title: Java Interface Naming Conventions
body: <p>I work on a Java web-app that uses Spring for dependency injection and JMock for mocking out these dependencies in our unit tests.</p>
  
  <p>Currently our team is at a point were we have a few different opinions in terms of how to name certain interfaces that we use. We have no issue with naming the interfaces in our domain that have multiple implementations, that is simple. However, when it comes to interfaces for which we only have one implementation and intend on only having one implementation in the future, we have hit a snag.</p>
  
  <p>The reason that we have such interfaces is purely for mocking, for example, we have services and repositories that we mock out in our unit tests and these services will be named "DocumentMappingService" or for repositories "EmployeeRepository". At the moment some of the guys just prefix the associated interface name with an "I", i.e. "IDocumentMappingService" and "IEmployeeRepository". Others name the interface as I have above and then append an "Impl" after the interface name for the implementing class.</p>
  
  <p>The third "faction" feels that both of these options are poor. Looking at literature such as the well-known "Growing object-oriented software, guided by tests" would lead one to believe that both of the before-mentioned options are poor and that the interface name should clearly define the contract and the implementing classes name should clearly specify how that contract has been implemented. We have found this quite difficult to do in the case I have mentioned above though.</p>
  
  <p>I was hoping that someone out there has had a similar issue before and has some suggestions ito which option is the best and why. Also, if you think that the "I" and "Impl" options are both poor, then please suggest a specific alternative convention.</p>