Featherweight Java with Dynamic and Static Overloading

Abstract. We propose FMJ (Featherweight Multi Java), an extension of Featherweight Java with encapsulated multi-methods thus providing dynamic overloading. Multi-methods (collections of overloaded methods associated to the same message, whose selection takes place dynamically instead of statically as in standard overloading) are a useful and flexible mechanism which enhances re-usability and separation of responsibilities. However, many mainstream languages, such as, e.g., Java, do not provide it, resorting to only static overloading. The proposed extension is conservative and type safe: both "message-not-understood'' and "message-ambiguous'' are statically ruled out. Possible ambiguities are checked during type checking only on method invocation expressions, without requiring to inspect all the classes of a program. A static annotation with type information guarantees that in a well-typed program no ambiguity can raise at run-time. This annotation mechanism also permits modeling static overloading in a smooth way. Our core language can be used as the formal basis for an actual implementation of dynamic (and static) overloading in Java-like languages.