In the UML class diagram the interface is equivalent to the interface concept in programming languages (a set of methods that the class that implement the interface should implement). I want to know if the interface in the component diagram has the same meaning. Are the interfaces mentioned in the component diagram the same interfaces that are detailed in the class diagram or should I treat every method in the component diagram as a separate interface?
There is actually no real difference between both. You can realize an interface with a class as well as with a component. Also you can show the realization in both cases with the socket/lollipop notation.
Components are not much different to classes. You can think of a component as a container that hosts a number of different classes. And if one of those classes realizes an interface you can expose that through the component (if you so wish). Additionally a component can have a lot of internal interfaces for its hosted classes.
Related
I have a couple of questions related to inheritance in Windows Runtime Components authored using C++/WinRT. Firstly why is there a restriction that classes, if they have a base class, must have ultimately derive from a class in the Windows.* namespace?
Secondly, what is the best way to work around this. In fact I don't really want consumers to be able to derive from my class, but I'd like to derive from a base class in the implementation in order not to rewrite code. At the moment my implementation class is a lightweight wrapper around a standard C++ class that uses standard C++ inheritance. But is there a way to simplify this? Can a list one implementation class as the base class of another? I am mainly familiar with C# where multiple inheritance is not possible, so am not sure about this sort of thing.
Any runtime class (that you declare in your application) that derives from a base class is known as a composable class. The ultimate base class of a composable class must be a type originating in a Windows.* namespace;.
And if you want to inherit custom runtimeclass, the Windows App Certification Kit tests will produce errors. From this thread, it seems that you can only declare a C++ native class and implement the interfaces. Let the implementation classes inherit from it.
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).
As we all know about abstraction and encapsulation fundamental that help to build robust and loosely coupled architecture.
When we start designing classes we need to keep in mind the specialized attributes and based on that structure need to build.Usually we go with interfaces or abstract classes for creating a foundation of architecture.
I am confused here with interfaces, weather they are lied under abstraction?
Why they said program to interface, not implementation.
Thanks in advance for clarifying.
If we talk about abstraction it is like hiding the implementation details from user. Like user need to only use the class or function that is abstracted.
If we use abstract class along with abstract method. We have to use all the abstract method (if any) of that abstract class while inheriting that class, But if we talk about interfaces it is not like that.
interface is to reduce redundant data to the inherited class. If we use interface then we can implement any feature or function of the class and write out the definition accordingly.
In short interface is not abstraction.
While certain guidelines state that you should use an interface when you want to define a contract for a class where inheritance is not clear (IDomesticated) and inheritance when the class is an extension of another (Cat : Mammal, Snake : Reptile), there are cases when (in my opinion) these guidelines enter a gray area.
For example, say my implementation was Cat : Pet. Pet is an abstract class. Should that be expanded to Cat : Mammal, IDomesticated where Mammal is an abstract class and IDomesticated is an interface? Or am I in conflict with the KISS/YAGNI principles (even though I'm not sure whether there will be a Wolf class in the future, which would not be able to inherit from Pet)?
Moving away from the metaphorical Cats and Pets, let's say I have some classes that represent sources for incoming data. They all need to implement the same base somehow. I could implement some generic code in an abstract Source class and inherit from it. I could also just make an ISource interface (which feels more "right" to me) and re-implement the generic code in each class (which is less intuitive). Finally, I could "have the cake and eat it" by making both the abstract class and the interface. What's best?
These two cases bring up points for using only an abstract class, only an interface and using both an abstract class and an interface. Are these all valid choices, or are there "rules" for when one should be used over another?
I'd like to clarify that by "using both an abstract class and an interface" that includes the case when they essentially represent the same thing (Source and ISource both have the same members), but the class adds generic functionality while the interface specifies the contract.
Also worth noting is that this question is mostly for languages that do not support multiple inheritance (such as .NET and Java).
As a first rule of thumb, I prefer abstract classes over interfaces, based on the .NET Design Guidelines. The reasoning applies much wider than .NET, but is better explained in the book Framework Design Guidelines.
The main reasoning behind the preference for abstract base classes is versioning, because you can always add a new virtual member to an abstract base class without breaking existing clients. That's not possible with interfaces.
There are scenarios where an interface is still the correct choice (particularly when you don't care about versioning), but being aware of the advantages and disadvantages enables you to make the correct decision.
So as a partial answer before I continue: Having both an interface and a base class only makes sense if you decide to code against an interface in the first place. If you allow an interface, you must code against that interface only, since otherwise you would be violating the Liskov Substitution Principle. In other words, even if you provide a base class that implements the interface, you cannot let your code consume that base class.
If you decide to code against a base class, having an interface makes no sense.
If you decide to code against an interface, having a base class that provides default functionality is optional. It is not necessary, but may speed up things for implementers, so you can provide one as a courtesy.
An example that springs to mind is in ASP.NET MVC. The request pipeline works on IController, but there's a Controller base class that you typically use to implement behavior.
Final answer: If using an abstract base class, use only that. If using an interface, a base class is an optional courtesy to implementers.
Update: I no longer prefer abstract classes over interfaces, and I haven't for a long time; instead, I favour composition over inheritance, using SOLID as a guideline.
