Abstract

Several recent studies have introduced lightweight ver-sions of Java: reduced languages in which complex fea-trues like threads and reflection are dropped to enable rigorous arguments about key properties such as type safety. We carry this process a step further, omitting almost all features of the full language (including inter-faces and even assignment) to obtain a small calculus, Featherweight Java, for which rigorous proofs are not only possible but easy. Featherweight Java bears a similar relation to full Java as the lambda-calculus does to languages such as ML and Haskell. It offers a similar computational “feel, ” providing classes, methods, fields, inheritance, and dynamic typecasts, with a semantics closely follow-ing Java’s. A proof of type safety for Featherweight Java thus illustrates many of the interesting features of a safety proof for the full language, while remaining pleasingly compact. The syntax, type rules, and opera-tional semantics of Featherweight Java fit on one page, making it easier to understand the consequences of ex-tensions and variations. As an illustration of its utility in this regard, we extend Featherweight Java with generic classes in the style of GJ (Bracha, Odersky, Stoutamire, and Wadler) and sketch a proof of type safety. The extended system formalizes for the first time some of the key features of GJ. Subject areas: theoretical foundations, language de-sign and implementation. 1

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