Type Inference for Java 5 Wildcards, F-Bounds, and Undecidability

We consider the problem of type checking for Java 5 with generics and wildcard types. Java supports type inference for methods with type parameters, but its presentation in the Java Language Specification lacks both clarity and a sense for how inference fits into the larger picture of Java’s type system. We show, in contrast, how this type inference can be cleanly integrated with subtyping of wildcard types, justifying the intuitions of programmers and logicians alike. In addition, we examine the decidability of Java typechecking and identify key points in the subtyping relation that both lead to undecidability and suggest a decidable conservative approximation.