Featherweight Jigsaw: A Minimal Core Calculus for Modular Composition of Classes

Abstract

Abstract. We present FJig, a simple calculus where basic building blocks are classes in the style of Featherweight Java, declaring elds, methods and one constructor. However, inheritance has been generalized to the much more exible notion originally proposed in Bracha's Jigsaw framework. That is, classes play also the role of modules, that can be composed by a rich set of operators, all of which can be expressed by a minimal core. We keep the nominal approach of Java-like languages, that is, types are class names. However, a class is not necessarily a structural subtype of any class used in its dening expression. The calculus allows the encoding of a large variety of dierent mechanisms for software composition in class-based languages, including standard inheritance, mixin classes, traits and hiding. Hence, FJig can be used as a unifying framework for analyzing existing mechanisms and proposing new extensions.We provide two dierent semantics of an FJig program: attening and direct semantics. The dierence is analogous to that between two intuitive models to understand inheritance: the former where inherited methods are copied into heir classes, and the latter where member lookup is performed by ascending the inheritance chain. Here we address equivalence of these two views for a more sophisticated composition mechanism.

Type safety for feature-oriented product lines

“…When looking up a method body, FFJ traverses the combined inheritance and refinement hierarchy of a class and selects the right-most and bottom-most (i.e., the least in the lexical order depicted in Figure 1) body of a method declaration or method refinement that is compatible. This kind of lookup is necessary since we model features directly in FFJ, instead of generating and evaluating FJ code [44].…”

“…Lagorio et al have shown that a flattening semantics and a direct semantics are equivalent [44]. An advantage of a "direct" semantics is that it allows a type checking and error reporting at a finer grain.…”

“…The evaluation terminates when a value (i.e., a term of the form new C(v)) is reached. Note that we use a direct semantics of class refinement [44]. That is, the field and method lookup mechanisms incorporate all refinements when a class is searched for fields and methods.…”

DeepFJig — Modular composition of nested classes.

“…Featherweight Jigsaw [LSZ09a,LSZ09b,LSZ12] (FJIG for short) is a simple calculus where basic building blocks are classes in the style of Featherweight Java (FJ for short) [IPW99], but inheritance has been replaced by the much more flexible notion originally proposed in Bracha's Jigsaw framework [Bra92]. That is, classes play also the role of modules, that can be composed by a rich set of operators, all of which can be expressed by a minimal core, composed by: sum, restrict, alias and redirect.…”

“…In this paper, we choose to explain the semantics of DEEPFJIG by flattening since this provides a more simple and intuitive understanding of the effect of the operators. However, a direct semantics could be provided for DEEPFJIG along the lines of that we have described for FJIG [LSZ09a,LSZ09b,LSZ12], even though method look-up is much more involved when there are many composition operators rather than just extends. Note also that an implementation could be based on one of these two approaches, or more likely adopt some even different or mixed optimized approach.…”