Problem with Java and

1. Problem with Java and how it causes a problem for DJ

2. Problem with Java and DJ • What is coming is not about a problem of DJ but about a problem with Java: the lack of parameterized classes. • The lack of parameterized classes forces the use of class Object which, as the mother of all classes, is too well connected. • This leads to unnecessary traversals and traversal graphs that are too big.

3. DJ Traversals are opportunistic • If there is a possibility that an object o may be on a path to success, o is visited. • The decision whether there is a possibility is made based on whether there is a path in the class graph. • The presence of class Object in a path in the class graph drastically increases the possibilities.

4. BList A * X B Vector * Object A X B

5. find all persons waiting at any bus stop on a bus route Traversal Strategy from BusRoute through BusStop to Person busStops BusRoute BusStopList buses 0..* BusStop BusList waiting 0..* passengers Bus PersonList Person 0..*

6. find all persons waiting at any bus stop on a bus route Traversal Strategy from BusRoute through BusStop to Person busStops BusRoute BusStopList Vector buses 0..* Object BusStop BusList waiting 0..* passengers Bus PersonList Person 0..*

7. find all persons waiting at any bus stop on a bus route Traversal Strategy from BusRoute through BusStop to Person busStops BusRoute BusStopList Vector buses 0..* Object BusStop BusList waiting passengers Bus PersonList visit all Bus-objects and stop at them. Person 0..*

8. ObjectGraph Traversal Bus17:Bus :Vector Route1:BusRoute buses Bus16:Bus busStops :Vector Bus15:Bus passengers CentralSquare:BusStop waiting :PersonList :PersonList Joan:Person Paul:Person Seema:Person Eric:Person

9. ObjectGraph Traversal Bus17:Bus :Vector Route1:BusRoute buses Bus16:Bus busStops :Vector Bus15:Bus passengers CentralSquare:BusStop waiting :PersonList :PersonList Joan:Person Paul:Person Seema:Person Eric:Person

10. Unnecessary Traversal • The traversal goes through the buses link although there will be no Bus-objects in that vector. • But in Java, when we use a Vector-object, we cannot express in the class graph that we have only Bus-objects in that collection.

11. Lack of parameterized classes in Java makes DJ harder to use • Consider the traversal: from A to B • Let’s assume that in the class graph between A and B there is a Java collection class. The intent is: A = List(B) which we cannot express in Java. Instead we have: A = Vector(Object). Object : A | B. Let’s assume we also have a class X=B.

12. Lack of parameterized classes in Java makes DJ harder to use • We have: A = Vector(Object). Object : A | B | X. X = B. • If the vector contains an X object it will be traversed!!! Vector * Object A X B

13. No X-object is allowed to be in vector A * X B Vector * Object A X B

14. Moral of the story • If the Collection objects contain only the objects advertised in the nice class graph of the application the traversal done by DJ will be correct. But unnecessary traversals still happen. • However, if the Collection objects contain additional objects (like an X-object) they will be traversed accidentally.

15. Moral of the story • Java should have parameterized classes. • Workaround: Use a JSR (Java Specification Request) 31 approach: use a schema notation with parameterization to express class graph and generate Java code from schema. For traversal computation, the schema will be used.

16. Size of traversal graph • DJ might create big traversal graphs when collection classes are involved. DJ will plan for all possibilities even though only a small subset will be realized during execution. • To reduce the size of the traversal graph, you need to use bypassing. In the example: from A bypassing {A,X} to B.