Object Constraint Language (OCL)

1
Object Constraint Language(OCL)

• Yih-Kuen Tsay
• Dept. of Information Management
• National Taiwan University

2
Outline

• Introduction
• Relation with UML Models
• Basic Values and Types
• Objects and Properties
• Collection Operations

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About OCL

• A formal language for writing expressions (such
  as invariants) on UML models.
• It can also be used to specify queries.
• OCL expressions are pure specifications without
  side effects.
• OCL is a typed language.
• Version 2.0 in the finalization process.

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Why OCL

• UML diagrams do not provide all the relevant
  aspects of a specification.
• Additional constraints expressed by a natural
  language are ambiguous.
• Traditional formal languages are precise, but
  hard to use.
• OCL tries to be formal and yet easy to use.

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Where to Use UML

• As a query language
• Invariants on classes and types.
• Pre- and post-conditions on operations
• Guards
• Target sets for messages and actions
• Constraints on operations
• Derivation rules for attributes

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Basic Form

• context TypeName inv
• 'this is an OCL expression with stereotype
  ltltinvariantgtgt in the
• context of TypeName' 'another string'

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Class Diagram Example
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Relation with UML Models

• Each OCL expression is written in the context of
  an instance of a specific type.
• The reserved word self is used to refer to the
  contextual instance.
• An explicit context declaration can be omitted if
  the OCL expression is properly placed in a
  diagram.

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Invariants

• Inside the class diagram example
• self.numberOfEmployees gt 50
• specifies that the number of employees must
  always exceed 50.
• Alternatively,
• context Company inv
• self.numberOfEmployees gt 50
• A different name can be used for self
• context c Company inv
• c.numberOfEmployees gt 50
• The invariant itself can also be given a name
  (after inv).

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Pre- and Post-Condtions

• Basic form
• context TypenameoperationName(param1 Type1,
  ... ) ReturnType
• pre param1 gt ...
• post result ...

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Pre- and Post-Condtions (cont.)

• Example
• context Personincome(d Date) Integer
• post result 5000
• Names may be given
• context TypenameoperationName(param1 Type1,
  ... ) ReturnType
• pre parameterOk param1 gt ...
• post resultOk result ...

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Package Context

• package PackageSubPackage
• context X inv
• ... some invariant ...
• context XoperationName(..)
• pre ... some precondition ...
• endpackage

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Operation Body Expression

• context PersongetCurrentSpouse() Person
• pre self.isMarried true
• body self.mariages-gtselect( m m.ended false
  ).spouse

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Initial and Derived Values

• context Personincome Integer
• init parents.income-gtsum() 1 -- pocket
  allowance
• derive if underAge
• then parents.income-gtsum() 1 -- pocket
  allowance
• else job.salary -- income from regular job
• endif

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Basic Types

• Boolean true, false
• Integer 1, -5, 2, 34, 26524, ...
• Real 1.5, 3.14, ...
• String 'To be or not to be...'

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Basic Operations

• Integer , , -, /, abs()
• Real , , -, /, floor()
• Boolean and, or, xor, not, implies, if-then-else
• String concat(), size(), substring()

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Other Types

• Classifiers
• All classifiers of a UML model are types in its
  OCL expressions.
• Enumerations

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Sub-expressions

• context Person inv
• let income Integer self.job.salary-gtsum() in
• if isUnemployed then
• income lt 100
• else
• income gt 100
• endif

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Definition Expressions

• context Person
• def income Integer self.job.salary-gtsum()
• def nickname String Little Red Rooster
• def hasTitle(t String) Boolean
  self.job-gtexists(title t)

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More about Types and Operations

• Type conformance
• Casting (re-typing)
• Precedence rules
• Infix operators
• Keywords
• Comments

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Properties

• A property is one of
• An Attribute
• context Person inv
• self.age gt 0
• An AssociationEnd
• An Operation with isQuery
• A Method with isQuery

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Previous Values in Post-Conditions

• context PersonbirthdayHappens()
• post age age_at_pre 1
• context CompanyhireEmployee(p Person)
• post employees employees_at_pre-gtincluding(p)
  and
• stockprice() stockprice_at_pre() 10

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Previous Values in Post-Conditions (cont.)

• a.b_at_pre.c -- takes the old value of property b of
  a, say x
• -- and then the new value of c of x.
• a.b_at_pre.c_at_pre -- takes the old value of property
  b of a, say x
• -- and then the old value of c of x.

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Collections

• Set
• Bag
• OrderedSet
• Sequence

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Collection Operations

• Select
• context Company inv
• self.employee-gtselect(age gt 50)-gtnotEmpty()
• Reject
• context Company inv
• self.employee-gtreject( isMarried )-gtisEmpty()

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ForAll Operation

• context Company
• inv self.employee-gtforAll( age lt 65 )
• inv self.employee-gtforAll( p p.age lt 65 )
• inv self.employee-gtforAll( p Person p.age lt
  65 )
• context Company inv
• self.employee-gtforAll( e1, e2 Person
• e1 ltgt e2 implies e1.forename ltgt e2.forename)

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Exists Operation

• context Company inv
• self.employee-gtexists( forename 'Jack' )
• UML OCL 2.0 Adopted Specification 29
• context Company inv
• self.employee-gtexists( p p.forename 'Jack' )
• context Company inv
• self.employee-gtexists( p Person p.forename
  'Jack' )

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Iterate Operation

• Reject, Select, forAll, Exists, and Collect can
  all be described in terms of interate.
• Example
• collection-gtcollect(x T x.property)
• -- is identical to
• collection-gtiterate(x T acc T2 Bag
• acc-gtincluding(x.property))