1. Academic Bug Patterns Stuart Hansen University of Wisconsin - Parkside

2. Bug Patterns • Recurring software defects • Term coined by Eric Allen Bug Patterns in Java, APress, 2002 • Academic Bug Patterns • Occur in student code • Not only “gotchas”

3. Motivation • Understanding our students’ bugs helps us teach them to • Find and remove them • Avoid them • Places empirical data behind our expertise • May give added insights

4. Object-Oriented Programming • New Programming Paradigm → New bug Patterns • Does OOP Really Match the Way we Think?, Les Hatton, IEEE Software 1998 • OOP programs are buggier and take longer to debug than procedural programs.

5. We are Experts Already • To a large extent we are already experts in academic bug patterns • CS1 and CS2 “gotchas” • Using symbolic constants • Problem decomposition • Textbooks include headings like • Common Programming Errors • Helpful Reminders

6. Survey • Two Classes • Data Structures and Algoritms • Event Driven Programming • Two Questions: • What was the most difficult bug/problem you encountered doing the assignment? • What did you do to resolve it?

7. Survey - continued • Intentionally wide open • Didn’t want to impose a taxonomy • Total of 56 surveys collected on 6 different assignments

8. Results • 19 Language and Tools Bugs • 13 Strong Coupling Bugs • 11 Local/Instance Bugs • 6 Blame the Instructor Bugs

9. Languages and Tools Pattern • 19/56 = 34% of all bugs reported • Lots of new stuff still being introduced • Swing • Microsoft .Net Programming • CORBA/RMI • Biggest student desire seems to be for brief cogent examples

10. Languages and Tools Sol’ns • Search the Internet • Ask someone in class • Only 1 student reported reading the text to find the solution!

11. Strong Coupling Bugs • 13/56 = 23% of all bugs reported • Event Driven Programming starts with lots of GUI material. • Model-View-Controller • Multi-threading • “When I came up one solution to a problem, it often caused new problems in other areas.”

12. Strong Coupling - Continued • Students wrote long paragraphs describing how they solved their problems. • No discernable patterns to their approaches. • We probably don’t teach enough design patterns early in our curriculum.

13. Local State/Object State Bug • 11/56 = 20% of all bugs reported • Students didn’t see whether data should be local, instance or belong to some sort of global data store. • Particularly true in game programming • Boggle and Connect 4 • Students have been introduced to recursion, but haven’t applied it much on their own.

14. Local/Object – Continued • Basic truth not in texts is that M-V-C requires redundant state! • “When I clicked the Undo button, the screen would clear the last move, but inside the memory the program did not actually clear the data.”

15. Brief Aside: • O-OP is gradually making its way into D.S. and Alg. courses. • Many texts still have Pascal look and feel. • Tension between O-OP and D.S. and Alg. • RedBlack trees require non-global, but more than local knowledge. • Null design pattern works, but an “if” may be clearer

16. Polymorphism Bugs • 8/56 = 14% of all bugs reported • Java GUIs are fundamentally polymorphic • Use inheritance to build interfaces • Register handlers • Also in D.S. and Algs. • 2D Trees, e.g. BSTs based on (X,Y) points. • Registration is easier polymorphism related pattern than others

17. Blame the Instructor - Bugs • 6/56 = 11% of all bugs reported • Anonymous survey is good opportunity to let loose on instructor • Two main complaints • Instructor code sometimes buggy • Program specs sometimes not clear enough

18. Summary • “See one, do one” is dominant learning model for our students. • Languages, tools and libraries are growing more and more complex. • Object-oriented design still poses major challenges.

19. Questions?