Encapsulation of primitive functions, together with the data upon which they act and to which they belong, is fundamental to object orientation. The trick is to get the right functionality encapsulated with the right data. If this is achieved then the network of objects, which collaborate to provide the required functionality of the software, will have the minimum possible essential connections between objects and maximum functional cohesion of the objects. From this maximal functional cohesion can be derived maximal maintainability, reuse and extension. To achieve this we need two things. Firstly, the right objects. Secondly, to set the interface to the objects in concrete. If a change to an object requires a change to the interface to the object, then the effect of that change will escape beyond the boundaries of the object and propagate around the network. This is to allow the whole reason for encapsulation to be negated in one go. Proper Analysis and Design defines the right objects for the system and their interfaces to the point where each object can be given to a programmer and coded independently of any other object in the system. If this is done well then the resulting objects will still be around in future versions of the software, and, furthermore, their interfaces will not have changed except for additions. We aim for maximum maintainability, re-use and extension.
