Kayak: Safe Semantic Refactoring to Java Streams

Refactorings are structured changes to existing software that leave its externally observable behaviour unchanged. Their intent is to improve readability, performance or other non-behavioural properties. State-of-the-art automatic refactoring tools are syntax-driven and, therefore, overly conservative. In this paper we explore semantics-driven refactoring, which enables much more sophisticated refactoring schemata. As an exemplar of this broader idea, we present Kayak, an automatic refactoring tool that transforms Java with external iteration over collections into code that uses Streams, a new abstraction introduced by Java 8. Our refactoring procedure performs semantic reasoning and search in the space of possible refactorings using automated program synthesis. Our experimental results support the conjecture that semantics-driven refactorings are more precise and are able to rewrite more complex code scenarios when compared to syntax-driven refactorings.