The naming of interfaces and corresponding implementation classes in languages that use them (of which Java and C# is the most used I guess) is a subject of sometimes heated debate. A growing distaste of the "best practice" (at least on Microsofts part) of using a capital "I" on interfaces prompted this rundown and a round of pro et contra. My ultimate goal is readability of code but a slight predicament emerges when considering a team: if the majority of team members are used to one particular convention is that, in that particular context, more readable for that team than another approach, all other things being equal? Even though the name of the interface reads more like natural English? The jury's still out on this one.

The different patterns I’ve found:

  1. Interfaces are prefixed with a capital I, where IReportGenerator is the interface and e.g. ReportGenerator is the implementation. Some argue for making the interface name read "I generate reports" and keep the I prefix, thus: IGenerateReports instead.
  2. Ditching the capital I and appending "imp" or "impl" to the implementation. Like so: UserManager and UserManagerImpl. Some use uppercase or lowercase "c" as prefix for concrete classes too but I find that horribly ugly, to be frank.
  3. Naming an interface for it's role and naming the implementing class(es) for what distinguishes it from (possibly) other implementations. E.g. UserManager for the interface and DatabaseUserManager or LDAPUserManager for the concrete implementations.
  4. Naming abstract class with the word "Abstract" as a prefix to make a distinction for abstract and concrete classes. Possibly combined with other conventions for interfaces as well. I would argue the context often reveals an abstract class to the reader.

C# vs Java

In C# the extension of an abstract base class and the implementation of an interface uses the same syntax and which one it is isn't always easy to discern. A naming convention makes this a bit easier:

public class DatabaseUserManager : UserManager {



public class DatabaseUserManager : IUserManager {


In java the syntax is different:

public class LDAPUserManager implements UserManager {



public class LDAPUserManager extends UserManager {


The need for a prefix for readability is less needed in Java, compared to C#.

Good names trumps conventions?

Clean Code by Robert C. Martin argues for good names: good names for variables, good names for methods and good names for interfaces and classes. The name should reveal intention. Choosing good names takes time but saves more than it takes. (Clean Code, page 18). The book also mentions not using the "I" prefix and prefers to encode the implementation, if at all.

The question is: if the names are as good as they can be, do we need a prefix or suffix or other kinds of encoding to indicate the type? In my mind we don't. I find that the I hurts readability and the "impl" certainly doesn't look good to me. So how to choose good names?

  1. Classes should have noun names or noun phrases, e.g. Customer, Policy or StreetAddress.
  2. Use name from the problem domain. E.g. If you're dealing with a customer or client object. In a medical system, should it be called Patient? Or if it's dealing with social benefits maybe BenefitsReceiver is a better name? The domain influences the choice of names. In stock-broking perhaps the name FuturesHolder is name candidate for a special kind of customer? (note: I'm not very familiar with either of these domains so the examples may be somewhat off. You get the point: know your domain and find good names from it.).
  3. Use names from the solution domain. If you use e.g. the Visitor pattern, make it a part of the name(s) so other programmers see it InsuranceCustomerVisitor is better than CustomerTraverser
  4. </ol>

    So unless some team coding standard absolutely makes me (and that is, I'm afraid, likely) I will not in the future prefix my interfaces with I and not adorn my implementations with c, impl or any other stuff. Just meaningful, names from the domain. Easier to write and easier to read.

    Lastly, I posted this question on twitter and @borud made the comment: "the only useful information conveyed by an I-prefix to interface types is information about the author of the code." That's a suitable end note I think :-)

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16 April 2010