Is it the best practice to extract an interface for every class?

Question

I have seen code where every class has an interface that it implements.
Sometimes there is no common interface for them all.
They are just there and they are used instead of concreate objects.
They do not offer a generic interface for two classes and are specific to the domain of the problem that the class solves.
Is there any reason to do that?

Answer 1

If is a part of the Dependency Inversion principle. Basically code depends on the interfaces and not on the implementations.

This allows you to easy swap the implementations in and out without affecting the calling classes. It allows for looser coupling which makes maintenance of the system much easier.

As your system grows and gets more complex, this principle keeps making more and more sense!

Answer 2

Interfaces define a behaviour. If you implement one or more interfaces then your object behaves like the one or other interfaces describes. This allows loose coupling between classes. It is really useful when you have to replace an implementation by another one. Communication between classes shall always be done using interfaces excepting if the classes are really tightly bound to each other.

Answer 3

There might be, if you want to be sure to be able to inject other implementations in the future. For some (maybe most) cases, this is overkill, but it is as with most habits - if you're used to it, you don't loos very much time doing it. And since you can never be sure what you'll want to replace in the future, extracting an interface on every class does have a point.

There is never only one solution to a problem. Thus, there could always be more than one implementation of the same interface.

Answer 4

It might seem silly, but the potential benefit of doing it this way is that if at some point you realize there's a better way to implement a certain functionality, you can just write a new class that implements the same interface, and change one line to make all of your code use that class: the line where the interface variable is assigned.

Doing it this way (writing a new class that implements the same interface) also means you can always switch back and forth between old and new implementations to compare them.

It may end up that you never take advantage of this convenience and your final product really does just use the original class that was written for each interface. If that's the case, great! But it really didn't take much time to write those interfaces, and had you needed them, they would've saved you a lot of time.

Answer 5

It's not "the best practice", but it is how you abstract the details of a class from people using the class. Interfaces define a contract - if you use interfaces everywhere, the underlying class can change so long as it implements the interface.

It's used to decouple parts of a program, typically layers or tiers. If coupling is not a problem, enforcing interfaces for all classes will be an unnecessary overhead.

It can also be used for defining rules for consumption. A method could take two "Transaction" classes for summing the amounts, or it could take two IAmount interfaces exposing the amounts - anything could then implement IAmount, thus generalising the method away from Transactions.

Answer 6

I don't think it's reasonable for Every class.

It's a matter of how much reuse you expect from what type of a component. Of course, you have to plan for more reuse (without the need to do major refactoring later) than you are really going to use at the moment, but extracting an abstract interface for every single class in a program would mean you have less classes than needed.

Answer 7

The interfaces are good to have since you can mock the classes when (unit-) testing.

I create interfaces for at least all classes that touches external resources (e.g. database, filesystem, webservice) and then write a mock or use a mocking framework to simulate the behavior.

Answer 8

There's no practical reason behind extracting Interfaces for each class in your project. That'd be an over-kill. The reason why they must be extracting interfaces would be the fact that they seem to implement an OOAD principle "Program to Interface, not to Implementation". You can find more information about this principle with an example here.

Answer 9

Having the interface and coding to the interface makes it a ton easier to swap out implementations. This also applies with unit testing. If you are testing some code that uses the interface, you can (in theory) use a mock object instead of a concrete object. This allows your test to be more focused and finer grained.

It is more common from what I have seen to switch out implementations for testing (mocks) then in actual production code. And yes it is wroth it for unit testing.

Answer 10

I like interfaces on things that could be implemented two different ways, either in time or space, i.e. either it could be implemented differently in the future, or there are 2 different code clients in different parts of the code which may want a different implementation.

The original writer of your code might have just been robo coding, or they were being clever and preparing for version resilience, or preping for unit testing. More likely the former because version resilience an uncommon need-- (i.e. where the client is deployed and can't be changed and a component will be deployed that must be compatible with the existing client)

I like interfaces on things that are dependencies worth isolation from some other code I plan to test. If these interfaces weren't created to support unit tests either, then I'm not sure they're such a good idea. Interface have a cost to maintain and when it comes time to make an object swappable with another, you might want to have an interface apply to only a few methods (so more classes can implement the interface), it might be better to use an abstract class (so that default behaviors can be implemented in an inheritance tree).

So pre-need interfaces is probably not a good idea.

Answer 11

No.

Interfaces are good for classes with complex behaviour, and are especially handy if you want to be able to create a mock or fake implementation class of that interface for use in unit tests.

But, some classes don't have a lot of behaviour and can be treated more like values and usually consist of a set of data fields. There's little point in creating interfaces for classes like this because doing so would introduce unnecessary overhead when there's little point in mocking or providing alternative implementations of the interface. For example, consider a class:

class Coordinate
{
  public Coordinate( int x, int y);
  public int X { get; }
  public int y { get; }
}

You're unlikely to want an interface ICoordinate to go with this class, because there's little point in implementing it in any other way than simply getting and setting X and Y values.

However, the class

class RoutePlanner
{
   // Return a new list of coordinates ordered to be the shortest route that
   // can be taken through all of the passed in coordinates.
   public List<Coordinate> GetShortestRoute( List<Coordinate> waypoints );
}

you probably would want an IRoutePlanner interface for because there are many different algorithms that could be used for planning a route. Also, if you had a third class:

class RobotTank
{
   public RobotTank( IRoutePlanner );
   public void DriveRoute( List<Coordinate> points );
}

By giving it an interface you could write a test method for RobotTank that creates one with a mock RoutePlanner that just returns a list of coordinates in no particular order. This would allow the test method to check that the tank navigates correctly between the coordinates without also testing the route planner. This means you can write a test that just tests one unit (the tank), without also testing the route planner.

You'll see though, it's quite easy to feed real Coordinates in to a test like this without needing to hide them behind an ICoordinate interface.

Answer 12

Hi!

Why do you need interfaces? Think practically and deeply. Interfaces are not really attached to classes, rather they are attached to services. The goal of interface is what you allow others to do with your code without serving them the code. So it relates to the service and its management.

See ya