multiple interfaces but can only inherit one class — Ogen.

I think the comparison between interface and class from Head First Java is vivid that A class defines who you are, and an interface tells what roles you could play — LittleLittleQ. Show 16 more comments. The key technical differences between an abstract class and an interface are: Abstract classes can have constants, members, method stubs methods without a body and defined methods , whereas interfaces can only have constants and methods stubs.

Peter Mortensen 29k 21 21 gold badges 97 97 silver badges bronze badges. Justin Johnson Justin Johnson And normally with classes you can instantiate an object from it unlike the abstract classes which CANNOT be instantiated.

Jiazzyuser If an abstract class implements an interface, it does not have to actually define the interface's methods. However, a concrete class must implement all interface methods that are not implemented by its parent class.

I'll add example to illustrate this point. Just as an interface can extend an interface without defining methods, an abstract class can inherit an abstract class without defining methods. Show 6 more comments. Vivek Vivek 4, 2 2 gold badges 16 16 silver badges 9 9 bronze badges. You need to say what language this applies to "Abstract class does not support multiple inheritance" is far from being universally true — Ben Voigt.

Last comparison is confusing as per table! Methods in interface can't be static but variables are static final Implemented methods in abstract class can be static — realPK. Member of the interface must be static final. Last statement is wrong.

I think "functionality" in this answer means "implementation. What is the targetted programming language here? Konamiman Konamiman This is the most correct answer, since PHP interfaces differ from other languages in that PHP interfaces ARE abstract classes under the hood, whereas other languages' interfaces are signatures that classes must match.

They behave the same as long as there are no errors though. True, for PHP it's the real best anwser. But it's harder to get from the text blob than from a simple snippet. 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. If you are looking for Java as programming language, here are a few more updates: Java 8 has reduced the gap between interface and abstract classes to some extent by providing a default method feature.

Have a look at this SE question for code examples to understand better. Community Bot 1 1 1 silver badge. Ravindra babu Ravindra babu Some important differences: In the form of a table: As stated by Joe from javapapers : 1.

A Java class can implement multiple interfaces but it can extend only one abstract class. I've edited your answer to provide correct attribution. You can't just drop a link at the bottom of your answer. You need to quote all of the language that was copied from another source, as well. Also, if that table was drawn from somewhere, you should clearly indicate where that is from. MageshBabu Perhaps defining a function in a class containing pure virtual function makes it an abstract class rather than the interface — cbinder.

With Java 8, the differences are less now. Check updated differences here: journaldev. The main point is that: Abstract is object oriented. It is concerned with the object's basic characteristics: what it has and what it can do. Hence objects which inherit from the same abstract class share the basic characteristics generalization. Interface is functionality oriented. It defines functionalities an object should have.

Regardless what object it is, as long as it can do these functionalities, which are defined in the interface, it's fine. It ignores everything else. Moch Yusup Moch Yusup 1, 13 13 silver badges 12 12 bronze badges. Thank you now we are getting somewhere with a good high level response. Funny how far deep into the comments you have to go to find a more understanding-based response.

The other answers are too technical. This is getting towards what I would feel is a 'right' answer. The whole point of OOP is semantics, and whether or not a private nested classes public getters are invoked through CPU expensive lookups is pretty much irrelevant here — Sentinel. I am constructing a building of floors The building's blueprint interface For example, Servlet I Building constructed up to floors - partially completed abstract Partial implementation, for example, generic and HTTP servlet Building construction completed- concrete Full implementation, for example, own servlet Interface We don't know anything about implementation, just requirements.

We can go for an interface. An addition, it uses concrete No restriction for abstract class method modifiers No restriction for abstract class variable modifiers We cannot declare other modifiers except abstract No restriction to initialize variables Taken from DurgaJobs Website. Chamod Pathirana 6 6 silver badges 14 14 bronze badges. Jaichander Jaichander 2 2 gold badges 11 11 silver badges 24 24 bronze badges.

An abstract class can have a constructor — vimal krishna. I completely disagree with this view. The blueprint is a completely different concept to 'interface. It is closer to 'class,' as the blueprint can be instantiated multiple times through its constructor, but even this is not close enough as the 'class' also contains the specification for how to construct the ctor , and the means to do so.

I will point out main differences, and the rest have already been explained: Abstract classes are useful for modeling a class hierarchy. Dhananjay Dhananjay 3, 2 2 gold badges 20 20 silver badges 19 19 bronze badges. Good answer, but the math metaphor is useless and made me waste roughly an equivalent amount of time reading it as writing this comment. Now multiply that by all the other people that have read this question. It's pretty simple actually. So an interface can only "declare" and not define the behavior you want the class to have.

One last thing, In Java, you can implement multiple interfaces, but you can only extend one Abstract Class or Class Interfaces on the other hand, you could simply do: interface C implements A, B So in effect Java supports multiple inheritance only in "declared behavior" ie interfaces, and only single inheritance with defined behavior.. Hopefully that makes sense.

Interface vs Class Interface is a contract between two objects. Again, comparing interfaces with abstract classes is not correct. In short the differences are the following: Syntactical Differences Between Interface and Abstract Class : Methods and members of an abstract class can have any visibility.

All methods of an interface must be public. An Abstract child class can have abstract methods. An interface extending another interface need not provide default implementation for methods inherited from the parent interface. A child class can only extend a single class. An interface can extend multiple interfaces. A class can implement multiple interfaces.

A child class can define abstract methods with the same or less restrictive visibility, whereas class implementing an interface must define all interface methods as public. Abstract Classes can have constructors but not interfaces.

Interfaces from Java 9 have private static methods. In Interfaces now: public static - supported public abstract - supported public default - supported private static - supported private abstract - compile error private default - compile error private - supported.

Pritam Banerjee Pritam Banerjee Following examples demonstrate this. An interface only allows you to define functionality, not implement it. And whereas a class can extend only one abstract class, it can take advantage of multiple interfaces. An abstract class is a special type of class that cannot be instantiated. An abstract class is designed to be inherited by subclasses that either implement or override its methods.

In other words, abstract classes are either partially implemented or not implemented at all. You can have functionality in your abstract class—the methods in an abstract class can be both abstract and concrete.

In terms of standard definition an Abstract class is, conceptually, a class that cannot be instantiated and is usually implemented as a class that has one or more pure virtual abstract functions. On other hand an Interface is a description of what member functions must a class, which inherits this interface, implement. In other words, an interface describes behaviour of the class.

As like of other general class design in C Abstract class also have its own implementation along with its declaration. On other hand an Interface can only have a signature, not the implementation.