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A Variation on Java Wildcards - Trading Expressiveness for Global Type Inference

Authors Andreas Stadelmeier , Martin Plümicke , Peter Thiemann

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Subject Classification

ACM Subject Classification
  • Software and its engineering → Syntax
Keywords
  • type inference
  • Java
  • subtyping
  • wildcards
  • capture conversion

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Abstract

In standard Java, wildcards behave like existential types: they must be opened before use in a method invocation, a process the compiler performs implicitly via capture conversion. We present Java-TX, a dialect of Java that sidesteps this existential encoding and treats wildcards as placeholders for unknown types, used directly without prior opening. This choice simplifies the type system and makes it compatible with an existing global type inference algorithm. The trade-off is that some method calls valid in standard Java become unavailable in Java-TX. In effect, we trade some of Java’s wildcard expressiveness for global type inference.
We explore the metatheory of Java-TX through Featherweight Java-TX (FJ-TX), a functional core calculus for Java-TX that extends Featherweight Generic Java with our wildcard interpretation. We prove type soundness for FJ-TX. Finally, we evaluate the practical impact of omitting capture conversion by conducting a study on open-source Java projects by calculating an underapproximation of how much existing Java code is compatible with the Java-TX type system.

Cite As Get BibTex

Andreas Stadelmeier, Martin Plümicke, and Peter Thiemann. A Variation on Java Wildcards - Trading Expressiveness for Global Type Inference. In 40th European Conference on Object-Oriented Programming (ECOOP 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 372, pp. 26:1-26:27, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026) https://doi.org/10.4230/LIPIcs.ECOOP.2026.26

Author Details

Andreas Stadelmeier
  • DHBW Stuttgart, Campus Horb, Germany
Martin Plümicke
  • DHBW Stuttgart, Campus Horb, Germany
Peter Thiemann
  • Institut für Informatik, Universität Freiburg, Germany

