Object and reference immutability using Java generics

  • Authors:

  • Yoav Zibin;Alex Potanin;Mahmood Ali;Shay Artzi;Adam Kie|un;Michael D. Ernst

  • Affiliations:

  • Massachusetts Institute of Technology, Cambridge, MA;Victoria University of Wellington, Wellington, New Zealand;Massachusetts Institute of Technology, Cambridge, MA;Massachusetts Institute of Technology, Cambridge, MA;Massachusetts Institute of Technology, Cambridge, MA;Massachusetts Institute of Technology, Cambridge, MA

  • Venue:
  • Proceedings of the the 6th joint meeting of the European software engineering conference and the ACM SIGSOFT symposium on The foundations of software engineering
  • Year:
  • 2007

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Abstract

A compiler-checked immutability guarantee provides useful documentation, facilitates reasoning, and enables optimizations. This paper presents Immutability Generic Java (IGJ), a novel language extension that expresses immutability without changing Java's syntax by building upon Java's generics and annotation mechanisms. In IGJ, each class has one additional type parameter that is Immutable, Mutable, or ReadOnly. IGJ guarantees both reference immutability (only mutable references can mutate an object) and object immutability (an immutable reference points to an immutable object). IGJ is the first proposal for enforcing object immutability within Java's syntax and type system, and its reference immutability is more expressive than previous work. IGJ also permits covariant changes of type parameters in a type-safe manner, e.g., a readonly list of integers is a subtype of a readonly list of numbers. IGJ extends Java's type system with a few simple rules. We formalize this type system and prove it sound. Our IGJ compiler works by type-erasure and generates byte-code that can be executed on any JVM without runtime penalty.