Chapter

ECOOP 2000 — Object-Oriented Programming

Volume 1850 of the series Lecture Notes in Computer Science pp 154-178

Date:

Jam - A Smooth Extension of Java with Mixins

  • Davide AnconaAffiliated withDipartimento di Informatica e Scienze dell’Informazione
  • , Giovanni LagorioAffiliated withDipartimento di Informatica e Scienze dell’Informazione
  • , Elena ZuccaAffiliated withDipartimento di Informatica e Scienze dell’Informazione

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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 instantiating the same mixin are considered subtypes of the corresponding type, 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 somehow similar to that based on multiple inheritance, but avoids the associated complications.

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, that we have developed a Jam to Java translator which makes Jam sources executable on every Java Virtual Machine.