CW 407

Marko van Dooren, Eric Steegmans
Combining the robustness of checked exceptions with the flexibility of unchecked exceptions using anchored exception declarations

Abstract

Ever since their invention 30 years ago, checked exceptions have been a point of much discussion. On the one hand, they increase the robustness of software by preventing the manifestation of unanticipated checked exceptions at run-time. On the other hand, they decrease the adaptability of software because they must be propagated explicitly, and must often be handled even if they cannot be signalled. We show that these problems are caused by a conflict between the exceptional interface of a method and the principle of abstraction. We then solve this conflict by introducing anchored exception declarations, which allow the exceptional behaviour of a method to be declared relative to that of others. We present their formal semantics, along with the necessary rules for ensuring compile-time safety , and give a proof of correctness. We show that anchored exception declarations do not violate the principle of information hiding when used properly, and provide a guideline for when to use them. We have implemented anchored exception declarations as an extension to the ClassicJava programming language, called Cappuccino.