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: