patterns in cs education Matthias Felleisen & Daniel Jackson NDIST · December 5, 2001

2 content experience reports (30 mins) ·Rice 2 freshman year courses ·MIT sophomore/junior software eng. shared (10 mins) ·conclusions ·provocations discussion (20 mins)

: lab in software engineering context ·required of all CS majors ·follows (Sussman/Abelson) ·our only programming course? ·8 weeks lectures & problem sets, 6-week team project content ·data abstraction ·specification conceptual models of problem component specifications ·medium-level design module dependences object models design patterns

4 talking about design to talk about design ·need succinct, precise language ·must escape from code ASAP strategy ·2 design notations module dependence diagram object model (invariant) ·introduce notations right at start & use pervasively ·gradually drop code from lectures documentation ·less textual, more graphical ·less uniform, more focused

5 relating problems & solutions hardest part of design? ·underlying conceptual model how to teach? ·simple notation for conceptual models same object modelling notation interpret as invariants about problem domain ·in early problem sets, have students write spec harder to do (good), harder to evaluate (bad) ·discuss strategies for problem -> solution transformation patterns?

6 using design patterns why teach design patterns? ·students (& teachers!) must know idioms ·writing style is exemplary ·inspire raising level of discourse ·books are great resource GoF, Design Patterns Joshua Bloch, Effective Java what happened? ·in course evals, students say they want more ·fit nicely with our design notations ·students need substantial examples of use this term, added 3 case studies JUnit, Java API, Tagger

7 example: observer D C CObserver D Subject Observer obssubj ! all o: Observer | o in o.subj.obs all s: Subject | s ! in s.^obs object model dependency diagram

8 conclusions why are patterns good for teaching? ·because they concretize abstract ideas are all patterns good? ·no, must be conveyed in a formal language how do patterns help? ·to communicate & understand do they help design code? ·no; key abstractions more vital do they never help with coding? ·yes, primarily in refactoring

9 conclusions, ctd do they understand why patterns are useful? ·yes, when they modify code (their own or others’) which patterns do students find useful? ·not small patterns (interpreter, composite, command) they think they’re obvious ·architectural patterns (MVC, kernel) harder, but more valuable

10 provocations patterns are the icing, not the cake ·principles are more essential teachers & researchers must know patterns ·they embody modern practice ·methods & tools must handle them & work at their level there aren’t very many patterns ·only GoF plus a few relevant to code ·of these, the architectural ones matter