Interface vs Base class

Question

When should I use an interface and when should I use a base class    Should it always be an interface if I don t want to actually define a base implementation of the methods   If I have a Dog and Cat class  Why would I want to implement IPet instead of PetBase  I can understand having interfaces for ISheds or IBarks  IMakesNoise    because those can be placed on a pet by pet basis  but I don t understand which to use for a generic Pet

User · Answer

When should I use an interface and when should I use a base class?

You should use interface if

You have pure abstract methods and don't have non-abstract methods
You don't have default implementation of non abstract methods (except for Java 8 language, where interface methods provides default implementation)
If you are using Java 8, now interface will provide default implementation for some non-abstract methods. This will make interface more usable compared to abstract classes.

Have a look at this SE question for more details.

Should it always be an interface if I don't want to actually define a base implementation of the methods?

Yes. It's better and cleaner. Even if you have a base class with some abstract methods, let's base class extends abstract methods through interface. You can change interface in future without changing the base class.

Example in java:

abstract class PetBase implements IPet {
// Add all abstract methods in IPet interface and keep base class clean. 
   Base class will contain only non abstract methods and static methods.
}

If I have a Dog and Cat class. Why would I want to implement IPet instead of PetBase? I can understand having interfaces for ISheds or IBarks (IMakesNoise?), because those can be placed on a pet by pet basis, but I don't understand which to use for a generic Pet.

I prefer to have base class implement the interface.

 abstract class PetBase implements IPet {
 // Add all abstract methods in IPet
 }

 /*If ISheds,IBarks is common for Pets, your PetBase can implement ISheds,IBarks. 
  Respective implementations of PetBase can change the behaviour in their concrete classes*/

 abstract class PetBase implements IPet,ISheds,IBarks {
 // Add all abstract methods in respective interfaces
 }

Advantages:

If I want to add one more abstract method in existing interfaces, I simple change interface without touching the abstract base class. If I want to change the contract, I will change interface & implementation classes without touching base class.
You can provide immutability to base class through interface. Have a look at this article

Refer to this related SE question for more details:

How should I have explained the difference between an Interface and an Abstract class?

User · Answer

It depends on your requirements   If IPet is simple enough  I would prefer to implement that   Otherwise  if PetBase implements a ton of functionality you don t want to duplicate  then have at it   The downside to implementing a base class is the requirement to override  or new  existing methods   This makes them virtual methods which means you have to be careful about how you use the object instance   Lastly  the single inheritance of  NET kills me   A naive example  Say you re making a user control  so you inherit UserControl   But  now you re locked out of also inheriting PetBase   This forces you to reorganize  such as to make a PetBase class member  instead

User · Answer

An inheritor of a base class should have an  is a  relationship   Interface represents An  implements a  relationship  So only use a base class when your inheritors will maintain the is a relationship

User · Answer

Interfaces should be small  Really small  If you re really breaking down your objects  then your interfaces will probably only contain a few very specific methods and properties   Abstract classes are shortcuts  Are there things that all derivatives of PetBase share that you can code once and be done with  If yes  then it s time for an abstract class   Abstract classes are also limiting  While they give you a great shortcut to producing child objects  any given object can only implement one abstract class  Many times  I find this a limitation of Abstract classes  and this is why I use lots of interfaces   Abstract classes may contain several interfaces  Your PetBase abstract class may implement IPet  pets have owners  and IDigestion  pets eat  or at least they should   However  PetBase will probably not implement IMammal  since not all pets are mammals and not all mammals are pets  You may add a MammalPetBase that extends PetBase and add IMammal  FishBase could have PetBase and add IFish  IFish would have ISwim and IUnderwaterBreather as interfaces   Yes  my example is extensively over-complicated for the simple example  but that s part of the great thing about how interfaces and abstract classes work together

User · Answer

In general  you should favor interfaces over abstract classes  One reason to use an abstract class is if you have common implementation among concrete classes  Of course  you should still declare an interface  IPet  and have an abstract class  PetBase  implement that interface Using small  distinct interfaces  you can use multiples to further improve flexibility  Interfaces allow the maximum amount of flexibility and portability of types across boundaries  When passing references across boundaries  always pass the interface and not the concrete type  This allows the receiving end to determine concrete implementation and provides maximum flexibility  This is absolutely true when programming in a TDD BDD fashion   The Gang of Four stated in their book  Because inheritance exposes a subclass to details of its parent s implementation  it s often said that  inheritance breaks encapsulation   I believe this to be true

