An implementation of roles as affordances: powerJava

This document shortly describes powerJava, a Java extension which provides the instructions to manage roles. After defined the environment in which we have worked, we will discuss the language’s new instructions and we will show an example. I. SOMETHING ABOUT ROLES Object orientation is a leading paradigm in programming languages, modeling, knowledge representation and databases. When we think to an object, we do it in terms of attributes and methods, and if we refer to object interaction, we do it in terms of public attributes and public methods: these are the only ways to realize it! In computer science literature, other kinds of interaction between entities have been proposed at levels higher than programming languages. We can properly speak about sessions, which contain the interaction state (as at web services level), but a very important concept is that of role. Steimann [1] provided an interesting role representation, giving three types of “role as”: roles as named places, role as specialization and/or generalization, roles as adjunct instances. Our approach is to consider roles as affordances; we consider roles as instances having a different identity respect to the players that play them. Inspired by research in cognitive science, this view sees the properties (attributes and operations) of an object as something not independent from whom is interacting with it. In this way, an object “affords” different ways of interaction to different kinds of objects. The notion of “affordance” has been made popular by Norman [3] (p. 9): “The term affordance refers to the perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could possibly be used. A chair affords (‘is for’) support, and, therefore, affords sitting.” How can we use the concept of “affordance” to introduce new modeling concepts in object oriented knowledge representation? The affordances of an object are not isolated, but they are associated with a given specie. So we have to consider sets of affordances. We will call a role type the different sets of interaction possibilities, the affordances of an object, which depend on the class of the interactant manipulating the object: the player of the role. To manipulate an object it is necessary to specify the role in which the interaction is made. A given role type can be instantiated, depending on a certain player of a role (which must have the required properties), and the role instance represents the state of the interaction with that role player. Just to better explain the possible use of roles as affordances, we introduce the following figures, which introduce different ways to interact with an object through the roles it offers. Figure 1 An object interacts with another one by means of the role