What does it mean to "program to an interface"?

Question

Hi,

I have seen this mentioned a few times and I am not totally clear on what it means. When and why would you do this?

I know what interfaces do, but the fact I am not clear on this makes me think I am missing out on using them correctly.

Is it just so if you were to do:

IInterface classRef = new ObjectWhatever()

You could use any class that implements IInterface? When would you need to do that? The only thing I can think of is if you have a method and you are unsure of what object will be passed expect for it implementing IInterface. I cannot think how often you would need to do that... (Also, how could you write a method that takes in a object that implements an interface? Is that possible?)

Sorry if I completely missed the point.

EDIT: Sorry to Bump, but it felt better than asking another question and since this is in relation to one of the comments before I though I might as well add it in here.

Does using a Interface hit performance, if so how much, and how can you avoid it without have to maintain two bits of code.

Answer 1

You should look into Inversion of Control:

• Martin Fowler: Inversion of Control Containers and the Dependency Injection pattern
• Wikipedia: Inversion of Control

In such a scenario, you wouldn't write this:

IInterface classRef = new ObjectWhatever();

You would write something like this:

IInterface classRef = container.Resolve<IInterface>();

This would go into a rule-based setup in the container object, and construct the actual object for you, which could be ObjectWhatever. The important thing is that you could replace this rule with something that used another type of object altogether, and your code would still work.

If we leave IoC off the table, you can write code that knows that it can talk to an object that does something specific, but not which type of object or how it does it.

This would come in handy when passing parameters.

As for your parenthesized question "Also, how could you write a method that takes in a object that implements a Interface? Is that possible?", in C# you would simply use the interface type for the parameter type, like this:

public void DoSomethingToAnObject(IInterface whatever) { ... }

this plugs right into the "talk to an object that does something specific". The method defined above knows what to expect from the object, that it implements everything in IInterface, but it doesn't care which type of object it is, only that it adheres to the contract, which is what an interface really is.

For instance, you're probably familiar with calculators and have probably used quite a few in your days, but most of the time they're all different. You, on the other hand, knows how a standard calculator should work, so you're able to use them all, even if you can't use the specific features that each calculator has that none of the other has.

This is the beauty of interfaces. You can write a piece of code, that knows that it will get objects passed to it that it can expect a certain behaviour from. It doesn't care one hoot what kind of object it is, only that it supports the behaviour needed.

Let me give you a concrete example.

We have a custom-built translation system for windows forms. This system loops through controls on a form, and translates text in each. The system knows how to handle basic controls, like the-type-of-control-that-has-a-Text-property, and similar basic stuff, but for anything basic, it falls short.

Now, since controls inherit from pre-defined classes that we have no control over, we could do one of three things:

1. Build support into our translation system to detect specifically which type of control it is working with, and translate the correct bits (maintenance nightmare)
2. Build support into base classes (impossible, since all the controls inherit from different pre-defined classes)
3. Add interface support

So we did nr. 3. All our controls implement ILocalizable, which is an interface that gives us one method, the ability to translate "itself" through a container of translation text/rules. As such, the form doesn't need to know which kind of control it has found, only that it implements the specific interface, and knows that there is a method there it can call to localize the control.

Answer 2

Programming to an interface is saying "I need this functionality and I don't care where it comes from."

Consider (in Java), the List interface versus the ArrayList and LinkedList concrete classes. If all I care about is that I have a data structure containing multiple data items that I should access via iteration, I'd pick List (and that's 99% of the time). If I know that I need constant-time insert/delete from either end of the list, I might pick the LinkedList concrete implementation (or more likely, use the Queue interface). If I know I need random access by index, I'd pick the ArrayList concrete class.

Answer 3

Disgustingly bad in general. I'm not making 6000 interfces w/ one implemenation and neither should you.

Answer 4

It makes your code a lot more extensible and easier to maintain when you have sets of similar classes. I am a junior programmer, so I am no expert, but I just finished a project that required something similar.

