Java Interfaces Implementation naming convention

Question

How do you name different classes   interfaces you create  Sometimes I don t have implementation information to add to the implementation name - like interface FileHandler and class SqlFileHandler   When this happens I usually name the interface in the  normal  name  like Truck and name the actual class TruckClass    How do you name interfaces and classes in this regard

User · Answer

I tend to follow the pseudo-conventions established by Java Core/Sun, e.g. in the Collections classes:

List - interface for the "conceptual" object
ArrayList - concrete implementation of interface
LinkedList - concrete implementation of interface
AbstractList - abstract "partial" implementation to assist custom implementations

I used to do the same thing modeling my event classes after the AWT Event/Listener/Adapter paradigm.

User · Answer

I use both conventions   If the interface is a specific instance of a a well known pattern  e g  Service  DAO   then it may not need an  I   e g UserService  AuditService  UserDao  all work fine without the  I   because the post-fix determines the meta pattern   But  if you have something one-off or two-off  usually for a callback pattern   then it helps to distinguish it from a class  e g  IAsynchCallbackHandler  IUpdateListener  IComputeDrone    These are special purpose interfaces designed for internal use  occasionally the IInterface calls out attention to the fact that an operand is actually an interface  so at first glance it is immediately clear   In other cases you can use the I to avoid colliding with other commonly known concrete classes  ISubject  IPrincipal vs Subject or Principal

User · Answer

I ve seen answers here that suggest that if you only have one implementation then you don t need an interface   This flies in the face of the Depencency Injection Inversion of Control principle  don t call us  we ll call you       So yes  there are situations in which you wish to simplify your code and make it easily testable by relying on injected interface implementations  which may also be proxied - your code doesn t know     Even if you only have two implementations - one a Mock for testing  and one that gets injected into the actual production code - this doesn t make having an interface superfluous   A well documented interface establishes a contract  which can also be maintained by a strict mock implementation for testing     in fact  you can establish tests that have mocks implement the most strict interface contract  throwing exceptions for arguments that shouldn t be null  etc  and catch errors in testing  using a more efficient implementation in production code  not checking arguments that should not be null for being null since the mock threw exceptions in your tests and you know that the arguments aren t null due to fixing the code after these tests  for example    Dependency Injection IOC can be hard to grasp for a newcomer  but once you understand its potential you ll want to use it all over the place and you ll find yourself making interfaces all the time - even if there will only be one  actual production  implementation   For this one implementation  you can infer  and you d be correct  that I believe the mocks for testing should be called Mock InterfaceName    I prefer the name Default InterfaceName    If a more specific implementation comes along  it can be named appropriately   This also avoids the Impl suffix that I particularly dislike  if it s not an abstract class  OF COURSE it is an  impl      I also prefer  Base InterfaceName   as opposed to  Abstract InterfaceName   because there are some situations in which you want your base class to become instantiable later  but now you re stuck with the name  Abstract InterfaceName    and this forces you to rename the class  possibly causing a little minor confusion - but if it was always Base InterfaceName   removing the abstract modifier doesn t change what the class was

User · Answer

TruckClass sounds like it were a class of Truck  I think that recommended solution is to add Impl suffix  In my opinion the best solution is to contain within implementation name some information  what s going on in that particular implementation  like we have with List interface and implementations  ArrayList or LinkedList   but sometimes you have just one implementation and have to have interface due to remote usage  for example   then   as mentioned at the beginning  Impl is the solution

User · Answer

I like interface names that indicate what contract an interface describes  such as  Comparable  or  Serializable   Nouns like  Truck  don t really describe truck-ness -- what are the Abilities of a truck   Regarding conventions  I have worked on projects where every interface starts with an  I   while this is somewhat alien to Java conventions  it makes finding interfaces very easy  Apart from that  the  Impl  suffix is a reasonable default name

User · Answer

Some people don t like this  and it s more of a  NET convention than Java  but you can name your interfaces with a capital I prefix  for example   IProductRepository - interface ProductRepository  SqlProductRepository  etc  - implementations   The people opposed to this naming convention might argue that you shouldn t care whether you re working with an interface or an object in your code  but I find it easier to read and understand on-the-fly   I wouldn t name the implementation class with a  Class  suffix   That may lead to confusion  because you can actually work with  class   i e  Type  objects in your code  but in your case  you re not working with the class object  you re just working with a plain-old object

User · Answer

Name your Interface what it is  Truck  Not ITruck because it isn t an ITruck it is a Truck    An Interface in Java is a Type  Then you have DumpTruck  TransferTruck  WreckerTruck  CementTruck  etc that implement Truck    When you are using the Interface in place of a sub-class you just cast it to Truck  As in List lt Truck gt   Putting I in front is just Hungarian style notation tautology that adds nothing but more stuff to type to your code     All modern Java IDE s mark Interfaces and Implementations and what not without this silly notation  Don t call it TruckClass that is tautology just as bad as the IInterface tautology     If it is an implementation it is a class  The only real exception to this rule  and there are always exceptions  could be something like AbstractTruck  Since only the sub-classes will ever see this and you should never cast to an Abstract class it does add some information that the class is abstract and to how it should be used  You could still come up with a better name than AbstractTruck and use BaseTruck or DefaultTruck instead since the abstract is in the definition  But since Abstract classes should never be part of any public facing interface I believe it is an acceptable exception to the rule  Making the constructors protected goes a long way to crossing this divide   And the Impl suffix is just more noise as well  More tautology  Anything that isn t an interface is an implementation  even abstract classes which are partial implementations  Are you going to put that silly Impl suffix on every name of every Class     The Interface is a contract on what the public methods and properties have to support  it is also Type information as well  Everything that implements Truck is a Type of Truck     Look to the Java standard library itself  Do you see IList  ArrayListImpl  LinkedListImpl  No  you see List and ArrayList  and LinkedList  Here is a nice article about this exact question  Any of these silly prefix suffix naming conventions all violate the DRY principle as well   Also  if you find yourself adding DTO  JDO  BEAN or other silly repetitive suffixes to objects then they probably belong in a package instead of all those suffixes  Properly packaged namespaces are self documenting and reduce all the useless redundant information in these really poorly conceived proprietary naming schemes that most places don t even internally adhere to in a consistent manner    If all you can come up with to make your Class name unique is suffixing it with Impl  then you need to rethink having an Interface at all  So when you have a situation where you have an Interface and a single Implementation that is not uniquely specialized from the Interface you probably don t need the Interface

User · Answer

The standard C  convention  which works well enough in Java too  is to prefix all interfaces with an I - so your file handler interface will be IFileHandler and your truck interface will be ITruck  It s consistent  and makes it easy to tell interfaces from classes

User · Answer

The name of the interface should describe the abstract concept the interface represents  Any implementation class should have some sort of specific traits that can be used to give it a more specific name   If there is only one implementation class and you can t think of anything that makes it specific  implied by wanting to name it -Impl   then it looks like there is no justification to have an interface at all

[java] Java Interfaces/Implementation naming convention

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