Interface naming in Java

Question

Most OO languages prefix their interface names with a capital I  why does Java not do this   What was the rationale for not following this convention   To demonstrate what I mean  if I wanted to have a User interface and a User implementation I d have two choices in Java         Class   User  Interface   UserInterface   Class   UserImpl  Interface   User      Where in most languages      Class   User  Interface   IUser   Now  you might argue that you could always pick a most descriptive name for the user implementation and the problem goes away  but Java s pushing a POJO approach to things and   most IOC containers use DynamicProxies extensively   These two things together mean that you ll have lots of interfaces with a single POJO implementation   So  I guess my question boils down to   Is it worth following the broader Interface naming convention especially in light of where Java Frameworks seem to be heading

User · Answer

Following good OO principles  your code should  as far as practical possible  depend on abstractions rather than concrete classes   For example  it is generally better to write a method like this   public void doSomething Collection someStuff                than this   public void doSomething Vector someStuff                If you follow this idea  then I maintain that your code will be more readable if you give interfaces names like  User  and  BankAccount   for example   rather than  IUser    UserInterface   or other variations     The only bits of code that should care about the actual concrete classes are the places where the concrete classes are constructed   Everything else should be written using the interfaces   If you do this  then the  ugly  concrete class names like  UserImpl  should be safely hidden from the rest of the code  which can merrily go on using the  nice  interface names

User · Answer

v  The  I  prefix is also used in the Wicket framework  where I got used to it quickly  In general  I welcome any convention that shortens cumbersome Java classnames   It is a hassle  though  that everything is alphabetized under  I  in the directories and in the Javadoc   Wicket coding practice is similar to Swing  in that many control widget instances are constructed as anonymous inner classes with inline method declarations   Annoyingly  it differs 180   from Swing in that Swing uses a prefix   J   for the implementing classes   The  Impl  suffix is a mangly abbreviation and doesn t internationalize well   If only we d at least gone with  Imp  it would be cuter  and shorter     Impl  is used for IOC  especially Spring  so we re sort of stuck with it for now   It gets a bit schizo following 3 different conventions in three different parts of one codebase  though

User · Answer

There is also another convention  used by many open source projects including Spring    interface User      class DefaultUser implements User      class AnotherClassOfUser implements User       I personally do not like the  I  prefix for the simple reason that its an optional convention  So if I adopt this does IIOPConnection mean an interface for IOPConnection  What if the class does not have the  I   prefix  do I then know its not an interface  the answer here is no  because conventions are not always followed  and policing them will create more work that the convention itself saves

User · Answer

There may be several reasons Java does not generally use the IUser convention    Part of the Object-Oriented approach is that you should not have to know whether the client is using an interface or an implementation class  So  even List is an interface and String is an actual class  a method might be passed both of them - it doesn t make sense to visually distinguish the interfaces  In general  we will actually prefer the use of interfaces in client code  prefer List to ArrayList  for instance   So it doesn t make sense to make the interfaces stand out as exceptions  The Java naming convention prefers longer names with actual meanings to Hungarian-style prefixes  So that code will be as readable as possible  a List represents a list  and a User represents a user - not an IUser

User · Answer

Bob Lee said once in a presentation       whats the point of an interface if you   have only one implementation    so  you start off with one implementation i e  without an interface  later on you decide  well  there is a need for an interface here  so you convert your class to an interface   then it becomes obvious  your original class was called User  your interface is now called User  maybe you have a UserProdImpl and a UserTestImpl  if you designed your application well  every class  except the ones that instantiate User  will be unchanged and will not notice that suddenly they get passed an interface   so it gets clear -  Interface User implementation UserImpl

User · Answer

In C  it is   public class AdminForumUser   UserBase  IUser   Java would say  public class AdminForumUser extends User implements ForumUserInterface   Because of that  I don t think conventions are nearly as important in java for interfaces  since there is an explicit difference between inheritance and interface implementation  I would say just choose any naming convention you would like  as long as you are consistant and use something to show people that these are interfaces  Haven t done java in a few years  but all interfaces would just be in their own directory  and that was the convention  Never really had any issues with it

User · Answer

In my experience  the  I  convention applies to interfaces that are intended to provide a contract to a class  particularly when the interface itself is not an abstract notion of the class     For example  in your case  I d only expect to see IUser if the only user you ever intend to have is User   If you plan to have different types of users - NoviceUser  ExpertUser  etc  - I would expect to see a User interface  and  perhaps  an AbstractUser class that implements some common functionality  like get setName      I would also expect interfaces that define capabilities - Comparable  Iterable  etc  - to be named like that  and not like IComparable or IIterable

User · Answer

Is this a broader naming convention in any real sense   I m more on the C   side  and not really up on Java and descendants   How many language communities use the I convention   If you have a language-independent shop standard naming convention here  use it   If not  go with the language naming convention

User · Answer

I prefer not to use a prefix on interfaces    The prefix hurts readability  Using interfaces in clients is the standard best way to program  so interfaces names should be as short and pleasant as possible  Implementing classes should be uglier to discourage their use  When changing from an abstract class to an interface a coding convention with prefix I implies renaming all the occurrences of the class --- not good

User · Answer

As another poster said  it s typically preferable to have interfaces define capabilities not types   I would tend not to  implement  something like a  User   and this is why  IUser  often isn t really necessary in the way described here   I often see classes as nouns and interfaces as adjectives   class Number implements Comparable        class MyThread implements Runnable      class SessionData implements Serializable         Sometimes an Adjective doesn t make sense  but I d still generally be using interfaces to model behavior  actions  capabilities  properties  etc     not types     Also  If you were really only going to make one User and call it User then what s the point of also having an IUser interface   And if you are going to have a few different types of users that need to implement a common interface  what does appending an  I  to the interface save you in choosing names of the implementations   I think a more realistic example would be that some types of users need to be able to login to a particular API   We could define a Login interface  and then have a  User  parent class with SuperUser  DefaultUser  AdminUser  AdministrativeContact  etc suclasses  some of which will or won t implement the Login  Loginable   interface as necessary

User · Answer

Is there really a difference between   class User implements IUser   and  class UserImpl implements User   if all we re talking about is naming conventions   Personally I prefer NOT preceding the interface with I as I want to be coding to the interface and I consider that to be more important in terms of the naming convention  If you call the interface IUser then every consumer of that class needs to know its an IUser  If you call the class UserImpl then only the class and your DI container know about the Impl part and the consumers just know they re working with a User   Then again  the times I ve been forced to use Impl because a better name doesn t present itself have been few and far between because the implementation gets named according to the implementation because that s where it s important  e g   class DbBasedAccountDAO implements AccountDAO class InMemoryAccountDAO implements AccountDAO

[java] Interface naming in Java

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