Jam---designing a Java extension with mixins

Abstract

In this paper we present Jam, an extension of the Java language supporting mixins, that is, parametric heir classes. A mixin declaration in Jam is similar to a Java heir class declaration, except that it does not extend a fixed parent class, but simply specifies the set of fields and methods a generic parent should provide. In this way, the same mixin can be instantiated on many parent classes, producing different heirs, thus avoiding code duplication and largely improving modularity and reuse. Moreover, as happens for classes and interfaces, mixin names are reference types, and all the classes obtained by instantiating the same mixin are considered subtypes of the corresponding type, and hence can be handled in a uniform way through the common interface. This possibility allows a programming style where different ingredients are "mixed" together in defining a class; this paradigm is somewhat similar to that based on multiple inheritance, but avoids its complication.The language has been designed with the main objective in mind to obtain, rather than a new theoretical language, a working and smooth extension of Java. That means, on the design side, that we have faced the challenging problem of integrating the Java overall principles and complex type system with this new notion; on the implementation side, it means that we have developed a Jam-to-Java translator which makes Jam sources executable on every Java Virtual Machine.

Type safety for feature-oriented product lines

“…Especially, calculi for mixins [28,18,1,36], traits [45], family polymorphism and virtual classes [35,27,32,20], path-dependent types [56,55], open and dependent classes [22,29], and nested inheritance [54] either support only the refinement of single classes or expect the classes that form a semantically coherent unit (i.e., that belong to a feature) to be located in a physical module that is defined in the host programming language. For example, a virtual class is by definition an inner class of the enclosing object, and a classbox is a package that aggregates a set of related classes.…”

“…A program that is decomposed into features is called henceforth a feature-oriented program. 1 Beside the decomposition of programs into feature modules, the concept of a feature is useful for distinguishing different, related programs which together make up a software product line [37,23]. Typically, programs of a common domain share a set of features but also differ in other features.…”

Software diversity: state of the art and perspectives

“…Since a subclass can be implemented before its superclass has been implemented, mixins remove most of the dependencies of the subclass on the superclass. Mixins have become a focus of active research in many communities and contexts: software engineering [77,207], programming language design [38,221,83,8], module systems [7,113], and distributed mobile code applications [32]. While mixins solve many problems encountered with multiple inheritance, their "linearization" strategy may still pose obstacles to code reuse.…”

O’Klaim: A Coordination Language with Mobile Mixins

“…Mixins [14,22,2] are (sub)class definitions parameterized over a superclass and were introduced as an alternative to standard class inheritance. A mixin could be seen as a function that, given one class as an argument, produces another class, by adding or overriding specific sets of methods.…”