Object and Reference Immutability using Java Generics

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 Javas syntax by building upon Javas 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 Javas 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

Efficient Subtyping Tests with PQ-Encoding

Subtyping tests, i.e., determining whether one type is a subtype of another, are a frequent operation during the execution of objectoriented programs. The challenge is in encoding the hierarchy in a small space, while simultaneously making sure that subtyping tests have efficient implementation. We present a new scheme for encoding multiple and single inheritance hierarchies, which, in the standardized hierarchies, reduces the footprint of all previously published schemes. The scheme is called PQ-encoding after PQ-trees, a data structure previously used in graph theory for finding the orderings that satisfy a collection of constraints. In particular, we show that in the traditional object layout model, the extra memory requirements for single inheritance hierarchies is zero. In the PQ-encoding subtyping tests are constant time, and use only two comparisons. Other than PQ-trees, PQ-encoding uses several novel optimization techniques. These techniques are applicable also in improving the performance of other, previously published, encoding schemes

Fast Algorithm for Creating Space Efficient Dispatching Tables with Application to Multi-Dispatching

The dispatching problem can be solved very efficiently in the singleinheritance (SI) setting. In this paper we show how to extend one such solution to the multiple-inheritance (MI) setting. This generalization comes with an increase to the space requirement by a small factor of . This factor can be thought of as a metric of the complexity of the topology of the inheritance hierarchy

Incremental Algorithms for Dispatching in Dynamically Typed Languages

A fundamental problem in the implementation of object-oriented languages is that of a frugal dispatching data structure, i.e., support for quick response to dispatching queries combined with compact representation of the type hierarchy and the method families. Previous theoretical algorithms tend to be impractical due to their complexity and large hidden constant. In contrast, successful practical heuristics, including Vitek and Horspool's compact dispatch tables (CT) [16] designed for dynamically typed languages, lack theoretical support. In subjecting CT to theoretical analysis, we are not only able to improve and generalize it, but also provide the first non-trivial bounds on the performance of such a heuristic