We define an algorithm for smarter recompilation of a small but significant Java-like language; such an algorithm is inspired by a type system previously defined by Ancona and Zucca. In comparison with all previous type systems for Java-like languages, this system enjoys the principal typings property, and is based on the two novel notions of local type assumption and entailment of type environments. The former allows the user to specify minimal requirements on the source fragments which need

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... be compiled in isolation, whereas the latter syntactically captures the concept of stronger type assumption. One of the most important practical advantages of this system is a better support for selective recompilation; indeed, it is possible to define an algorithm directly driven by the typing rules which is able to avoid the unnecessary recompilation steps which are usually performed by the Java compilers. The algorithm is smarter in the sense that it never forces useless recompilations, that is, recompilations which would generate the same binary fragment obtained from the previous compilation of the same source fragment. Finally, we show that the algorithm can actually speed up the overall recompilation process, since checking for recompilation is always strictly less expensive than recompiling the same fragment.