Class Family Accord - Abstract

An Accord class family is a hierarchy of classes for which another class hierarchy has been designated as corresponding. Class  A0 is the top of the class family, class A1 extends A0, class An extends An-1.

The partner of this class family is B0 at the top, Bn extends Bn-1. Class family A and B are have an Accord relationship if, by design intention, An corresponds to Bn. In each level, there is at least one method that overrides or defines a variant with a behavior representative of the progression of requirements.

The intention of this design concept is to maintain this correspondence when, as requirements evolve, the design calls for extending An and Bn into An+1 and Bn+1. The reason for maintaining this relationship would be that A has new or refined behaviors that only make sense with reference to the state or behaviors of B at the same level.

To realize this relationship in the Java programming language, a designer could simply document the intention. However, coding would inevitably require explicit down cast to force references to the intended levels. This white paper suggests a set of Java annotations to make the Accord relationship between class families explicit and generate the necessary dispatch code and casts. The resulting generated code would in effect provide a parametric override capability.

At a minimum an annotation @Accord  designates a class as the head or subclass in a class family. Its attribute has an attribute, partner, to identify the other class family. Methods that are intended to follow the progression are annotated as @Covariant. The effect is to make the method be a covariant override. Its parameter referring to a class at the same inheritance level in the partner family is treated a covariant. A prototype precompiler is (to be) provided for research purposes.