Object and Reference Immutability using Java Generics
Author(s)
Zibin, Yoav
Potanin, Alex
Artzi, Shay
Kiezun, Adam
Ernst, Michael D.
Advisor(s)
Michael Ernst
Date Issued
March 16, 2007
Series/Report no.
Massachusetts Institute of Technology Computer Science and Artificial Intelligence Laboratory
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 generic 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, and its reference immutability is more expressive than previous work. IGJ also permits covariant changes of generic arguments 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.
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