User · Answer

Interfaces have the distinct advantage of being somewhat  hot swappable  for classes   Changing a class from one parent to another will often result in a great deal of work  but Interfaces can often be removed and changed without a great deal of effect on the implementation class   This is especially useful in cases where you have several narrow sets of behaviour that you  may  want a class to implement   This works especially well in my field  game programming   Base classes can get bloated with tons of behaviours that  may  be needed by inherited objects   With interfaces different behaviours can be added or removed to objects easily and readily   For example  if I create an interface for  IDamageEffects  for objects that I want to reflect damage  then I can easily apply that to various game objects  and easily change my mind later   Say I design an initial class that I want to use for  static  decorative objects and I initially decide they are non-destructible   Later on  I may decide it would be more fun if they could blow up so I alter the class to implement the  IDamageEffects  interface   This is much easier to do than switching base classes or creating a new object hierarchy

User · Answer

Thanks for answering by Jon Limjap but I want to add some explanation for concept of Interface and Abstract Base Classes  Interface Types vs  Abstract Base Classes  Adapted from the Pro C  5 0 and the  NET 4 5 Framework book   The interface type might seem very similar to an abstract base class  Recall that when a class is marked as abstract  it may define any number of abstract members to provide a polymorphic interface to all derived types  However  even when a class does define a set of abstract members  it is also free to define any number of constructors  field data  nonabstract members  with implementation   and so on  Interfaces  on the other hand  contain only abstract member definitions  The polymorphic interface established by an abstract parent class suffers from one major limitation in that only derived types support the members defined by the abstract parent  However  in larger software systems  it is very common to develop multiple class hierarchies that have no common parent beyond System Object  Given that abstract members in an abstract base class apply only to derived types  we have no way to configure types in different hierarchies to support the same polymorphic interface  By way of example  assume you have defined the following abstract class   public abstract class CloneableType      Only derived types can support this     polymorphic interface   Classes in other    hierarchies have no access to this abstract    member     public abstract object Clone        Given this definition  only members that extend CloneableType are able to support the Clone   method  If you create a new set of classes that do not extend this base class  you can   t gain this polymorphic interface  Also  you might recall that C  does not support multiple inheritance for classes  Therefore  if you wanted to create a MiniVan that is-a Car and is-a CloneableType  you are unable to do so      Nope  Multiple inheritance is not possible in C     for classes  public class MiniVan   Car  CloneableType       As you would guess  interface types come to the rescue  After an interface has been defined  it can be implemented by any class or structure  in any hierarchy  within any namespace or any assembly  written in any  NET programming language   As you can see  interfaces are highly polymorphic  Consider the standard  NET interface named ICloneable  defined in the System namespace  This interface defines a single method named Clone     public interface ICloneable   object Clone

User · Answer

Conceptually  an interface is used to formally and semi-formally define a set of methods that an object will provide  Formally means a set of method names and signatures  and semi-formally means human readable documentation associated with those methods   Interfaces are only descriptions of an API  after all  API stands for application programming interface   they can t contain any implementation  and it s not possible to use or run an interface  They only make explicit the contract of how you should interact with an object   Classes provide an implementation  and they can declare that they implement zero  one or more Interfaces  If a class is intended to be inherited  the convention is to prefix the class name with  Base    There is a distinction between a base class and an abstract base classes  ABC   ABCs mix interface and implementation together  Abstract outside of computer programming means  summary   that is  abstract    interface   An abstract base class can then describe both an interface  as well as an empty  partial or complete implementation that is intended to be inherited   Opinions on when to use interfaces versus abstract base classes versus just classes is going to vary wildly based on both what you are developing  and which language you are developing in  Interfaces are often associated only with statically typed languages such as Java or C   but dynamically typed languages can also have interfaces and abstract base classes  In Python for example  the distinction is made clear between a Class  which declares that it implements an interface  and an object  which is an instance of a class  and is said to provide that interface  It s possible in a dynamic language that two objects that are both instances of the same class  can declare that they provide completely different interfaces  In Python this is only possible for object attributes  while methods are shared state between all objects of a class  However  in Ruby  objects can have per-instance methods  so it s possible that the interface between two objects of the same class can vary as much as the programmer desires  however  Ruby doesn t have any explicit way of declaring Interfaces    In dynamic languages the interface to an object is often implicitly assumed  either by introspecting an object and asking it what methods it provides  look before you leap  or preferably by simply attempting to use the desired interface on an object and catching exceptions if the object doesn t provide that interface  easier to ask forgiveness than permission   This can lead to  false positives  where two interfaces have the same method name  but are semantically different  However  the trade-off is that your code is more flexible since you don t need to over specify up-front to anticipate all possible uses of your code

User · Answer

Modern style is to define IPet and PetBase   The advantage of the interface is that other code can use it without any ties whatsoever to other executable code   Completely  clean    Also interfaces can be mixed   But base classes are useful for simple implementations and common utilities   So provide an abstract base class as well to save time and code

User · Answer

Juan   I like to think of interfaces as a way to characterize a class   A particular dog breed class  say a YorkshireTerrier  may be a descended of the parent dog class  but it is also implements IFurry  IStubby  and IYippieDog   So the class defines what the class is but the interface tells us things about it   The advantage of this is it allows me to  for example  gather all the IYippieDog s and throw them into my Ocean collection   So now I can reach across a particular set of objects and find ones that meet the criteria I am looking at without inspecting the class too closely   I find that interfaces really should define a sub-set of the public behavior of a class   If it defines all the public behavior for all the classes that implement then it usually does not need to exist   They do not tell me anything useful   This thought though goes counter to the idea that every class should have an interface and you should code to the interface   That s fine  but you end up with a lot of one to one interfaces to classes and it makes things confusing   I understand that the idea is it does not really cost anything to do and now you can swap things in and out with ease   However  I find that I rarely do that   Most of the time I am just modifying the existing class in place and have the exact same issues I always did if the public interface of that class needs changing  except I now have to change it in two places   So if you think like me you would definitely say that Cat and Dog are IPettable   It is a characterization that matches them both   The other piece of this though is should they have the same base class   The question is do they need to be broadly treated as the same thing   Certainly they are both Animals  but does that fit how we are going to use them together     Say I want to gather all Animal classes and put them in my Ark container   Or do they need to be Mammals   Perhaps we need some kind of cross animal milking factory   Do they even need to be linked together at all   Is it enough to just know they are both IPettable   I often feel the desire to derive a whole class hierarchy when I really just need one class    I do it in anticipation someday I might need it and usually I never do   Even when I do  I usually find I have to do a lot to fix it   That   s because the first class I am creating is not the Dog  I am not that lucky  it is instead the Platypus   Now my entire class hierarchy is based on the bizarre case and I have a lot of wasted code    You might also find at some point that not all Cats are IPettable  like that hairless one    Now you can move that Interface to all the derivative classes that fit   You will find that a much less breaking change that all of a sudden Cats are no longer derived from PettableBase

User · Answer

By def  interface provides a layer to communicate with other code  All the public properties and methods of a class are by default implementing implicit interface  We can also define an interface as a role  when ever any class needs to play that role  it has to implement it giving it different forms of implementation depending on the class implementing it  Hence when you talk about interface  you are talking about polymorphism and when you are talking about base class  you are talking about inheritance   Two concepts of oops

User · Answer

Interfaces and base classes represent two different forms of relationships   Inheritance  base classes  represent an  is-a  relationship  E g  a dog or a cat  is-a  pet  This relationship always represents the  single  purpose of the class  in conjunction with the  single responsibility principle     Interfaces  on the other hand  represent additional features of a class  I d call it an  is  relationship  like in  Foo is disposable   hence the IDisposable interface in C

User · Answer

Regarding C   in some senses interfaces and abstract classes can be interchangeable  However  the differences are  i  interfaces cannot implement code  ii  because of this  interfaces cannot call further up the stack to subclass  and iii  only can abstract class may be inherited on a class  whereas multiple interfaces may be implemented on a class

User · Answer

Another option to keep in mind is using the  has-a  relationship  aka  is implemented in terms of  or  composition    Sometimes this is a cleaner  more flexible way to structure things than using  is-a  inheritance   It may not make as much sense logically to say that Dog and Cat both  have  a Pet  but it avoids common multiple inheritance pitfalls   public class Pet       void Bathe        void Train Trick t      public class Dog       private Pet pet       public void Bathe     pet Bathe          public void Train Trick t    pet Train t        public class Cat       private Pet pet       public void Bathe     pet Bathe          public void Train Trick t    pet Train t         Yes  this example shows that there is a lot of code duplication and lack of elegance involved in doing things this way   But one should also appreciate that this helps to keep Dog and Cat decoupled from the Pet class  in that Dog and Cat do not have access to the private members of Pet   and it leaves room for Dog and Cat to inherit from something else--possibly the Mammal class   Composition is preferable when no private access is required and you don t need to refer to Dog and Cat using generic Pet references pointers   Interfaces give you that generic reference capability and can help cut down on the verbosity of your code  but they can also obfuscate things when they are poorly organized   Inheritance is useful when you need private member access  and in using it you are committing yourself to highly coupling your Dog and Cat classes to your Pet class  which is a steep cost to pay   Between inheritance  composition  and interfaces there is no one way that is always right  and it helps to consider how all three options can be used in harmony   Of the three  inheritance is typically the option that should be used the least often

User · Answer

In addition to those comments that mention the IPet PetBase implementation  there are also cases where providing an accessor helper class can be very valuable   The IPet PetBase style assumes that you have multiple implementations thus increasing the value of PetBase since it simplifies implementation   However  if you have the reverse or a blend of the two where you have multiple clients  providing a class help assist in the usage of the interface can reduce cost by making it easier to use an interface

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I recommend using composition instead of inheritence whenever possible   Use interfaces but use member objects for base implementation   That way  you can define a factory that constructs your objects to behave in a certain way   If you want to change the behavior then you make a new factory method  or abstract factory  that creates different types of sub-objects   In some cases  you may find that your primary objects don t need interfaces at all  if all of the mutable behavior is defined in helper objects   So instead of IPet or PetBase  you might end up with a Pet which has an IFurBehavior parameter   The IFurBehavior parameter is set by the CreateDog   method of the PetFactory   It is this parameter which is called for the shed   method   If you do this you ll find your code is much more flexible and most of your simple objects deal with very basic system-wide behaviors   I recommend this pattern even in multiple-inheritence languages

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It is explained well in this Java World article   Personally  I tend to use interfaces to define interfaces - i e  parts of the system design that specify how something should be accessed   It s not uncommon that I will have a class implementing one or more interfaces   Abstract classes I use as a basis for something else   The following is an extract from the above mentioned article JavaWorld com article  author Tony Sintes  04 20 01       Interface vs  abstract class      Choosing interfaces and abstract classes is not an either or proposition  If you need to change your design  make it an interface  However  you may have abstract classes that provide some default behavior  Abstract classes are excellent candidates inside of application frameworks       Abstract classes let you define some behaviors  they force your subclasses to provide others  For example  if you have an application framework  an abstract class may provide default services such as event and message handling  Those services allow your application to plug in to your application framework  However  there is some application-specific functionality that only your application can perform  Such functionality might include startup and shutdown tasks  which are often application-dependent  So instead of trying to define that behavior itself  the abstract base class can declare abstract shutdown and startup methods  The base class knows that it needs those methods  but an abstract class lets your class admit that it doesn t know how to perform those actions  it only knows that it must initiate the actions  When it is time to start up  the abstract class can call the startup method  When the base class calls this method  Java calls the method defined by the child class       Many developers forget that a class that defines an abstract method can call that method as well  Abstract classes are an excellent way to create planned inheritance hierarchies  They re also a good choice for nonleaf classes in class hierarchies       Class vs  interface   Some say you should define all classes in terms of interfaces  but I think recommendation seems a bit extreme  I use interfaces when I see that something in my design will change frequently      For example  the Strategy pattern lets you swap new algorithms and processes into your program without altering the objects that use them  A media player might know how to play CDs  MP3s  and wav files  Of course  you don t want to hardcode those playback algorithms into the player  that will make it difficult to add a new format like AVI  Furthermore  your code will be littered with useless case statements  And to add insult to injury  you will need to update those case statements each time you add a new algorithm  All in all  this is not a very object-oriented way to program       With the Strategy pattern  you can simply encapsulate the algorithm behind an object  If you do that  you can provide new media plug-ins at any time  Let s call the plug-in class MediaStrategy  That object would have one method  playStream Stream s   So to add a new algorithm  we simply extend our algorithm class  Now  when the program encounters the new media type  it simply delegates the playing of the stream to our media strategy  Of course  you ll need some plumbing to properly instantiate the algorithm strategies you will need       This is an excellent place to use an interface  We ve used the Strategy pattern  which clearly indicates a place in the design that will change  Thus  you should define the strategy as an interface  You should generally favor interfaces over inheritance when you want an object to have a certain type  in this case  MediaStrategy  Relying on inheritance for type identity is dangerous  it locks you into a particular inheritance hierarchy  Java doesn t allow multiple inheritance  so you can t extend something that gives you a useful implementation or more type identity

User · Answer

I have a rough rule-of-thumb  Functionality  likely to be different in all parts  Interface   Data  and functionality  parts will be mostly the same  parts different  abstract class   Data  and functionality  actually working  if extended only with slight changes  ordinary  concrete  class  Data and functionality  no changes planned  ordinary  concrete  class with final modifier   Data  and maybe functionality  read-only  enum members   This is very rough and ready and not at all strictly defined  but there is a spectrum from interfaces where everything is intended to be changed to enums where everything is fixed a bit like a read-only file