References

  1. John Altidor, Shan Shan Huang, and Yannis Smaragdakis. Taming the wildcards: combining definition- and use-site variance. In Mary W. Hall and David A. Padua, editors, Proceedings of the 32nd ACM SIGPLAN Conference on Programming Language Design and Implementation, PLDI 2011, San Jose, CA, USA, June 4-8, 2011, pages 602-613. ACM, 2011. URL: https://doi.org/10.1145/1993498.1993569.
  2. Nada Amin and Ross Tate. Java and Scala’s type systems are unsound: the existential crisis of null pointers. In Eelco Visser and Yannis Smaragdakis, editors, OOPSLA 2016, pages 838-848. ACM, 2016. URL: https://doi.org/10.1145/2983990.2984004.
  3. Kevin Bierhoff. Wildcards need witness protection. Proc. ACM Program. Lang., 6(OOPSLA2), 2022. URL: https://doi.org/10.1145/3563301.
  4. Nicholas Cameron, Sophia Drossopoulou, and Erik Ernst. A model for Java with wildcards. ECOOP 2008 - Object-Oriented Programming, 22nd European Conference, Paphos, Cyprus, July 7-11, 2008, Proceedings, 5142:2-26, 2008. URL: https://doi.org/10.1007/978-3-540-70592-5_2.
  5. Robert Dyer, Hridesh Rajan, Hoan Anh Nguyen, and Tien N. Nguyen. Mining billions of AST nodes to study actual and potential usage of Java language features. In Proceedings of the 36th International Conference on Software Engineering, ICSE 2014, Hyderabad, India, May 31 - June 7, 2014, pages 779-790. ACM, 2014. URL: https://doi.org/10.1145/2568225.2568295.
  6. James Gosling, Bill Joy, Guy Steele, Gilad Bracha, Alex Buckley, and Daniel Smith. The Java® Language Specification. Addison-Wesley, Java SE 21 edition, 2023. URL: https://docs.oracle.com/javase/specs/jls/se21/jls21.pdf.
  7. Atsushi Igarashi, Benjamin Pierce, and Philip Wadler. Featherweight Java: A minimal core calculus for Java and GJ. Proceedings of the ACM SIGPLAN Conference OOPSLA, 1999. Google Scholar
  8. Atsushi Igarashi, Benjamin C. Pierce, and Philip Wadler. Featherweight Java: a minimal core calculus for Java and GJ. ACM Transactions on Programming Languages and Systems (TOPLAS), 23(3):396-450, 2001. URL: https://doi.org/10.1145/503502.503505.
  9. Ameya Ketkar, Nikolaos Tsantalis, and Danny Dig. Understanding type changes in Java. In Proceedings of the 28th ACM Joint Meeting on European Software Engineering Conference and Symposium on the Foundations of Software Engineering, ESEC/FSE 2020, pages 629-641, New York, NY, USA, 2020. Association for Computing Machinery. URL: https://doi.org/10.1145/3368089.3409725.
  10. Ruben Kraft and Martin Plümicke. Ein Language-Server für Java-TX. In Stefan Brunthaler, editor, 23. Kolloquium Programmiersprachen und Grundlagen der Programmierung - Vorläufiger Tagungsband. Universität der Bundeswehr München, September 2025. (in german). Google Scholar
  11. Stas Negara, Mihai Codoban, Danny Dig, and Ralph E. Johnson. Mining fine-grained code changes to detect unknown change patterns. In Proceedings of the 36th International Conference on Software Engineering, ICSE 2014, pages 803-813, New York, NY, USA, 2014. Association for Computing Machinery. URL: https://doi.org/10.1145/2568225.2568317.
  12. Martin Pluemicke. Completing the functional approach in object-oriented languages. In A Second Soul: Celebrating the Many Languages of Programming - Festschrift in Honor of Peter Thiemann’s Sixtieth Birthday, Freiburg, Germany, 30th August 2024, volume 413 of Electronic Proceedings in Theoretical Computer Science, pages 43-56. Open Publishing Association, 2024. URL: https://doi.org/10.4204/EPTCS.413.4.
  13. Martin Plümicke. Java type unification with wildcards. In Applications of Declarative Programming and Knowledge Management, 17th International Conference, INAP 2007, and 21st Workshop on Logic Programming, WLP 2007, Würzburg, Germany, October 4-6, 2007, Revised Selected Papers, volume 5437 of Lecture Notes in Computer Science, pages 223-240. Springer, 2007. URL: https://doi.org/10.1007/978-3-642-00675-3_15.
  14. Martin Plümicke. Optimization of the Java Type Unification. In Sibylle Schwarz and Mario Wenzel, editors, Proceedings of the 37th Workshop on (Constraint) Logic Programming (WLP 2023), 2023. URL: https://dbs.informatik.uni-halle.de/wlp2023/WLP2023_Pl%C3%BCmicke_Optimization%20of%20the%20Java%20Type%20Unification.pdf.
  15. Martin Plümicke. Featherweight-Java-TX: A Minimal Core Calculus for Java-TX (FJ-TX). In Daniel Holle, Jens Knoop, Martin Plümicke, Peter Thiemann, and Baltasar Trancón y Widemann, editors, 40. Workshop der GI-Fachgruppe "Programmiersprachen und Rechenkonzepte", number 02/2024 in INSIGHTS - Schriftenreihe der Fakultät Technik, pages 93-101, Bad Honnef, Germany, April 2024. URL: https://www.dhbw-stuttgart.de/forschung-transfer/technik/schriftenreihe-insights.
  16. Martin Plümicke and Andreas Stadelmeier. Introducing Scala-like function types into Java-TX. In Proceedings of the 14th International Conference on Managed Languages and Runtimes, ManLang 2017, pages 23-34, New York, NY, USA, 2017. ACM. URL: https://doi.org/10.1145/3132190.3132203.
  17. Andreas Stadelmeier, Martin Plümicke, and Peter Thiemann. Global Type Inference for Featherweight Generic Java. 36th European Conference on Object-Oriented Programming (ECOOP 2022), 222:28:1-28:27, 2022. URL: https://doi.org/10.4230/LIPIcs.ECOOP.2022.28.
  18. Mads Torgersen, Erik Ernst, and Christian Plesner Hansen. Wild FJ. In Philip Wadler, editor, Proceedings of FOOL 12, Long Beach, California, USA, 2012. ACM, School of Informatics, University of Edinburgh. URL: http://homepages.inf.ed.ac.uk/wadler/fool/.
  19. Mads Torgersen, Erik Ernst, Christian Plesner Hansen, Peter von der Ahé, Gilad Bracha, and Neal Gafter. Adding wildcards to the Java programming language. Journal of Object Technology, 3(11):97-116, December 2004. URL: https://doi.org/10.5381/jot.2004.3.11.a5.
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