The process in which the newly created class uses elements of a more general class of already existing is inheritance, but is this also apply to the polymorphism?. I can't find in the internet an satisfying answer.
Inheritance is derived from the overall concept of polymorphism. Inheritance would be more specifically a type of Ad hoc polymorphism. The concept of an object oriented language in general is to allow features such as inheritance and abstraction. So inheritance allows the specialization of classes in say a hierarchical manner so then subclasses may inherit from a parent or more "general class", while Polymorphism allows you to use an object without knowing its exact type such as calling an inherited or virtual method and the language being able to get the correct method from many derivations or implementations of such method.
No, Polymorphism is where two or more classes that either implement the same interface or extend the same parent can be substituted for each other.
An example is how a List and a HashTable are both Collections and may be substituted for each other when the object is defined as the general type (Collection)
Related
https://www.tutorialspoint.com/java/java_polymorphism.htm#:~:text=Polymorphism%20is%20the%20ability%20of,is%20considered%20to%20be%20polymorphic.
As per the link above, in which they say:
Polymorphism is the ability of an object to take on many forms.
I'm having trouble figuring out whether to take this literally or not.
From my knowledge of polymorphism, classes are polymorphic when they can have multiple children, each implementing a parent class method in a different way.
A function can be polymorphic as we can overload and override them so functions can display different behaviors.
I was told and taught that another form of polymorphism applies to objects that inherit from multiple parent classes.
In C++ for example an object may behave differently depending on its reference type if its methods aren't virtual.
Another example that comes to mind is in Java with multiple interfaces, I can look at an object as different types causing different expected behaviors.
Are these examples really considered polymorphism at play or is this just inheritance and Polymorphism best defines the first two examples (classes and methods)
Thanks
The literal meaning of the word "polymorphism" is what the tutorial says. However it makes very little sense to use the literal meaning of a technical term. In OOP, "polymorphism" means subtyping. There are other ways the term "polymorphism" is used in programming (ad-hoc and parametric polymorphism) but they are not related to OOP specifically.
"Polymorphic object" is not a standard term across the OOP literature. It has a very specific meaning in C++, and it's not "an object with several base classes". It is simply an object with at least one virtual function. From the OOP perspective, a C++ "polymorphic object" is just an object.
Deriving from more than one base classes is called multiple inheritance. I have never heard anyone using the term "polymorphism" for this.
I am struggling to understand both abstract and interface approach. Since i get the idea what is the purpose to use one over another is clear. I was trying to found whatever example of using them both in action however all tutorials are how to use interface over abstract or vice versa showing usage either for one or another. I would really love to see practical example which could show both in action best on some real life example. Additional comments why in specific case you used one over another appreciated. Generics are very welcome to see as well in such example.
I'll propose foloowing example. We got some engine to get files from diffrent locations which could be taken using diffrent protocols as follows. I would like to understand on this example how this could be accomplished with both interfaces and abstract.
'As all of protocol has to close and open would it be good to put in abstract?
abstract class Collector
Protected Id
Protected Name
MustInherit Sub OpenConnection
MustInherit Sub CloseConection
End Class
'?
class Ftp : Collector
class Sftp: Collector
class Soap: Collector
'Interface?
Public Interface IRepository(Of T, Tkey)
Function GetAllFiles() As IEnumerable(Of T)
Function GetAllById(Tkey) as IEnumerable(Of T)
End Interface
Some key distinctions:
An abstract class can contain some implementation. An interface cannot.
In .NET, a class can not inherit from multiple base classes.
A class can implement multiple interfaces
The choice of which approach is really up to you. In general, it's a choice between the Composition pattern or Inheritance.
Composition uses Interfaces. Think of an object as having X.
Inheritance uses Classes. Think of an object as being X.
In either case, an abstract class or an interface is just a Type, through which you will access and manipulate them. For example, if you have some code that wants to perform Insert/Update/Delete operations, it doesn't need to know that the object it is operating on is a FTP client--only that the object has the ability to support these operations. (and that is exactly what IRepository specifies)
You definitely can combine both. There's no reason a concrete FtpClient class couldn't inherit from an abstract Protocol class and also implement the IRepository interface. It could even use generics!
Interfaces are great for decoupling your code, and also great for unit test mocks.
There is also a good summary of pros & cons on Wikipedia (Composition_over_inheritance). Pros:
To favor composition over inheritance is a design principle that gives the design higher flexibility. It is more natural to build business-domain classes out of various components than trying to find commonality between them and creating a family tree. For example, a gas pedal and a wheel share very few common traits, yet are both vital components in a car. What they can do and how they can be used to benefit the car is easily defined. Composition also provides a more stable business domain in the long term as it is less prone to the quirks of the family members. In other words, it is better to compose what an object can do (HAS-A) than extend what it is (IS-A).