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I usually don t implement either until I need one   I favor interfaces over abstract classes because that gives a little more flexibility   If there s common behavior in some of the inheriting classes I move that up and make an abstract base class   I don t see the need for both  since they essentially server the same purpose  and having both is a bad code smell  imho  that the solution has been over-engineered

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Make a list of the things your objects must be  have  or do and the things your objects can  or might  be  have  or do  Must indicates your base types and can indicates your interfaces    E g   your PetBase must Breathe  and your IPet might DoTricks   Analysis of your problem domain will help you define the precise hierarchy structure

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Let s take your example of a Dog and a Cat class  and let s illustrate using C    Both a dog and a cat are animals  specifically  quadruped mammals  animals are waaay too general   Let us assume that you have an abstract class Mammal  for both of them   public abstract class Mammal   This base class will probably have default methods such as    Feed Mate   All of which are behavior that have more or less the same implementation between either species  To define this you will have   public class Dog   Mammal public class Cat   Mammal   Now let s suppose there are other mammals  which we will usually see in a zoo   public class Giraffe   Mammal public class Rhinoceros   Mammal public class Hippopotamus   Mammal   This will still be valid because at the core of the functionality Feed   and Mate   will still be the same   However  giraffes  rhinoceros  and hippos are not exactly animals that you can make pets out of  That s where an interface will be useful   public interface IPettable       IList lt Trick gt  Tricks get  set       void Bathe        void Train Trick t       The implementation for the above contract will not be the same between a cat and dog  putting their implementations in an abstract class to inherit will be a bad idea    Your Dog and Cat definitions should now look like   public class Dog   Mammal  IPettable public class Cat   Mammal  IPettable   Theoretically you can override them from a higher base class  but essentially an interface allows you to add on only the things you need into a class without the need for inheritance   Consequently  because you can usually only inherit from one abstract class  in most statically typed OO languages that is    exceptions include C    but be able to implement multiple interfaces  it allows you to construct objects in a strictly as required basis

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This is pretty  NET specific  but the Framework Design Guidelines book argues that in general classes give more flexibility in an evolving framework   Once an interface is shipped  you don t get the chance to change it without breaking code that used that interface   With a class however  you can modify it and not break code that links to it  As long you make the right modifications  which includes adding new functionality  you will be able to extend and evolve your code   Krzysztof Cwalina says on page 81      Over the course of the three versions of the  NET Framework  I have talked about this guideline with quite a few developers on our team   Many of them  including those who initially disagreed with the guidelines  have said that they regret having shipped some API as an interface   I have not heard of even one case in which somebody regretted that they shipped a class    That being said there certainly is a place for interfaces   As a general guideline always provide an abstract base class implementation of an interface if for nothing else as an example of a way to implement the interface   In the best case that base class will save a lot of work

User · Answer

When I first started learning about object-oriented programming  I made the easy and probably common mistake of using inheritance to share common behavior - even where that behavior was not essential to the nature of the object   To further build on an example much used in this particular question  there are lots of things that are petable - girlfriends  cars  fuzzy blankets    - so I might have had a Petable class that provided this common behavior  and various classes inheriting from it   However  being petable is not part of the nature of any of these objects  There are vastly more important concepts that are essential to their nature - the girlfriend is a person  the car is a land vehicle  the cat is a mammal     Behaviors should be assigned first to interfaces  including the default interface of the class   and promoted to a base class only if they are  a  common to a large group of classes that are subsets of a larger class - in the same sense that  cat  and  person  are subsets of  mammal    The catch is  after you understand object-oriented design sufficiently better than I did at first  you ll normally do this automatically without even thinking about it  So the bare truth of the statement  code to an interface  not an abstract class  becomes so obvious you have a hard time believing anyone would bother to say it - and start trying to read other meanings into it   Another thing I d add is that if a class is purely abstract - with no non-abstract  non-inherited members or methods exposed to child  parent  or client - then why is it a class  It could be replaced  in some cases by an interface and in other cases by Null

User · Answer

Prefer interfaces over abstract classes  Rationale  the main points to consider  two already mentioned here  are     Interfaces are more flexible  because a class can implement multiple interfaces   Since Java does not have multiple inheritance  using abstract classes prevents your users from using any other class hierarchy  In general  prefer interfaces when there are no default implementations or state  Java collections offer good examples of this  Map  Set  etc    Abstract classes have the advantage of allowing better forward compatibility   Once clients use an interface  you cannot change it  if they use an abstract class  you can still add behavior without breaking existing code  If compatibility is a concern  consider using abstract classes  Even if you do have default implementations or internal state  consider offering an interface and an abstract implementation of it  This will assist clients  but still allow them greater freedom if desired  1   Of course  the subject has been discussed at length elsewhere  2 3      1  It adds more code  of course  but if brevity is your primary concern  you probably should have avoided Java in the first place    2  Joshua Bloch  Effective Java  items 16-18    3  http   www codeproject com KB ar

