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On Subtyping, Wildcards, and Existential Types

Nicholas Cameron, Sophia Drossopoulou

Conference or Workshop Paper
Formal Techniques for Java Programs
July, 2009
ACM Digital Library

Wildcards are an often confusing part of the Java type system: the behaviour of wildcard types is not fully specified bysubtyping, due to wildcard capture, and the rules for type

checking are often misunderstood. Their very formulation seems somehow `different' from the rest of the Java type system, which is based on a simple, nominal hierarchy.

We investigate subtyping in models for Java with and without generics and wildcards. We separate subclassing from subtyping, unify subtyping for class and wildcard types

using existential types, and show that Java wildcards emerge naturally from the combination of inclusion and parametric polymorphism.

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