The Runabout

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.