I work on client side software that talks to a server running a medical device. We are developing a new version of this device that has some new components that the customer must configure at times. There are two types of new components, and they are different, but they are also very similar. Basically, I had to create two config forms, two lists classes, two of everything.

I decided that it would be best to create an abstract base class for each control type that would hold almost all of the real logic, and then derived types to take care of the differences between the two components. However, the base classes would not have been able to perform operations on these components if I had to worry about types all of the time (well, they could have, but there would have been an "if" statement or switch in every method).

I defined a simple interface for these components and all of the base classes talk to this interface. Now when I change something, it pretty much 'just works' everywhere and I have no code duplication.

Answer 5

To add to the existing posts, sometimes coding to interfaces helps on large projects when developers work on separate components simultaneously. All you need is to define interfaces upfront and write code to them while other developers write code to the interface you are implementing.

Answer 6

Using interfaces is a key factor in making your code easily testable in addition to removing unnecessary couplings between your classes. By creating an interface that defines the operations on your class, you allow classes that want to use that functionality the ability to use it without depending on your implementing class directly. If later on you decide to change and use a different implementation, you need only change the part of the code where the implementation is instantiated. The rest of the code need not change because it depends on the interface, not the implementing class.

This is very useful in creating unit tests. In the class under test you have it depend on the interface and inject an instance of the interface into the class (or a factory that allows it to build instances of the interface as needed) via the constructor or a property settor. The class uses the provided (or created) interface in its methods. When you go to write your tests, you can mock or fake the interface and provide an interface that responds with data configured in your unit test. You can do this because your class under test deals only with the interface, not your concrete implementation. Any class implementing the interface, including your mock or fake class, will do.

EDIT: Below is a link to an article where Erich Gamma discusses his quote, "Program to an interface, not an implementation."

http://www.artima.com/lejava/articles/designprinciples.html

Answer 7

If you program in Java, JDBC is a good example. JDBC defines a set of interfaces but says nothing about the implementation. Your applications can be written against this set of interfaces. In theory, you pick some JDBC driver and your application would just work. If you discover there's a faster or "better" or cheaper JDBC driver or for whatever reason, you can again in theory re-configure your property file, and without having to make any change in your application, your application would still work.

Answer 8

The specific example I used to give to students is that they should write

List myList = new ArrayList(); // programming to the List interface

instead of

ArrayList myList = new ArrayList(); // this is bad

These look exactly the same in a short program, but if you go on to use myList 100 times in your program you can start to see a difference. The first declaration ensures that you only call methods on myList that are defined by the List interface (so no ArrayList specific methods). If you've programmed to the interface this way, later on you can decide that you really need

List myList = new TreeList();

and you only have to change your code in that one spot. You already know that the rest of your code doesn't do anything that will be broken by changing the implementation because you programmed to the interface.

Answer 9

Imagine you have a product called 'Zebra' that can be extended by plugins. It finds the pluggins by searching for DLLs in some directory. It loads all those DLLs and uses reflection to find any classes that implement IZebraPlugin, and then calls the methods of that interface to communicate with the plugins. This makes it completely independent of any specific plugin class - it doesn't care what the classes are. It only cares that they fulfill the interface specification. Interfaces are a way of defining points of extensibility like this. Code that talks to an interface is more loosely coupled - in fact it is not coupled at all to any other specific code. It can inter-operate with plugins written years later by people who have never met the original developer.

You could instead use a base class with virtual functions - all plugins would be derived from the base class. But this is much more limiting because a class can only have one base class, whereas it can implement any number of interfaces.

Answer 10

Programming to Interfaces is awesome, it promotes loose coupling. As @lassevk mention Inversion of Control is a great use of this.

I found this presentation on this to be absolutly awesome Loose Coupling presentation

it goes through hard coded (strong coupled exmaple) then looks at interfaces, finally progressing to a IoC/DI tool (NInject)

HTH

bones

Answer 11

There are some wonderful answers on here to this questions that get into all sorts of great detail about interfaces and loosely coupling code, inversion of control and so on. There are some fairly heady discussions, so I'd like to take the opportunity to break things down a bit for understanding why an interface is useful.

When I first started getting exposed to interfaces, I too was confused about their relevance. I didn't understand why you needed them. If we're using a language like Java or C#, we already have inheritance and I viewed interfaces as a weaker form of inheritance and thought, "why bother?" In a sense I was right, you can think of interfaces as sort of a weak form of inheritance, but beyond that I finally understood their use as a language construct by thinking of them as means of classifying common traits or behaviors that were exhibited by potentially many non-related classes of objects.

For example -- say you have a SIM game and have the following classes:

 class HouseFly inherits Insect {
   void FlyAroundYourHead();
   void LandOnThings();
 }

 class Telemarketer inherits Person {
   void CallDuringDinner();
   void ContinueTalkingWhenYouSayNo();
 }

Clearly, these two objects have nothing in common in terms of direct inheritance. But, you could say they are both annoying.

Let's say our game needs to have some sort of random thing that annoys the game player when they eat dinner. This could be a HouseFly or a Telemarketer or both -- but how do you allow for both with a single function? And how do you ask each different type of object to "do their annoying thing" in the same way?

The key to realize is that both a Telemarketer and HouseFly share a common loosely interpreted behavior even though they are nothing alike in terms of modeling them. So, let's make an interface that both can implement:

 interface IPest {
    void BeAnnoying();
 }


 class HouseFly inherits Insect implements IPest {
   void FlyAroundYourHead();
   void LandOnThings();

   void BeAnnoying() {
     FlyAroundYourHead();
     LandOnThings();
   }
 }

 class Telemarketer inherits Person implements IPest {
   void CallDuringDinner();
   void ContinueTalkingWhenYouSayNo();

   void BeAnnoying() {
      CallDuringDinner();
      ContinueTalkingWhenYouSayNo();
   }
 }

We now have two classes that can each be annoying in their own way. And they do not need to derive from the same base class and share common inherent characteristics -- they simply need to satisfy the contract of IPest -- that contract is simple. You just have to BeAnnoying. In this regard, we can model the following:

 class DinningRoom {


   DinningRoom(Person[] dinningPeople, IPest[] pests) { ... }

   void ServeDinner() {
     when dinningPeople are eating,

       foreach pest in pests
         pest.BeAnnoying();
   }
 }

Here we have a dinning room that accepts a number of dinners and a number of pests -- note the use of the interface. This means that in our little world, a memeber of the pests array could actually be a Telemarketer object or a HouseFly object.

The ServeDinner method is called when dinner is served and our people in the dinning room are supposed to eat. In our little game, that's when our pests do their work -- each pest is instructed to be annoying by way of the IPest interface. In this way, we can easily have both Telemarketers and HouseFlys be annoying in each of their own ways -- we care only that we have something in the DinningRoom object that is a pest, we don't really care what it is and they could have nothing in common with other.

This very contrived pseudo-code example (that dragged on a lot longer than I anticipated) is simply meant to illustrate the kind of thing that finally turned the light on for me in terms of when we might use an interface. I apologize in advance for the silliness of the example, but hope that it helps in your understanding. And, to be sure, the other posted answers you've received here really cover the gamut of the use of interfaces today in design patterns and development methodologies.

Answer 12

In Java these concrete classes all implement the CharSequence interface:

CharBuffer, String, StringBuffer, StringBuilder

These concrete classes do not have a common parent class other than Object, so there is nothing that relates them, other than the fact they each have something to do with arrays of characters, representing such, or manipulating such. For instance, the characters of String cannot be changed once a String object is instantiated, whereas the characters of StringBuffer or StringBuilder can be edited.

Yet each one of these classes is capable of suitably implementing the CharSequence interface methods:

char charAt(int index)
int length()
CharSequence subSequence(int start, int end)
String toString()

In some cases Java class library classes that used to accept String have been revised to now accept the CharSequence interface. So if you have an instance of StringBuilder, instead of extracting a String object (which means instantiating a new object instance), can instead just pass the StringBuilder itself as it implements the CharSequence interface.

The Appendable interface that some classes implement has much the same kind of benefit for any situation where characters can be appended to an instance of the underlying concrete class object instance. All of these concrete classes implement the Appendable interface:

BufferedWriter, CharArrayWriter, CharBuffer, FileWriter, FilterWriter, LogStream, OutputStreamWriter, PipedWriter, PrintStream, PrintWriter, StringBuffer, StringBuilder, StringWriter, Writer

Answer 13

It sounds like you understand how interfaces work but are unsure of when to use them and what advantages they offer. Here are a few examples of when an interface would make sense:

// if I want to add search capabilities to my application and support multiple search
// engines such as google, yahoo, live, etc.

interface ISearchProvider
{
    string Search(string keywords);
}

then I could create GoogleSearchProvider, YahooSearchProvider, LiveSearchProvider etc.

// if I want to support multiple downloads using different protocols
// HTTP, HTTPS, FTP, FTPS, etc.
interface IUrlDownload
{
    void Download(string url)
}

// how about an image loader for different kinds of images JPG, GIF, PNG, etc.
interface IImageLoader
{
    Bitmap LoadImage(string filename)
}

then create JpegImageLoader, GifImageLoader, PngImageLoader, etc.

Most add-ins and plugin sytems work off interfaces.

Another popular use is for the Repository pattern. Say I want to load a list of zip codes from different sources

interface IZipCodeRepository
{
    IList<ZipCode> GetZipCodes(string state);
}

then I could create an XMLZipCodeRepository, SQLZipCodeRepository, CSVZipCodeRepository, etc. For my web applications I often create XML repositories early on so I can get something up and running prior to the Sql Database being ready. Once the database is ready I write an SQLRepository to replace the XML version. The rest of my code remains unchanged since it runs soley off of interfaces.

Methods can accept interfaces such as:

PrintZipCodes(IZipCodeRepository zipCodeRepository, string state)
{
    foreach (ZipCode zipCode in zipCodeRepository.GetZipCodes(state))
    {
        Console.WriteLine(zipCode.ToString());
    }
}

Answer 14

Advanced reading on this topic: Decoupling classes with inferred interfaces

Answer 15

So, just to get this right, the advantage of a interface is that I can separate the calling of a method from any particular class. Instead creating a instance of the interface, where the implementation is given from whichever class I choose that implements that interface. Thus allowing me to have many classes, which have similar but slightly different functionality and in some cases (the cases related to the intention of the interface) not care which object it is.

For example, I could have a movement interface. A method which makes something 'move' and any object (Person, Car, Cat) that implements the movement interface could be passed in and told to move. Without the method every knowing the type of class it is.

Answer 16

It is also good for Unit Testing, you can inject your own classes (that meet the requirements of the interface) into a class that depends on it

Answer 17

In addition to the already selected answer (and the various informative posts here), I would highly recommend grabbing a copy of Head First Design Patterns. It is a very easy read and will answer your question directly, explain why it is important, and show you many programming patterns you can use to make use of that principle (and others).

Answer 18

In simple terms... If I'm writing a new class Swimmer to add the functionality swim() and need to use an object of class say Dog, and this Dog class implements interface Animal which declares swim()[To better understand...you may draw a diagram as to what I am talking about]. At the top of the hierarchy(Animal) it's very abstract while at the bottom (Dog) it's very concrete. The way I think about "programming to interfaces" is that, as I write Swimmer class, I want to write my code against the interface that's as far up that hierarchy which in this case is Animal object. An interface is free from implementation details and thus makes your code loosely-coupled. The implementation details can be changed with time, however it would not affect the remaining code since all you are interacting is with the interface and not the implementation. You don't care what the implementation is like...all you know is that there will be a class that would implement the interface.