We introduce JPred, a backward-compatible extension to Java supporting predicate dispatch. While prior languages with predicate dispatch have been extensions to toy or non-mainstream languages, we show how predicate dispatch can be naturally added to a traditional OO language. While prior languages with predicate dispatch have required the whole program to be available for typechecking and compilation, JPred retains Java's modular typechecking and compilation strategies. While prior languages with predicate dispatch have included special-purpose algorithms for reasoning about predicates, JPred employs general-purpose, off-the-shelf decision procedures. As a result, JPred's type system is more flexible, allowing several useful programming idioms that are spuriously rejected by those other languages. After describing the JPred language and type system, we present a case study illustrating the utility of JPred in a real-world application, including its use in the detection of several errors.
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