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AP/DJ
• AP: a generic technology
• DJ: a low-learning-curve implementation of
AP
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
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.
Law of Demeter
FRIENDS
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
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.
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
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
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
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
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);
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
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;} …)
}
}
Using DJ
TraversalGraph getT()
{return new TraversalGraph(
classGraph1,
new Strategy(
“from BusRoute
via BusStop to
Person”));}
More info
• DJ Home Page:
www.ccs.neu.edu/research/demeter/DJ