1. Object Constraint Language Design by Contract and Queries Dan Massey Y&L Consulting

2. OCL Topics Object Constraint Language (OCL) OCL for Design by Contract OCL for Queries Another OCL Example

3. OCL Topics Object Constraint Language (OCL) OCL for Design by Contract OCL for Queries Another OCL Example

4. Object Constraint Language OCL 2.0 Product of the Object Management Group (OMG) Add-on to the Unified Modeling Language (UML™) 2.0 specification UML combined with OCL forms a solid base for Model Driven Architecture (MDA ® )

5. Model Driven Architecture Work with Platform Independent Model (PIM) Transform PIM into Platform Specific Model (PSM) Transform PSM into implementation

6. Model Constraints Originally viewed as a way to constrain or to restrict values in a model In addition OCL supports: Query expressions Derived values Conditions Business rules

7. Reasons to use OCL Express concepts that are not supported by UML diagrams Make models more precise Support transformation tools and code generation (MDA)

8. Approach We’re going to explore OCL by example to give you ideas as to where it might apply in your work. At the end I’ll point you toward sources of more complete information.

9. OCL Topics Object Constraint Language (OCL) OCL for Design by Contract OCL for Queries Another OCL Example

10. Design by Contract Why do we use contracts? We know the rules. We know what happens if we break them. Design by Contract does the same for software. Provide a formal specification of behavior.

11. More Design by Contract winningBid():Bid is a signature, not a specification. It’s like giving someone a phone without providing the format for a correct phone number. Are there any conditions that must be met for a call to winningBid() to succeed?

12. And… Design by Contract Specify preconditions, postconditions, and invariants. Defensive programming? We’ll cover each of these in turn. Bertrand Meyer is the father of DbC http://www2.inf.ethz.ch/personal/meyer/

13. Auction Domain A partial auction domain to get us started. Includes “Auction,” “Bid,” and “Money.” We have the structure, but what are the rules?

14. OCL Expression Context Every OCL expression takes place in a context: context Auction context Auction::winningBid():Bid

15. Preconditions Preconditions are the conditions that must be true prior to successfully invoking the method in question. Often preconditions that fail result in exceptions.

16. Example Preconditions context Auction::placeBid(bidder:Id, bid:Money) pre : not bidder.oclIsUndefined() pre : not bid.oclIsUndefined() pre : bid.currency = self.bids->first().amount.currency pre : bid > self.currentBid().money

17. Simple Preconditions context Auction::placeBid(bidder:Id, bid:Money) pre : not bidder.oclIsUndefined() pre : not bid.oclIsUndefined() This operation requires a bidder identifier and a bid.

18. Implementing Preconditions … public void placeBid(Id bidder, Money bid) { if (bidder == null) { throw new NullPointerException( “bidder may not be null”); } …

19. More Involved Preconditions context Auction::placeBid(bidder:Id, bid:Money) pre : bid.currency = self.bids->first().amount.currency pre : bid > self.currentBid().money The bid must be in the same currency as the starting bid. The bid must be higher than the current high bid.

20. Implementing Preconditions … if (bid.getAmount().getCurrency() != ((Bid) bids.first()).getAmount().getCurrency()) { throw new IllegalArgumentException( “currencies must match”); } …

21. Assertion Temptation // Don’t do this. Anyone who runs your app // can “turn off” assertions. … assert bidder != null : “bidder may not be null”; assert bid.getAmount().getCurrency() == ((Bid) bids.first()).getAmount().getCurrency() : “currencies must match”; …

22. OCL Comments context Auction::placeBid(bidder:Id, bid:Money) -- currencies must match pre : bid.currency = self.bids->first().amount.currency pre alwaysTrue : 1 = 1 -- and /**/ are valid comment forms in OCL

23. More Preconditions Preconditions are not limited to checking parameters. They can specify any condition that must be true prior to executing the operation. What are some other possible preconditions?

