TamingWildcardsinJava’sTypeSystem-CornellUniversity.PDFVIP

Taming Wildcards in Java’s Type System Ross Tate Alan Leung Sorin Lerner University of California, San Diego University of California, San Diego University of California, San Diego Abstract Java generics were a long-awaited improvement to Java and Wildcards have become an important part of Java’s type system have been tremendously useful, leading to a significant reduction since their introduction 7 years ago. Yet there are still many open in the amount of unsafe type casts found in Java code. However, problems with Java’s wildcards. For example, there are no known while Java generics improved the language, they also made type sound and complete algorithms for subtyping (and consequently checking extremely complex. In particular, Java generics came type checking) Java wildcards, and in fact subtyping is suspected with wildcards, a sophisticated type feature designed to address the to be undecidable because wildcards are a form of bounded exis- limitations of plain parametric polymorphism [18]. tential types. Furthermore, some Java types with wildcards have no Wildcards are a simple form of existential types. For exam- joins, making inference of type arguments for generic methods par- ple, represents a “list of unknowns”, namely a list of ob- ticularly difficult. Although there has been progress on these fronts, jects, all of which have the same unknown static