Object Constraint Language

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Object Constraint Language

• Design by Contract and Queries
• Dan Massey
• YL Consulting

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OCL Topics

• Object Constraint Language (OCL)
• OCL for Design by Contract
• OCL for Queries
• Another OCL Example

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OCL Topics

• Object Constraint Language (OCL)
• OCL for Design by Contract
• OCL for Queries
• Another OCL Example

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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)

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Model Driven Architecture

• Work with Platform Independent Model (PIM)
• Transform PIM into Platform Specific Model (PSM)
• Transform PSM into implementation

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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

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Reasons to use OCL

• Express concepts that are not supported by UML
  diagrams
• Make models more precise
• Support transformation tools and code generation
  (MDA)

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Approach

• Were going to explore OCL by example to give you
  ideas as to where it might apply in your work.
• At the end Ill point you toward sources of more
  complete information.

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OCL Topics

• Object Constraint Language (OCL)
• OCL for Design by Contract
• OCL for Queries
• Another OCL Example

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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.

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More Design by Contract

• winningBid()Bid is a signature, not a
  specification. Its 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?

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And Design by Contract

• Specify preconditions, postconditions, and
  invariants. Defensive programming?
• Well cover each of these in turn.
• Bertrand Meyer is the father of DbC
• http//www2.inf.ethz.ch/personal/meyer/

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Auction Domain

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

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OCL Expression Context

• Every OCL expression takes place in a context
• context Auction
• context AuctionwinningBid()Bid

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Preconditions

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

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Example Preconditions

• context AuctionplaceBid(bidderId,
  bidMoney)
• pre not bidder.oclIsUndefined()
• pre not bid.oclIsUndefined()
• pre bid.currency
• self.bids-gtfirst().amount.currency
• pre bid gt self.currentBid().money

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Simple Preconditions

• context AuctionplaceBid(bidderId,
  bidMoney)
• pre not bidder.oclIsUndefined()
• pre not bid.oclIsUndefined()
• This operation requires a bidder identifier and a
  bid.

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Implementing Preconditions

• public void placeBid(Id bidder,
• Money bid)
• if (bidder null)
• throw new NullPointerException(
• bidder may not be null)

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More Involved Preconditions

• context AuctionplaceBid(bidderId,
  bidMoney)
• pre bid.currency
• self.bids-gtfirst().amount.currency
• pre bid gt 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.

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Implementing Preconditions

• if (bid.getAmount().getCurrency() !
• ((Bid) bids.first()).getAmount()
• .getCurrency())
• throw new IllegalArgumentException(
• currencies must match)

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Assertion Temptation

• // Dont 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

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OCL Comments

• context AuctionplaceBid(bidderId,
  bidMoney)
• -- currencies must match
• pre bid.currency
• self.bids-gtfirst().amount.currency
• pre alwaysTrue 1 1
• -- and // are valid comment forms in OCL

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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?

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More Preconditions

• context AuctionwinningBid()Bid
• -- auction must end to have a winner
• pre TimeStampnow() gt self.expires()
• context Bidgt(bidBid)Boolean
• -- _gt is our name for gt
• pre not bid.oclIsUndefined()

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Invariants

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

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Invariants

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

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Example Invariants

• context Auction
• inv self.expires() gt self.start
• context Bid
• inv self.amount.amount gt 0.0
• / Note this could be specified in
• the Auction context, but dont. /

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Postconditions

• Postconditions are the conditions that your
  operation guarantees will be true when it has
  completed.
• The contract You meet the preconditions and Ill
  meet the postconditions.

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PostConditions

• context AuctionplaceBid(bidderId,
• bidMoney)
• post self.currentBid().amount bid
• Notice that the postcondition assumes serial or
  single-threaded execution. This postcondition
  isnt necessarily enforceable in an unprotected
  multi-threaded context.

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PostConditions

• context AuctionplaceBid(bidderId,
• bidMoney)
• post self.currentBid() gt
• self.currentBid_at_pre()
• Postconditions can reach back in time.
• _at_pre references the value before the operation.
• Another way to state a precondition

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Implementing PostConditions

• public void placeBid(Id bidder,
• Money bid)
• Bid preCurrentBid this.currentBid()
• if (this.currentBid() lt preCurrentBid)

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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.

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OCL Topics

• Object Constraint Language (OCL)
• OCL for Design by Contract
• OCL for Queries
• Another OCL Example

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OCL as Query Language

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

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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.

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Example Test Data Set

• Heres some example test result data from a
  logician competition.

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Derived Values

• context TestResultcompositeScore Integer
• derive (2 logicScore) illogicScore

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OCL Collection Operations

• context TestSessionnumberOfResults
  Integer
• derive results-gtsize()

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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

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OCL Collections Classification
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OCL select() Query

• context TestSessionnumberOfScoresOverTarget(
  target Integer) Integer
• body results-gtselect( compositeScore gt
  target)-gtsize()

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Translating OCL Queries

• Lets look at what a transformation tool might do
  with one of our queries in SQL.
• SELECT COUNT() FROM test_results WHERE
  composite_score gt target

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Returning a Collection

• context TestSessiontestResultsForTeam( testTeam
  Team) Set(TestResult)
• body results-gtselect(team testTeam)

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Building Up Queries

• context TestSessionaverageScoreForTeam( testTea
  m Team) Real
• body testResultsForTeam(testTeam).compositeScore
• -gtsum() / testResultsForTeam(testTeam)-gtsize()

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Which Logicians Won?

• context TestSessionbestResults()
  Set(TestResult)
• body results-gtselect(compositeScore
• results-gtsortedBy(compositeScore) -gtlast().compo
  siteScore)

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OCL Resources

• The Object Constraint Language, 2edJos Warmer
  and Anneke Kleppe
• Klasse Objectens OCL pagehttp//www.klasse.nl/oc
  l/
• OCL 2.0 Specification at OMGhttp//www.omg.org/cg
  i-bin/doc?ptc/2003-10-14

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Questions and Answers