1. David Evans http://www.cs.virginia.edu/~evans
Lecture 21: Crosscutting Aspect-Oriented Programming
Background
just got here last week
finished degree at MIT week before
Philosophy of advising students
don’t come to grad school to implement someone else’s idea
can get paid more to do that in industry
learn to be a researcher
important part of that is deciding what problems and ideas are worth spending time on
grad students should have their own project
looking for students who can come up with their own ideas for research
will take good students interested in things I’m interested in – systems, programming languages & compilers, security
rest of talk – give you a flavor of the kinds of things I am interested in
meant to give you ideas (hopefully even inspiration!) but not meant to suggest what you should work on
CS655: Programming Languages
University of Virginia
Computer Science
David Evans

2. Adaptive Programming: Demeter
Instance of AOP [Lieberherr92]
Separate the program text and the class structure
Program is independent of class graph
Accomplish tasks by traversals
Specification for what parts of received object should be traversed
Code fragments for what to execute when specific object types are encountered
15 January 2019
University of Virginia CS 655

3. University of Virginia CS 655
Law of Demeter
Law of Demeter: a method should talk only to its friends:
arguments and part objects (computed or stored)
newly created objects
Dilemma:
Small method problem of OO (if followed)
Unmaintainable code (if not followed)
Traversal strategies are the solution to this dilemma
Demeter = Greek Goddess of Agriculture (grow software from small blocks)
15 January 2019
University of Virginia CS 655

4. AP Example: UML Class Diagram
Slide adapted from Karl Lieberherr talk
AP Example: UML Class Diagram
busStops
BusRoute
BusStopList
buses
0..*
BusStop
BusList
waiting
0..*
passengers
Example for developing strategies and writing adaptive programs
Bus
PersonList
Person
0..*
1/15/2019
AOP/Demeter

5. Collaborating Classes
Slide adapted from Karl Lieberherr talk
Collaborating Classes
Find all persons waiting at any bus stop on a bus route
busStops
BusRoute
BusStopList
OO solution:
one method
for each red
class
buses
0..*
BusStop
BusList
waiting
0..*
passengers
Bus
PersonList
Person
0..*
1/15/2019
AOP/Demeter

6. Java Solution (excerpt)
class BusRoute {
BusStopList busstops;
void printWaitingPassengers () {
busstops->printWaitingPassengers (); } }
class BusStopList {
BusStop stops[];
for (int i = 0; i < stops.length; i++)
stops[i].printWaitingPassengers ();
15 January 2019
University of Virginia CS 655

7. University of Virginia CS 655
Java Solution (cont.)
class BusStop {
PersonList waiting;
void printWaitingPassengers () {
waiting.print (); } }
class PersonList {
Person people[];
void print () {
for (int i = 0; i < people.length; i++) people[i].print (); }
class Person {
String name;
void print () { System.stdout.println (name); } }
15 January 2019
University of Virginia CS 655

8. University of Virginia CS 655
Demeter Approach
Devise a traversal strategy
Specify code for different types of objects reached on a traversal
Example: code prints name if object is a Person
Independent of class graph
15 January 2019
University of Virginia CS 655

9. Slide adapted from Karl Lieberherr talk
Traversal Strategy
First try: from BusRoute to Person
busStops
BusRoute
BusRoute
BusStopList
BusStopList
buses
0..*
BusStop
BusStop
BusList
BusList
waiting
0..*
passengers
Bus
Bus
PersonList
PersonList
Person
Person
0..*
1/15/2019
AOP/Demeter

10. Slide adapted from Karl Lieberherr talk
Traversal Strategy
from BusRoute through BusStop to Person
busStops
BusRoute
BusStopList
buses
0..*
BusStop
BusList
waiting
0..*
passengers
Bus
PersonList
Person
0..*
1/15/2019
AOP/Demeter

11. Robustness of Strategy
Slide adapted from Karl Lieberherr talk
Robustness of Strategy
from BusRoute bypassing Bus to Person
villages
BusRoute
BusStopList
buses
VillageList
busStops
0..*
0..*
BusStop
BusList
Village
waiting
0..*
passengers
Bus
PersonList
Person
0..*
1/15/2019
AOP/Demeter

12. Filter out noise in class diagram
Slide adapted from Karl Lieberherr talk
Filter out noise in class diagram
only three out of seven classes
are mentioned in traversal
strategy!
from BusRoute through BusStop to Person
explain why strategy better
replaces traversal methods for the classes
BusRoute VillageList Village BusStopList BusStop
PersonList Person
1/15/2019
AOP/Demeter

13. University of Virginia CS 655
Summary
Aspect-Oriented Programming and Adaptive Programming provide programmers with new expressive options
Active research area (Separation of Concerns Workshops at OOPSLA, ICSE, ECOOP, etc.)
Many directions to explore
15 January 2019
University of Virginia CS 655