• What does the product do? DJ is a tool which allows you to write generic Java programs that are easy to evolve and reuse. DJ is easy to learn and use by Java Programmers and allows them to use high-level actions like traverse, fetch, gather, find, add, delete, etc. DJ builds on the observation pioneered by the Demeter project that navigation through object structures is ubiquitous and therefore warrants a special purpose language for navigation from which correct traversals can be generated automatically. The special purpose language is a traversal strategy language for specifying navigation through objects. The product is the result of 10 years of research/development in Adaptive Programming (AP) which started with the Law of Demeter. The traversal strategy language allows for a better way to follow the Law of Demeter by replacing the many small traversal methods with simple traversal strategies.
• Why is the product useful? The product has the following benefits compared to Java without traversal support:
  1. Programs become more flexible and adapt automatically to a wide variety of structural changes. If the adaptation is not the desired one, for typical programs it is much easier to change the traversal strategy than the corresponding object-oriented program. The flexibility of software written with DJ facilitates rapid application development.
  2. Programs are more readable since traversal strategies are a part of the program, i.e., the programmer's high-level intentions are an integral part of the program (traversal strategies), rather than having to be written as comments or external documentation. This means that a part of the rationale capture for the program is an integral part of the program itself. Without the explicit traversal strategies, the programs are harder to read since often a large number of function calls would need to be studied to get the equivalent information in the strategies.
  3. Programs become simpler because traversal strategies are simpler than the detailed traversals.
  4. DJ integrates Generic Programming and Traversal-Visitor style programming giving you the benefit of both. DJ is a programming tool that integrates Generic Programming (GP) (best known from the C++ Standard Template Library) using the Java Collections Framework with Traversal-Visitor Programming (best known from the Demeter work and the Visitor Design pattern). Both Generic Programming and Traversal-Visitor Programming are made more generic by using structure-shy traversal strategies leading to a programming system that is easier to use than Java alone. The additional genericity comes from a parameterization over the UML class diagram that is produced by DJ from the Java program. The interface to the Java Collections Framework is very simple by providing adaptor methods from traversals to containers. It needs to be stressed, however, that DJ is already very useful without using Generic Programming by only using the three actions traverse, fetch and gather (which probably give you 60% of the benefits of DJ).
  The product has the following disadvantages compared to Java without traversal support:
  1. At a minimum, three concepts must be learned: class graph, traversal strategy graph, and traversal graph. Advanced users also need to learn the concept of object graph slice and the Java collection class library. The traversal strategy graph language must be learned. Traversal strategies specify navigation intent and are expressed as Java strings that must be syntactically correct.
  2. DJ uses reflection to interpret and execute the navigation. Therefore in the current implementation of DJ all traversals run slower than hard-coded or generated traversal code. A future version of DJ will have an option to compile the traversals that are statically known.
• Platforms. DJ is implemented in Java (using Demeter/Java, the big sister of DJ) and available in 100% pure Java source form. It runs on all platforms where Java runs.
• How to get it. We distribute DJ as a research prototype. (www.ccs.neu.edu/research/demeter/DJ). For general information on Demeter, see: www.ccs.neu.edu/research/demeter.

  DJ patterns . For theoretical foundations, see the book www.ccs.neu.edu/research/demeter/biblio/dem-book.html .

Publications
Aspect-Oriented Programming with Adaptive Methods.
Dynamic Adaptive Programming in Java.

JXPath implements some of the Demeter ideas using XPath as navigation language. For further information: JXPath Jakarta Project

Supported by DARPA and Rome Laboratory under agreement F30602-96-0239 (Evolution of Software Via Adaptive Programming), Karl Lieberherr (PI), lieber@ccs.neu.edu.

Acknowledgements: Implemented by Josh Marshall and Doug Orleans using the AP Library by Doug Orleans. Lars Hansen proposed an early idea and Johan Ovlinger nurtured it with his TAO system inside Demeter/Java. Cristina Lopes implemented at Xerox PARC a similar system for a version of Lisp: AP/S++: A CASE-study of a MOP for Purposes of Software Evolution (ftp://ftp.ccs.neu.edu/pub/people/lieber/reflection-adaptive.ps). Several people recommended the development of DJ, among them Harold Ossher (IBM), Luis Blando (GTE Labs), David Wagstaff (Novell).

We develop DJ using a Bazaar approach. Please send us your bug fixes and improvements to lieber@ccs.neu.edu and we will integrate them.

version June 11, 1999