User · Answer

Use your own judgement and be smart  Don t always do what others  like me  are saying  You will hear  prefer interfaces to abstract classes  but it really depends  It depends what the class is   In the above mentioned case where we have a hierarchy of objects  interface is a good idea  Interface helps to work with collections of these objects and it also helps when implementing a service working with any object of the hierarchy  You just define a contract for working with objects from the hierarchy   On the other hand when you implement a bunch of services that share a common functionality you can either separate the common functionality into a complete separate class or you can move it up into a common base class and make it abstract so that nobody can instantiate the base class   Also consider this how to support your abstractions over time  Interfaces are fixed  You release an interface as a contract for a set of functionality that any type can implement  Base classes can be extended over time  Those extensions become part of every derived class

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Source  http   jasonroell com 2014 12 09 interfaces-vs-abstract-classes-what-should-you-use   C  is a wonderful language that has matured and evolved over the last 14 years  This is great for us developers because a mature language provides us with a plethora of language features that are at our disposal   However  with much power becomes much responsibility  Some of these features can be misused  or sometimes it is hard to understand why you would choose to use one feature over another  Over the years  a feature that I have seen many developers struggle with is when to choose to use an interface or to choose to use an abstract class  Both have there advantages and disadvantages and the correct time and place to use each  But how to we decide     Both provide for reuse of common functionality between types  The most obvious difference right away is that interfaces provide no implementation for their functionality whereas abstract classes allow you to implement some    base    or    default    behavior and then have the ability to    override    this default behavior with the classes derived types if necessary   This is all well and good and provides for great reuse of code and adheres to the DRY  Don   t Repeat Yourself  principle of software development  Abstract classes are great to use when you have an    is a    relationship   For example  A golden retriever    is a    type of dog  So is a poodle  They both can bark  as all dogs can  However  you might want to state that the poodle park is significantly different than the    default    dog bark  Therefor  it could make sense for you to implement something as follows   public abstract class Dog         public virtual void Bark                   Console WriteLine  Base Class implementation of Bark               public class GoldenRetriever   Dog         the Bark method is inherited from the Dog class    public class Poodle   Dog        here we are overriding the base functionality of Bark with our new implementation      specific to the Poodle class   public override void Bark            Console WriteLine  Poodle s implementation of Bark              Add a list of dogs to a collection and call the bark method   void Main         var poodle   new Poodle        var goldenRetriever   new GoldenRetriever         var dogs   new List lt Dog gt         dogs Add poodle       dogs Add goldenRetriever        foreach  var dog in dogs               dog Bark                Output will be     Poodle s implementation of Bark    Base Class implementation of Bark        As you can see  this would be a great way to keep your code DRY and allow for the base class implementation be called when any of the types can just rely on the default Bark instead of a special case implementation  The classes like GoldenRetriever  Boxer  Lab could all could inherit the    default     bass class  Bark at no charge just because they implement the Dog abstract class   But I   m sure you already knew that   You are here because you want to understand why you might want to choose an interface over an abstract class or vice versa  Well one reason you may want to choose an interface over an abstract class is when you don   t have or want to prevent a default implementation  This is usually because the types that are implementing the interface not related in an    is a    relationship  Actually  they don   t have to be related at all except for the fact that each type    is able    or has    the ablity    to do something or have something   Now what the heck does that mean  Well  for example  A human is not a duck   and a duck is not a human  Pretty obvious  However  both a duck and a human have    the ability    to swim  given that the human passed his swimming lessons in 1st grade       Also  since a duck is not a human or vice versa  this is not an    is a    realationship  but instead an    is able    relationship and we can use an interface to illustrate that      Create ISwimable interface public interface ISwimable         public void Swim          Have Human implement ISwimable Interface public class Human   ISwimable       public void Swim                    Human s implementation of Swim         Console WriteLine  I m a human swimming                Have Duck implement ISwimable interface public class Duck  ISwimable        public void Swim                       Duck s implementation of Swim           Console WriteLine  Quack  Quack  I m a Duck swimming                Now they can both be used in places where you just need an object that has the ability  to swim   public void ShowHowYouSwim ISwimable somethingThatCanSwim         somethingThatCanSwim Swim       public void Main           var human   new Human          var duck   new Duck           var listOfThingsThatCanSwim   new List lt ISwimable gt            listOfThingsThatCanSwim Add duck         listOfThingsThatCanSwim Add human          foreach  var something in listOfThingsThatCanSwim                     ShowHowYouSwim something                  So at runtime the correct implementation of something Swim   will be called     Output      Quack  Quack  I m a Duck swimming      I m a human swimming    Using interfaces like the code above will allow you to pass an object into a method that    is able    to do something   The code doesn   t care how it does it   All it knows is that it can call the Swim method on that object and that object will know which behavior take at run-time based on its type   Once again  this helps your code stay DRY so that you would not have to write multiple methods that are calling the object to preform the same core function  ShowHowHumanSwims human   ShowHowDuckSwims duck   etc    Using an interface here allows the calling methods to not have to worry about what type is which or how the behavior is implemented  It just knows that given the interface  each object will have to have implemented the Swim method so it is safe to call it in its own code and allow the behavior of the Swim method be handled within its own class   Summary   So my main rule of thumb is use an abstract class when you want to implement a    default    functionality for a class hierarchy or and the classes or types you are working with share a    is a    relationship  ex  poodle    is a    type of dog    On the other hand use an interface when you do not have an    is a    relationship but have types that share    the ability    to do something or have something  ex  Duck    is not    a human  However  duck and human share    the ability    to swim    Another difference to note between abstract classes and interfaces is that a class can implement one to many interfaces but a class can only inherit from ONE abstract class  or any class for that matter   Yes  you can nest classes and have an inheritance hierarchy  which many programs do and should have  but you cannot inherit two classes in one derived class definition  this rule applies to C   In some other languages you are able to do this  usually only because of the lack of interfaces in these languages    Also remember when using interfaces to adhere to the Interface Segregation Principle  ISP   ISP states that no client should be forced to depend on methods it does not use  For this reason interfaces should be focused on specific tasks and are usually very small  ex  IDisposable  IComparable     Another tip is if you are developing small  concise bits of functionality  use interfaces  If you are designing large functional units  use an abstract class   Hope this clears things up for some people   Also if you can think of any better examples or want to point something out  please do so in the comments below

