Static detection of brittle parameter typing

To avoid receiving incorrect arguments, a method specifies the expected type of each formal parameter. However, some parameter types are too general and have subtypes that the method does not expect as actual argument types. For example, this may happen if there is no common supertype that precisely describes all expected types. As a result of such brittle parameter typing, a caller may accidentally pass arguments unexpected by the callee without any warnings from the type system. This paper presents a fully automatic, static analysis to find brittle parameter typing and unexpected arguments given to brittle parameters. First, the analysis infers from callers of a method the types that arguments commonly have. Then, the analysis reports potentially unexpected arguments that stand out by having an unusual type. We apply the approach to 21 real-world Java programs that use the Swing API, an API providing various methods with brittle parameters. The analysis reveals 15 previously unknown bugs and code smells where programmers pass arguments that are compatible with the declared parameter type but nevertheless unexpected by the callee. The warnings reported by the analysis have 47% precision and 83% recall.