From dougo@ccs.neu.edu Wed Jun 12 02:27:35 1996
To: Karl Lieberherr <lieber@ccs.neu.edu>
Cc: dougo@ccs.neu.edu
Subject: Re: schedule

Karl Lieberherr writes:
 > please can you do a plan for Demeter/Java and predict when
 > which features will be available on the net.

 > 	    "PRE-MEETING HOMEWORK: Each project needs to submit a 1-2
 > 	    page summary of what it will be producing in each cluster
 > 	    area where it is participating and when prototypes will be
 > 	    available. (Similar to "Innovative Claims" Section of
 > 	    proposal, but more specific since you're now planning on
 > 	    doing, rather than proposing, the work.) The description
 > 	    should emphasize usable "features" rather than technology


Demeter/Java is a tool which takes a class dictionary and a set of
method specifications and produces a Java source program.  A class
dictionary is a textual representation of a class graph together with
syntactic annotations.  There can be three types of
method specifications:

    (1) Ordinary methods.  These are Java methods attached to the
        classes in the class graph which are copied verbatim to the
        output program.

    (2) Traversal methods.  A traversal method is a succinct subgraph
        specification of a path to follow through the class graph.
	The arguments to a traversal method are one or more visitor
	objects, which have visitor methods that correspond to classes
	along the path.

    (3) Visitor methods.  A visitor class may have "before" or "after"
        methods corresponding to other classes in the class graph.
        For each object being visited in a traversal, the visitor
        methods on the visitor objects which correspond to the class
        of that object are called, either immediately before or
        immediately after the traversal of the object has taken place.
	The bodies of the visitor methods are copied verbatim to the
	output program.

The traversal specifications are mapped onto the class graph and
expanded out into ordinary Java methods, with calls to the appropriate
visitor methods in the appropriate places.  The final output is a
complete Java program, i.e. a list of classes and their parts and
methods, which can be compiled and run as normal.  The advantage to
Demeter/Java is that the structure and behavior are loosely coupled,
allowing ease of evolution in one independent of the other.

Demeter/Java is different from Demeter/C++ not only in that it
produces Java code instead of C++ code, but it also uses a newer
technology.  A traversal method can be thought of as a form of
a propagation pattern, with the visitor methods corresponding to
object wrappers; the data members of visitor objects correspond to
transportation variables, but with the added benefits of encapsulation
and genericity (reusability).

A prototype implementing this minimal description will be available in
July.  It will be written in Demeter/C++ and utilize the state-passing
algorithm proposed in [Lieberherr, Palsberg, Patt-Shamir, Pradhan].
By August it will be ported to Demeter/Java; in other words, the
compiler will be able to compile itself.  By September a small library
of utility functions will be available, including functions to parse,
print, and compare object trees.  Also by September, a prototype
implementation of a class graph drawing tool will be available,
written in Java using the AWT library.

Future research will generalize the language of class dictionaries to
be the Unified Modeling Language.  It will also allow the
behavior to be specified using the context object relation described
in [Seiter-Keszenheimer, Lieberherr, Palsberg].

--Doug