(While I could edit the above text directly, it would radically change the nature of the post, and since a few people have found it valuable enough to up-vote it, I'd rather let the original text stand, and instead add this note. The latter part of the post is still meaningful, so it would be a shame to delete it, too.)
I tend to use base classes (abstract or not) to describe what something is, while I use interfaces to describe the capabilities of an object.
A Cat is a Mammal but one of it's capabilities is that it is Pettable.
Or, to put it a different way, classes are nouns, while interfaces map closer to adjectives.
From MSDN, Recommendations for Abstract Classes vs. Interfaces
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.
If you want to provide the option of replacing your implementation completely, use an interface. This applies especially for interactions between major components, these should always be decoupled by interfaces.
There may also be technical reasons for prefering an interface, for example to enable mocking in unit tests.
Internally in a component it may be fine to just use an abstract class directly to access a hierarchy of classes.
If you use an interface and have a hierarchy of implementing classes then it is good practice to have an abstract classe which contain the common parts of the implementation. E.g.
interface Foo
abstract class FooBase implements Foo
class FunnyFoo extends FooBase
class SeriousFoo extends FooBase
You could also have more abstract classes inheriting from each other for a more complicated hierarchy.
Refer to below SE question for generic guidelines:
Interface vs Abstract Class (general OO)
Practical use case for interface:
Implementation of Strategy_pattern: Define your strategy as an interface. Switch the implementation dynamically with one of concrete implementations of strategy at run time.
Define a capability among multiple unrelated classes.
Practical use case for abstract class:
Implementation of Template_method_pattern: Define a skeleton of an algorithm. The child classes can't change strucutre of the algortihm but they can re-define a part of the implementation in child classes.
When you want share non-static and non-final variables among multiple related classes with "has a" relation.
Use of both abstradt class and interface:
If you are going for an abstract class, you can move abstract methods to interface and abstract class can simply implement that interface. All use cases of abstract classes can fall into this category.
I always use these guidelines:
Use interfaces for multiple TYPE inheritance (as .NET/Java don't use multiple inheritance)
Use abstract classes for a re-usable implementation of a type
The rule of the dominant concern dictates that a class always has a main concern and 0 or more others (see http://citeseer.ist.psu.edu/tarr99degrees.html). Those 0 or more others you then implement through interfaces, as the class then implements all the types it has to implement (its own, and all interfaces it implements).
In a world of multiple implementation inheritance (e.g. C++/Eiffel), one would inherit from classes which implement the interfaces. (In theory. In practise it might not work that well.)
There is also something called the DRY principle - Don't Repeat Yourself.
In your example of data sources you say there is some generic code that is common between different implementations. To me it seems that the best way to handle that would be to have an abstract class with the generic code in it and some concrete classes extending it.
The advantage is that every bug fix in generic code benefits all concrete implementations.
If you go interface only you will have to maintain several copies of the same code which is asking for trouble.
Regarding abstract + interface if there is no immediate justification for it I would not do it. Extracting interface from abstract class is an easy refactoring, so I would do it only when it is actually needed.
Abstract classes are described as being useful for a family of objects (e.g. could be used for animals which are mammals). However, what difference is there between using an interface or abstract class for representing a family of related objects?
My process is to use an abstract class when I want to define common functionality but with the option for future extensions and an interface for custom functionality (implementations).
For example, I wrote an abstract class to encapsulate some database functionality which will be used heavily in a small web app at work. I wrote an abstract class with virtual methods which can be overrided with custom functionality in the future (e.g. logging, or some reporting of the database events which may be required).
Is this the right way to go? Is there any significance in choosing one construct (abstract or interface) to represent a family?
An abstract class should be used when there is common state and behavior between all types. An interface should be used when all types will have a common interface but will not share state or behavior.
Here is an example.
German Shepherd, Golden Retriever, Beagle
These three objects are all dogs, and as such they share certain common state (carnivorous, 4 legs, etc.) and they also share certain overridable behavior (bark, pant, etc.). In this instance it would make the most sense to create an abstract Dog class to hold this common state and behavior and create subtypes of Dog for each type of dog.
Pencil, Pen, Chalk
These objects have no common state and they cannot share behavior. Yet you may notice that they do have something in common - they are cabaple of writing. These objects are best build separately and without a base class and then tied together with a Writable interface that exposes each type's Write method.
I would suggest using interfaces so that you can implement new functionality in your database utility at some future point.
As always, the primary design principle when it comes to development is
Design towards an interface, not an implementation
With abstract classes, you can provide implementation that is needed and shared by all the classes in your hierarchy. Therefore, you're reusing code. You may allow the derived classes to override the default behavior or not but at least you're providing a baseline functionality like breathing for a new born animal. However, with interfaces, you can't provide any implementation. You simply define a contract that all classes that inherits that interface should honor and provide implementation for. This may lead to repetitive and duplicate code among the hierarchy of classes.
Interfaces are not very good for extensibility and you need to worry about versioning. You decide to make change to an existing interface but you will soon realize that there are a lot of classes in existence you may need to modify. Think about adding Breath method to IMammal interface that's already being used by many mammals. You will need to go and provide Breath implementation for each one. With an abstract class, you can simply add Breath method and provide some baseline implementation without having to worry about existing derived classes. So abstract classes are more flexible in term of the development of your hierarchy and the api.