Pluggable types enable users to enforce multiple type systems in one programming language. We have developed a suite of tools, called the JavaCOP framework, that allows developers to create pluggable type systems for Java. JavaCOP provides a simple declarative language in which program constraints are defined over a program's abstract syntax tree. The JavaCOP compiler automatically enforces these constraints on programs during compilation. The JavaCOP framework also includes a dataflow analysis API in order to support type systems which depend on flow-sensitive information. Finally, JavaCOP includes a novel test framework which helps users gain confidence in the correctness of their pluggable type systems. We demonstrate the framework by discussing a number of pluggable type systems which have been implemented in JavaCOP in order to detect errors and enforce strong invariants in programs. These type systems range from general-purpose checkers, such as a type system for nonnull references, to domain-specific ones, such as a checker for conformance to a library's usage rules.

Pluggable types have been proposed to support multiple type systems in the same programming language. We have designed and implemented JavaCOP, a program constraint system for implementing practical pluggable type systems for Java. JavaCOP enforces user-defined typing constraints written in a declarative and expressive rule language. We have validated our design by (re)implementing a range of type systems and program checkers. By using a program constraint system to implement pluggable types, programmers are able to check that their programs will operate correctly in restricted environments, adhere to strict programming rules, avoid null pointer errors or scoped memory exceptions, and meet style guidelines, while programming language researchers can easily experiment with novel type systems.

When researchers extend a conventional communication application to mobile devices, the communication becomes ubiquitous. We recently examined a mobile communication application from a usability perspective. We found that despite the potentially high utility of the mobile communication application, it had usability issues that required user interface redesign. This paper is an alert for mobile and ubiquitous multimedia designers that they must not ignore usability considerations when designing software and hardware solutions for ubiquitous mobility.