Statically Scoped Object Adaptation with Expanders

This paper introduces the expander, a new object-oriented (OO) programming language construct designed to support object adap-tation. Expanders allow existing classes to be noninvasively up-dated with new methods, fields, and superinterfaces. Each client can customize its view of a class by explicitly importing any num-ber of associated expanders. This view then applies to all instances of that class, including objects passed to the client from other com-ponents. A form of expander overriding allows expanders to inter-act naturally with OO-style inheritance. We describe the design, implementation, and evaluation of eJava, an extension to Java supporting expanders. We illustrate eJava’s syntax and semantics through several examples. The statically scoped nature of expander usage allows for a modular static type system to prevent several important classes of errors. We describe this modular static type system informally, formalize eJava and its type system in an extension to Featherweight Java, and prove a type soundness theorem for the formalization. We also describe a modu-lar compilation strategy for expanders, which we have implemented using the Polyglot extensible compiler framework. Finally, we il-lustrate the practical benefits of eJava by using this implementation in two case studies. 1