MultiJava: Modular Open Classes and Symmetric Multiple Dispatch for Java by Curtis Clifton, Gary T. Leavens, Craig Chambers, and Todd Millstein Abstract We present MultiJava, a backward-compatible extension to Java supporting open classes and symmetric multiple dispatch. Open classes allow one to add to the set of methods that an existing class supports without creating distinct subclasses or editing existing code. Unlike the "Visitor" design pattern, open classes do not require advance planning, and open classes preserve the ability to add new subclasses modularly and safely. Multiple dispatch offers several well-known advantages over the single dispatching of conventional object-oriented languages, including a simple solution to some kinds of "binary method" problems. MultiJava's multiple dispatch retains Java's existing class-based encapsulation properties. We adapt previous theoretical work to allow compilation units to be statically typechecked modularly and safely, ruling out any link-time or run-time type errors. We also present a novel compilation scheme that operates modularly and incurs performance overhead only where open classes or multiple dispatching are actually used. Keywords: Open classes, open objects, extensible classes, extensible external methods, external methods, multimethods, generic functions, object-oriented programming languages, single dispatch, multiple dispatch, encapsulation, modularity, static typechecking, subtyping, inheritance, Java language, MultiJava language, separate compilation. 1999 CR Categories: D.1.5 [Programming Techniques] Object-oriented programming; D.3.2 [Programming Languages] Language Classifications --- object-oriented languages; D.3.3 [Programming Languages] Language Constructs and Features --- abstract data types, classes and objects, control structures, inheritance, modules, packages, patterns, procedures, functions, and subroutines; D.3.4 [Programming Languages] Processors --- compilers; D.3.m [Programming Languages] Miscellaneous --- open classes, multimethods, generic functions. Copyright (c) ACM, 2000. To appear in OOPSLA 2000. Permission to make digital or hard copies of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee.