The Expression Problem Revisited Four New Solutions Using Generics

Abstract

The expression problem (aka the extensibility problem) refers to a fundamental dilemma of programming: To which degree can your application be structured in such a way that both the data model and the set of virtual operations over it can be extended without the need to modify existing code, without the need for code repetition and without runtime type errors. Over the years, many approaches to this problem have been proposed, each having different characteristics of type safety and reusability. While many of these rely on exotic or problem specific language extensions, this paper investigates the solution space within the framework of the soonto-be mainstream generic extensions of C# and the Java programming language. Four new solutions are presented which, though quite different, all rely on techniques that can be used in everyday programming. © Springer-Verlag 2004.