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. 1.

Abstract. This paper presents a variation of the visitor pattern which allows programmers to write visitor-like code in a concise way. The Runabout is a library extension that adds a limited form of multi-dispatch to Java. While the Runabout is not as expressive as a general multiple dispatching facility, the Runabout can be significantly faster than existing implementations of multiple dispatch for Java, such as MultiJava. Unlike MultiJava, the Runabout does not require changes to the syntax and the compiler. This paper illustrates how to use the Runabout, details its implementation, and provides benchmarks comparing its performance with other approaches. Furthermore, the effect of an automatic static program transformation tool that translates bytecode using the Runabout to equivalent bytecode is evaluated. The tool uses double dispatch and runtime type checks to achieve the same semantics that the Runabout has. The performance comparisons on large benchmarks that make extensive use multiple dispatch show that using the Runabout does not result in a significant loss of performance for realistic applications and that, depending on the application and platform, small performance gains are also possible. 1