AP/DJ

1. AP/DJ • AP: a generic technology • DJ: a low-learning-curve implementation of AP

2. From Mike Mannion • Regarding DOM navigation, I would be very interested in taking a look at the library you mention (DJ). Can I use XPath expressions to navigate a DOM within Java? That would be very useful indeed!

3. Law of Demeter Principle • Each unit should only have limited knowledge about other units: only about units “closely” related to the current unit. • “Each unit should only talk to its friends.” “Don’t talk to strangers.” • Main Motivation: Control information overload. We can only keep a limited set of items in short-term memory.

4. Law of Demeter FRIENDS

5. Application to OO • Unit = method • closely related = • methods of class of this/self and other argument classes • methods of immediate part classes (classes that are return types of methods of class of this/self) • In the following we talk about this application of the Law of Demeter Principle to OO

6. Rumbaugh and the Law of Demeter Quote: Avoid traversing multiple links or methods. A method should have limited knowledge of an object model. A method must be able to traverse links to obtain its neighbors and must be able to call operations on them, but it should not traverse a second link from the neighbor to a third class.

7. Law of Demeter(alternative formulation) A method should have limited knowledge of an object model. AP is a reaction to this view of the Law of Demeter

8. Agreement that LoD Good Idea • How to follow LoD: good solutions exist but not widely known. Two approaches to following LoD: • OO approach • Adaptive approaches • DJ • APPC • Demeter/Java

9. DJ • An implementation of AP using only the DJ library and the Java Generic Library (JGL) • All programs written in pure Java • Intended as prototyping tool: makes heavy use of introspection in Java • Integrates Generic Programming (a la STL) and Adaptive programming

10. Integration of Generic and Adaptive Programming • A traversal specification turns an object graph into a container. • Can invoke 50+ generic algorithms on those containers. Examples: add, delete, find, etc. • What is gained: genericity not only with respect to data structure implementations but also with respect to class graph

11. Sample DJ code // Find the user with the specified uid Container libUsers = new Container(library, "from Library to User"); User user = libUsers.find("uid", uid);

12. In addition: Traverse method: excellent support for Visitor Pattern // class ClassGraph Object traverse(Object o, Strategy s, Visitor v); traverse navigates through Object o following traversal specification s and executing the before and after methods in visitor v ClassGraph is computed using introspection

13. Using DJ: BusRoute example public int count (){ // class BusRoute getT().traverse( this, new Visitor(){ int r; public void before(Person host) { r++; } public void start() { r = 0;} …) } }

14. Using DJ TraversalGraph getT() {return new TraversalGraph( classGraph1, new Strategy( “from BusRoute via BusStop to Person”));}

15. More info • DJ Home Page: www.ccs.neu.edu/research/demeter/DJ