Jam---designing a Java extension with mixins

Reviewer: Axel T. Schreiner

A mixin is similar to an abstract class: it establishes a reference type, and contains field and method declarations, and in particular, method implementations. Unlike an abstract class, a mixin does not declare a parent class, but instead declares a set of fields and methods that it expects to inherit. Mixins are designed to reuse code in otherwise unrelated classes, and to establish a common reference type for classes instantiating the same mixin. This paper discusses the design, rationale, and implementation of Jam, a mixin system for Java 1.0 (without inner classes), and provides a formal definition of the type system of a slightly reduced subset of the extended language and of its translation to Java. It concludes with a close look at related work: other mixin designs, parametric types, and the system's relationship to modules. The paper should be interesting to both language designers and users. While about two-thirds of the paper is devoted to formal treatments, the informal sections on the language definition, rationale, translation, and implementation are an excellent overview of the design complications, and of the design choices made to overcome them. This makes using mixins that much easier. The paper emphasizes that Jam is a working extension of Java, and the authors provide a download of their system. A slightly more recent paper [1] reports on how traits (essentially mixins without a reference type and with more composition operations) were used to improve the architecture of the Smalltalk collection classes. Mixins compare favorably with traits; it would have been instructive to see a similar, real-life application of Jam as part of this paper. Online Computing Reviews Service