Initial design is simplified by identifying system object behaviors in separate interfaces instead of creating a hierarchical relationship to distribute behaviors among business-domain classes via inheritance. This approach more easily accommodates future requirements changes that would otherwise require a complete restructuring of business-domain classes in the inheritance model. Additionally, it avoids problems often associated with relatively minor changes to an inheritance-based model that includes several generations of classes.
Cons:
One common drawback of using composition instead of inheritance is that methods being provided by individual components may have to be implemented in the derived type, even if they are only forwarding methods. In contrast, inheritance does not require all of the base class's methods to be re-implemented within the derived class. Rather, the derived class only needs to implement (override) the methods having different behavior than the base class methods. This can require significantly less programming effort if the base class contains many methods providing default behavior and only a few of them need to be overridden within the derived class.
I don't understand why you want to have an example combining both. Let's just say both are valid ways to build solid software architecture. They're just two tools - like having a kitchen knife and a meat cleaver. You won't necessarily use them together but see the pro's and con's when looking at the dinner you want to serve.
So usually you take abstract/MustInherit classes if you want to provide a common denominator. Sub-classes derive from the abstract one and have to implement the methods just like they would if they implemeted an interface. The good thing here is that abstract classes can provide "base logic" which can be developed centrally and all the sub-classes can make use of that. In the best case, abstract classes provide kind of "hooks" to plug in special logic in the sub-classes.
Interfaces describe what a class has to fulfill. So everything an interface defines has to be implemented in classes implementing the interface. There's no reusable logic built-in in this approach like in abstract base classes but the big "pro" for interfaces is that they don't take away the single base type you can derive from like abstract classes do. So you can derive from anything or nothing and still implement an interface. AND: You can implement multiple interfaces.
One word to the "reusable logic" with interfaces. While this is not really wroing, the .NET framework allows use to write extension methods on types (and interfaces) to attach externally developed code. This allows code reuse with interfaces like having a method implemented in there. So for example, you could write an extension method None() for the interface IEnumerable which is checking whether the enumerable is empty.
public static bool None(this IEnumerable values)
{
return !values.Any();
}
With this, None() can be used on any IEnumerable in your code base having access to the extension method (in fact, Any(), Select(), Where(), etc. are extension methods as well, lying in the System.Linq namespace).
I could not find the main difference. And I am very confused when we could use inheritance and when we can use subtyping. I found some definitions but they are not very clear.
What is the difference between subtyping and inheritance in object-oriented programming?
In addition to the answers already given, here's a link to an article I think is relevant.
Excerpts:
In the object-oriented framework, inheritance is usually presented as a feature that goes hand in hand with subtyping when one organizes abstract datatypes in a hierarchy of classes. However, the two are orthogonal ideas.
Subtyping refers to compatibility of interfaces. A type B is a subtype of A if every function that can be invoked on an object of type A can also be invoked on an object of type B.
Inheritance refers to reuse of implementations. A type B inherits from another type A if some functions for B are written in terms of functions of A.
However, subtyping and inheritance need not go hand in hand. Consider the data structure deque, a double-ended queue. A deque supports insertion and deletion at both ends, so it has four functions insert-front, delete-front, insert-rear and delete-rear. If we use just insert-rear and delete-front we get a normal queue. On the other hand, if we use just insert-front and delete-front, we get a stack. In other words, we can implement queues and stacks in terms of deques, so as datatypes, Stack and Queue inherit from Deque. On the other hand, neither Stack nor Queue are subtypes of Deque since they do not support all the functions provided by Deque. In fact, in this case, Deque is a subtype of both Stack and Queue!
I think that Java, C++, C# and their ilk have contributed to the confusion, as already noted, by the fact that they consolidate both ideas into a single class hierarchy. However, I think the example given above does justice to the ideas in a rather language-agnostic way. I'm sure others can give more examples.
A relative unfortunately died and left you his bookstore.
You can now read all the books there, sell them, you can look at his accounts, his customer list, etc. This is inheritance - you have everything the relative had. Inheritance is a form of code reuse.
You can also re-open the book store yourself, taking on all of the relative's roles and responsibilities, even though you add some changes of your own - this is subtyping - you are now a bookstore owner, just like your relative used to be.
Subtyping is a key component of OOP - you have an object of one type but which fulfills the interface of another type, so it can be used anywhere the other object could have been used.
In the languages you listed in your question - C++, Java and C# - the two are (almost) always used together, and thus the only way to inherit from something is to subtype it and vice versa. But other languages don't necessarily fuse the two concepts.
Inheritance is about gaining attributes (and/or functionality) of super types. For example:
class Base {
//interface with included definitions
}
class Derived inherits Base {
//Add some additional functionality.
