The Java language lacks the important notions of ownership (an object owns its representation to prevent unwanted aliasing) and immutability (the division into mutable, immutable, and readonly data and references). Programmers are prone to design errors, such as representation exposure or violation of immutability contracts. This paper presents Ownership Immutability Generic Java (OIGJ), a backward-compatible purely-static language extension supporting ownership and immutability. We formally defined a core calculus for OIGJ, based on Featherweight Java, and proved it sound. We also implemented OIGJ and performed case studies on 33,000 lines of code.

Are you a practitioner who is tired of null pointer exceptions, unintended side effects, SQL injections, concurrency errors, mistaken equality tests, and other run-time errors that appear during testing or in the field? A pluggable type system can guarantee the absence of these errors, and many more.

This paper introduces the Checker Framework, which supports adding pluggable type systems to the Java language in a backward-compatible way. A type system designer defines type qualifiers and their semantics, and a compiler plug-in enforces the semantics. Programmers can write the type qualifiers in their programs and use the plug-in to detect or prevent errors. The Checker Framework is useful both to programmers who wish to write error-free code, and to type system designers who wish to evaluate and deploy their type systems.

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.