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Object Constraint Language

Object Constraint Language. The OCL is a language of typed expressions. An OCL expression is valid if it is written according to the rules (formal grammar) of OCL. A constraint is a valid OCL expression of type Boolean.

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Object Constraint Language

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  1. Object Constraint Language • The OCL is a language of typed expressions. • An OCL expression is valid if it is written according to the rules (formal grammar) of OCL. • A constraint is a valid OCL expression of type Boolean. • A constraint is a restriction on one or more values of (part of) an object-oriented model or system.

  2. Constraint examples self.transaction -> forAll(t:Transaction | t.value > 100) Account Transaction 1 0..* SuperSaverAccount self.balance > 0

  3. Design by contract put (element: T, key: STRING) is -- insert element with given key require count < capacity do .. insertion algorithm ... ensure count <= capacity; item (key) = element; count = old count + 1 end --put

  4. The requirements of OCL 1. The OCL must enable us to express extra, necessary, information on object models. 2. The OCL must be a precise and unambiguous language that can be easily read by developers and customers. 3. The OCL must be a declarative language, its expressions can have no side-effects. 4. OCL must be a typed language so that OCL expressions can be type checked for correctness.

  5. Types in OCL • The types in OCL are as follows: • Predefined types • Basic types - Integer, Real, String and Boolean • Collection types - Collection, Set, Bag, Sequence • Meta types • OclAny, OclExpression, OclType • User-defined model types • Enumeration and all classes, types and interfaces

  6. The context of an OCL expression VocabularyElement 0..5 Hint Vocabulary Element self.hintCount <= 5 and self.hintCount >= 0

  7. Real and Integer

  8. Multiplicity constraints Vocabulary VocabElement Hint 1 0..* 1 0..5 Equivalent constraints expressed on the classes VocabElementself.hint -> size >= 0 and self.hint -> size <= 5 VocabElementself.vocabulary -> size = 1 Hintself.vocabElement -> size = 1

  9. String

  10. Using Enumerations Customer gender: enum{male, female}name: Stringtitle: StringdateOfBirth: Date gender = #male implies title = ‘Mr. ‘

  11. Boolean

  12. Simple boolean constraints Customer name: Stringtitle: Stringage: IntegerisMale: Boolean title = if isMale then ‘Mr.’ else ‘Ms.’ endif age >= 18 and age < 66 name.size < 100

  13. The implication operation Probably the most counter-intuitive logical operator. A B A implies B T T T T F F F ? T Torches with broken bulbs?

  14. Operations on Collections

  15. Collection hierarchy Collection Set Bag Sequence minussymmetricDifferenceasSequenceasBag asSequenceasSet firstlastat(int)appendprependasBagasSet

  16. Navigating Associations Account Transaction 1 0..* self.transaction returns a set of transactions Book Member borrower self.borrower returns a set of members

  17. Navigating to collections Customer Account Transaction Customerself.account produces a set of Accounts Customer self.account.transaction produces a bag of transactions If we want to use this as a set we have to do the following self.account.transaction -> asSet

  18. The subset constraint Flight Person pilot crew 1..* flightAttendant 0..* Flightself.crew -> includes( self.pilot ) Flightself.crew -> includesAll(self.flightAttendants)

  19. Iteration over collections Account Transaction Accountself.transactions -> select( value > 500 ) Accountself.transactions -> reject( not(value > 500 )) Accountself.transactions -> sum(collect ( value ))

  20. Abstract classes The fact that the Hint class is abstract can be expressed this way. Hint Hint.allInstances -> select(oclType = Hint) -> isEmpty This constraint uses some of the meta facilities which I have mostly glossed over so far.

  21. OCL references Warmer, J. Kleppe, A. The Object Constraint Language: Precise Modeling with UML Addison-Wesley 1999 Object Constraint Language Specification, version 1.1 OMG document ad970808 The OCL parser can be found at http://www.software.ibm.com/ad/ocl The Klasse Objecten web site is http://www.klasse.nl/Engels/ocl.htm

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