Taming wildcards in Java's type system

Abstract

Wildcards have become an important part of Java's type system since their introduction 7 years ago. Yet there are still many open problems with Java's wildcards. For example, there are no known sound and complete algorithms for subtyping (and consequently type checking) Java wildcards, and in fact subtyping is suspected to be undecidable because wildcards are a form of bounded existential types. Furthermore, some Java types with wildcards have no joins, making inference of type arguments for generic methods particularly difficult. Although there has been progress on these fronts, we have identified significant shortcomings of the current state of the art, along with new problems that have not been addressed.