//Reuse Base without having to explicitly forward
//the functions in Base
}
Here, a Derived cannot be used where a Base is expected, but is able to act similarly to a Base, while adding behaviour or changing some aspect of Bases behaviour. Typically, Base would be a small helper class that provides both an interface and an implementation for some commonly desired functionality.
Subtype-polymorphism is about implementing an interface, and so being able to substitute different implementations of that interface at run-time:
class Interface {
//some abstract interface, no definitions included
}
class Implementation implements Interface {
//provide all the operations
//required by the interface
}
Here, an Implementation can be used wherever an Interface is required, and different implementations can be substituted at run-time. The purpose is to allow code that uses Interface to be more widely useful.
Your confusion is justified. Java, C#, and C++ all conflate these two ideas into a single class hierarchy. However, the two concepts are not identical, and there do exist languages which separate the two.
If you inherit privately in C++, you get inheritance without subtyping. That is, given:
class Derived : Base // note the missing public before Base
You cannot write:
Base * p = new Derived(); // type error
Because Derived is not a subtype of Base. You merely inherited the implementation, not the type.
Subtyping doesn't have to be implemented via inheritance. Some subtyping that is not inheritance:
Ocaml's variant
Rust's lifetime anotation
Clean's uniqueness types
Go's interface
in a simple word: subtyping and inheritance both are polymorphism, (inheritance is a dynamic polymorphism - overriding). Actually, inheritance is subclassing, it means in inheritance there is no warranty to ensure capability of the subclass with the superclass (make sure subclass do not discard superclass behavior), but subtyping(such as implementing an interface and ... ), ensure the class does not discard the expected behavior.
I know that interfaces cannot contain method body and we can implement in another classes and can write our custom logic. But the same thing can also implement by using inheritance with classes. Then why interfaces come into picture. If we want to override any method definition we can do in inheritance of classes and can write our custom code. What is the exact purpose of interfaces?
One reason is that a class may implement multiple interfaces but only derive from a single class.
Another is, that hierarchically totally unrelated classes may implement the same interface. In statically typed languages without interfaces, one can often observe very deep inheritance hierarchies, created only because they could not simply implement an interface and had to force unrelated classes to derive. This often tends to violate the "Is a" - principle of inheritance. Such implementations also tend to drag around unused code, just because it is needed further down the inheritance tree.
tl;dr - it can be done but the results are often ugly and unmaintainable
Interfaces - The object can do this.
Class - This is how the object does this.
Also interfaces can be used to avoid the diamond problem
This question already has answers here:
Why can't I seem to grasp interfaces?
(26 answers)
Closed 3 years ago.
With regards to OOP, how would you describe an interface?
What I mean is, sub-classing can be described as "Has-A", and inheritance could be "Is-A". A member method could be "Can-Do".
Is there any way this could be extended (no pun intended) to describe what an interface does?
I think of objects as nouns, methods as verbs, and interfaces as adjectives (of course this analogy is oversimplified, but frequently works well enough).
Example: an interface Serializable works like an adjective, in that it applies some qualities to an object that implement that interface, but does not change what that object is. We can say, "this is a serializable object." But we don't say, "this object is a serializable," nor do we say, "this object has a serializable."
I also like Federico's answer that an interface is "CAN-DO".
An interface is a group of related operations that the class supports. Together, the methods in an interface describe what the class can do.
Just like a noun can take multiple adjectives, a class can implement multiple interfaces, as long as they don't conflict. The union of all the interfaces a class implements is the sum of what the class can do.
In practical terms, an interface is a set of method signatures, without the code implementing those methods. Just the method name and arguments. Depending on the language, a method signature may also include return type, and exceptions thrown.
An interface consists of methods, but not data members.
BTW, I wouldn't say sub-classing is HAS-A. My understanding is that sub-classing is the same as inheritance, so these are both IS-A. Whereas HAS-A is called Aggregation or Composition.
Composition is where an object owns another object. Destroying the outer object also destroys the inner objects. Example: University composes Departments. Close the University, and the Departments disappear.
Aggregation is where an object includes another object, but does not own it. Destroying the outer object does not destroy the inner objects. Example: University employs Professors, but closing the University does not kill the Professors.
An interface is an abstract base class with all pure virtual members.
So looking at your Has-A/Is-A, it should be similar to whatever you would be apply for an abstract base class.
Interfaces generally exist in languages that do not fully support multiple inheritance as a way to more safely offer some of the same benefits.
Acts-As-A.
As is your description for methods, I would also describe an interface as a "Can-Do". An interface is a contract like "all the classes that implement me, can do these things".
Joel that isn't exaclty what an interface is. It is like a abstract base class in a way but it has no implementation of the methods and properties.
This pretty much sums up what an interface is.
http://www.c-sharpcorner.com/UploadFile/rmcochran/csharp_interrfaces03052006095933AM/csharp_interrfaces.aspx?ArticleID=cd6a6952-530a-4250-a6d7-54717ef3b345