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Also keep in mind not to get swept away in OO  see blog  and always model objects based on behavior required  if you were designing an app where the only behavior you required was a generic name and species for an animal then you would only need one class Animal with a property for the name  instead of millions of classes for every possible animal in the world

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There are other advantages to inheritance as well - such as the ability for a variable to be able to hold an object of either the parent class or the inherited class  without having to declare it as something generic  like  Object      For example  in  NET WinForms  most UI components derive from System Windows Forms Control  so a variable declared as that could  hold  just about any UI element - be it a button  a ListView  or what have you  Now  granted  you won t have access to all the properties or methods of the item  but you ll have all the basic stuff - and that can be useful

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You should use a base class if there really isn t any reason for other developers to desire using their own base class in addition to your type s members and you foresee versioning issues  see http   haacked com archive 2008 02 21 versioning-issues-with-abstract-base-classes-and-interfaces aspx    If inheriting developers have any reason to use their own base class to implement your type s interface and you don t see the interface changing  then go with an interface  In this case  you can still throw in a default base class that implements the interface for sake of convenience

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Previous comments about using abstract classes for common implementation is definitely on the mark   One benefit I haven t seen mentioned yet is that the use of interfaces makes it much easier to implement mock objects for the purpose of unit testing   Defining IPet and PetBase as Jason Cohen described enables you to mock different data conditions easily  without the overhead of a physical database  until you decide it s time to test the real thing

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Don t use a base class unless you know what it means  and that it applies in this case   If it applies  use it  otherwise  use interfaces   But note the answer about small interfaces   Public Inheritance is overused in OOD and expresses a lot more than most developers realize or are willing to live up to   See the Liskov Substitutablity Principle  In short  if A  is a  B then A requires no more than B and delivers no less than B  for every method it exposes

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The case for Base Classes over Interfaces was explained well in the Submain  NET Coding Guidelines      Base Classes vs  Interfaces    An interface type is a partial   description of a value  potentially   supported by many object types  Use   base classes instead of interfaces   whenever possible  From a versioning   perspective  classes are more flexible   than interfaces  With a class  you can   ship Version 1 0 and then in Version   2 0 add a new method to the class  As long as the method is not abstract    any existing derived classes continue   to function unchanged       Because interfaces do not support   implementation inheritance  the   pattern that applies to classes does   not apply to interfaces  Adding a   method to an interface is equivalent   to adding an abstract method to a base   class  any class that implements the   interface will break because the class   does not implement the new method    Interfaces are appropriate in the   following situations          Several unrelated classes want to support the protocol    These classes already have established base classes  for   example    some are user interface  UI  controls    and some are XML Web services     Aggregation is not appropriate or practicable  In all other   situations    class inheritance is a better model

