The implications of method placement on API learnability

To better understand what makes Application Programming Interfaces (APIs) hard to use and how to improve them, recent research has begun studying programmers' strategies and use of APIs. It was found that method placement --- on which class or classes a method is placed --- can have large usability impact in object-oriented APIs. This was because programmers often start their exploration of an API from one "main" object, and were slower finding other objects that were not referenced in the methods of the main object. For example, while mailServer.send(mailMessage) might make sense, if programmers often begin their API explorations from the MailMessage class, then this makes it harder to find the MailServer class than the alternative mailMessage.send(mailServer). This is interesting because many real APIs place methods essential to common objects on other, helper objects. Alternate versions of three different APIs were compared, and it was found that programmers gravitated toward the same starting classes and were dramatically faster --- between 2 to 11 times --- combining multiple objects when a method on the starting class referred to the other class.