Controlling Visibility of Class Extensions

Unanticipated changes to complex software systems can§introduce §anomalies such as duplicated code, suboptimal§inheritance §relationships and a proliferation of run-time§downcasts. Refactoring §to eliminate these anomalies may not be an option, at§least in §certain stages of software evolution.§§A class extension is a method that is defined in a§module, but §whose class is defined elsewhere. Class extensions§offer a §convenient way to incrementally modify existing§classes when §subclassing is inappropriate. Unfortunately existing§approaches §suffer from various limitations. Either class§extensions have a global §impact or they have a purely local impact, with§negative results for §collaborating clients. Furthermore, conflicting class§extensions are §either disallowed, or resolved by linearization, with§subsequent §negative effects.§§To solve these problems we present classboxes, a§module system §for object-oriented languages that provides for§behavior refinement §(i.e., method addition and replacement). Moreover,§the changes §made by a classbox are only visible to that classbox§(or classboxes §that import it), a feature we call local rebinding.