24. More Preconditions context Auction::winningBid():Bid -- auction must end to have a winner pre : TimeStamp::now() >= self.expires() context Bid::>(bid:Bid):Boolean -- _gt is our name for “>” pre : not bid.oclIsUndefined()

25. Invariants Invariants are conditions that are always true. They are always true. Think conservation of energy, not “the color of the sky is blue.”

26. Invariants If an invariant is violated something is wrong with the implementation of your system. What are some invariants in our domain?

27. Example Invariants context Auction inv : self.expires() > self.start context Bid inv : self.amount.amount > 0.0 /* Note this could be specified in the Auction context, but don’t. */

28. Postconditions Postconditions are the conditions that your operation guarantees will be true when it has completed. The contract: You meet the preconditions and I’ll meet the postconditions.

29. PostConditions context Auction::placeBid(bidder:Id, bid:Money) post: self.currentBid().amount = bid Notice that the postcondition assumes serial or single-threaded execution. This postcondition isn’t necessarily enforceable in an unprotected multi-threaded context.

30. PostConditions context Auction::placeBid(bidder:Id, bid:Money) post : self.currentBid() > self.currentBid@pre() Postconditions can reach back in time. @pre references the value before the operation. Another way to state a precondition…

31. Implementing PostConditions … public void placeBid(Id bidder, Money bid) { … Bid preCurrentBid = this.currentBid(); … if (this.currentBid() <= preCurrentBid) { …

32. OCL Contracts Add more formal specifications to your models. Specify what will happen, without the how. Start at component boundaries. You can start out with simple contracts.

33. OCL Topics Object Constraint Language (OCL) OCL for Design by Contract OCL for Queries Another OCL Example

34. OCL as Query Language You can think of OCL as equivalent to SQL when it comes to querying object models. Borland’s ECO framework uses OCL for querying (as well as constraints, derived values, etc.) Other query alternatives include OQL and Xquery

35. Test Results Mini Domain Below is a simple domain for describing test results from test sessions at a competition for teams of logicians. This is our domain for examining OCL queries.

36. Example Test Data Set Here’s some example test result data from a logician competition.

37. Derived Values context TestResult::compositeScore : Integer derive: (2 * logicScore) + illogicScore

38. OCL Collection Operations context TestSession::numberOfResults : Integer derive: results->size()

39. OCL Collection Choices Set: no duplicates, not ordered, result of navigating on association OrderedSet: not sorted, just ordered Bag: may contain duplicates, not ordered, usually the result of navigating multiple associations Sequence: an ordered Bag

40. OCL Collections Classification DuplicatesNo Duplicates Not OrderedBagSet OrderedSequenceOrderedSet

41. OCL select() Query context TestSession::numberOfScoresOverTarget( target : Integer) : Integer body: results->select( compositeScore > target)->size()

42. Translating OCL Queries Let’s look at what a transformation tool might do with one of our queries in SQL. SELECT COUNT(*) FROM test_results WHERE composite_score > :target; teamlogic_scoreillogic_scorecomposite_scorename Blue244795A Appell Blue243280B Barry Red352393C Cross Green185086D Dolly Red4944143E Elms Blue3742116F Fisher

43. Returning a Collection context TestSession::testResultsForTeam( testTeam : Team) : Set(TestResult) body: results->select(team = testTeam)

44. Building Up Queries context TestSession::averageScoreForTeam( testTeam : Team) : Real body: testResultsForTeam(testTeam).compositeScore ->sum() / testResultsForTeam(testTeam)->size()

45. Which Logicians Won? context TestSession::bestResults() : Set(TestResult) body: results->select(compositeScore = results->sortedBy(compositeScore) ->last().compositeScore)

46. OCL Resources The Object Constraint Language, 2ed Jos Warmer and Anneke Kleppe Klasse Objecten’s OCL page http://www.klasse.nl/ocl/ http://www.klasse.nl/ocl/ OCL 2.0 Specification at OMG http://www.omg.org/cgi-bin/doc?ptc/2003-10-14

47. Questions and Answers