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One important difference is that you can only inherit one base class  but you can implement many interfaces   So you only want to use a base class if you are absolutely certain that you won t need to also inherit a different base class   Additionally  if you find your interface is getting large then you should start looking to break it up into a few logical pieces that define independent functionality  since there s no rule that your class can t implement them all  or that you can define a different interface that just inherits them all to group them

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Here is the basic and simple definiton of interface and base class    Base class   object inheritance  Interface   functional inheritance    cheers

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Well  Josh Bloch said himself in Effective Java 2d   Prefer interfaces over abstract classes  Some main points         Existing classes can be easily retrofitted to implement a new   interface  All you have to do is add   the required methods if they don   t yet   exist and add an implements clause to   the class declaration     Interfaces are ideal for defining mixins  Loosely speaking  a   mixin is a type that a class can   implement in addition to its    primary   type    to declare that it provides   some optional behavior  For example    Comparable is a mixin interface that   allows a class to declare that its   instances are ordered with respect to   other mutually comparable objects    Interfaces allow the construction of nonhierarchical type   frameworks  Type hierarchies are   great for organizing some things  but   other things don   t fall neatly into a   rigid hierarchy     Interfaces enable safe  powerful functionality enhancements via the   wrap- per class idiom  If you use   abstract classes to define types  you   leave the programmer who wants to add   functionality with no alternative but   to use inheritance           Moreover  you can combine the virtues   of interfaces and abstract classes by   providing an abstract skeletal   implementation class to go with each   nontrivial interface that you export    On the other hand  interfaces are very hard to evolve  If you add a method to an interface it ll break all of it s implementations   PS   Buy the book  It s a lot more detailed

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I ve found that a pattern of Interface   Abstract   Concrete works in the following use-case   1   You have a general interface  eg IPet  2   You have a implementation that is less general  eg Mammal  3   You have many concrete members  eg Cat  Dog  Ape    The abstract class defines default shared attributes of the concrete classes  yet enforces the interface   For example   public interface IPet       public boolean hasHair         public boolean walksUprights         public boolean hasNipples        Now  since all mammals have hair and nipples  AFAIK  I m not a zoologist   we can roll this into the abstract base class  public abstract class Mammal   implements IPet         override      public walksUpright             throw new NotSupportedException  Walks Upright not implemented                  override      public hasNipples   return true         override      public hasHair   return true    And then the concrete classes merely define that they walk upright   public class Ape extends Mammal          override     public walksUpright return true     public class Catextends Mammal          override     public walksUpright return false      This design is nice when there are lots of concrete classes  and you don t want to maintain boilerplate just to program to an interface   If new methods were added to the interface  it would break all of the resulting classes  so you are still getting the advantages of the interface approach   In this case  the abstract could just as well be concrete  however  the abstract designation helps to emphasize that this pattern is being employed

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Use Interfaces to enforce a contract ACROSS families of unrelated classes  For example  you might have common access methods for classes that represent collections  but contain radically different data i e  one class might represent a result set from a query  while the other might represent the images in a gallery  Also  you can implement multiple interfaces  thus allowing you to blend  and signify  the capabilities of the class   Use Inheritance when the classes bear a common relationship and therefore have a similair structural and behavioural signature  i e  Car  Motorbike  Truck and SUV are all types of road vehicle that might contain a number of wheels  a top speed

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Interfaces   Defines the contract between 2 modules  Cannot have any implementation    Most languages allow you to implement multiple interfaces   Modifying an interface is a breaking change  All implementations need to be recompiled modified  All members are public  Implementations have to implement all members  Interfaces help in Decoupling  You can use mock frameworks to mock out anything behind an interface Interfaces normally indicate a kind of behavior Interface implementations are decoupled   isolated from each other   Base classes   Allows you to add some default implementation that you get for free by derivation   Except C    you can only derive from one class  Even if could from multiple classes  it is usually a bad idea  Changing the base class is relatively easy  Derivations do not need to do anything special Base classes can declare protected and public functions that can be accessed by derivations Abstract Base classes can t be mocked easily like interfaces Base classes normally indicate type hierarchy  IS A  Class derivations may come to depend on some base behavior  have intricate knowledge of parent implementation   Things can be messy if you make a change to the base implementation for one guy and break the others

[oop] Interface vs Base class

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