The Object Constraint Language OCL The Object Constraint Language

Content HISTORY COLLECTION TYPES AND QUERING IN OCL FORMAL LANGUAGE - STATEMENT EXAMPLES CONSTRAINTS AND OCL EXAMPLES

History Formal Languages like Z, have for a long time (ca. 1970) been used to describe things in a precise and unambiguous way. OCL was developed by IBM (1995) and is a part of UML. It emphasize precision and simplicity. There is no use of special mathematical symbols.

Aligning OCL with the OO-Concepts Level Class Property Ass. OCL 2 (metamodel) the class concept the property concept association concept an OCL construct as a concept 1 a specific class a specific property a specific association OCL Formulas an object of a class a slot with value a link between objects a formula instantiated

Content HISTORY COLLECTION TYPES AND QUERING IN OCL FORMAL LANGUAGE - STATEMENT EXAMPLES CONSTRAINTS AND OCL EXAMPLES

Some Collection Types in OCL A Set is the mathematical set (no duplicate elements). A Bag is like a set, which may contain duplicates (i.e., the same element may be in a bag twice or more). A Sequence is like a Bag in which the elements are ordered. Both Bags and Sets have no order defined on them.

model instance (example) Self [2]: ”Each OCL expression is written in the context of an instance of a specific type. In an OCL expression the name self is used to refer to the contextual instance.” context Gang inv: self.member… member Gang GangMember 1..* 0..* model instance (example) When self is A:Gang then self.member is: Set{@M1,@M2} :Set(GangMember) self.member self M1:GangMember A:Gang M3:GangMember M2:GangMember B:Gang M4:GangMember M5:GangMember

One More Navigation Example Model alternative Quiz Question AnswerAlternative * * * 1..* contex Quiz inv: self.question.alternative… incomplete constraint self self.question self.question.alternative quiz2:Quiz q3:Question a4:AnswerAlternative When the complete constraint is checked, then self is bound to one Quiz object at a time and then the constraint is checked for that object. self self.question self.question.alternative What correspond to self in Java? Example function as quering – not constraint becaouse? quiz1:Quiz q1:Question a1:AnswerAlternative a2:AnswerAlternative q2:Question a3:AnswerAlternative

Example… Defining classes – content of file gang1.use: model Gangs class Gang attributes name : String isArmy : Boolean end class GangMember -- associations association Membership between Gang[1] role gang GangMember[1..*] role member After opening of specification (gang1.use) in the tool and having created a class diagram view:

content of command file gang1_objects.cmd: The Example continues… We need to define some objects to “navigate” – content of command file gang1_objects.cmd: -- Create objects: -- Gang A !create a:Gang !set a.name := 'A' !set a.isArmy := false -- Member m1 !create m1: GangMember !set m1.name := 'M1' -- Member m2 !create m2: GangMember !set m2.name := 'M2' -- Membership !insert (a,m1) into Membership !insert (a,m2) into Membership -- Gang B !create b:Gang !set b.name := 'B' !set b.isArmy := false -- Member m3 !create m3: GangMember !set m3.name := 'M3' -- Member m4 !create m4: GangMember !set m4.name := 'M4' -- Member m5 !create m5: GangMember !set m5.name := 'M5' !insert (b,m3) into Membership !insert (b,m4) into Membership !insert (b,m5) into Membership After running command file (gang1_objects.cmd) and the creation of an object diagram view:

The Example continues… Using OCL as a Query Language: Finding all members no matter which Gang they belong to OCL is case sensitive! Select in menu! To display the OCL Evaluate Window: State/OCL Evaluate OCL expression.

Single navigation of an association results in a Set. Alternative way to find all members no matter which Gang they belong to Single navigation of an association results in a Set. Combined navigations results in a Bag

Allowing a Member to be Member of Several Gangs belonging to two gangs

Allowing a Member to be Member of Several Gangs belonging to two gangs

asSet

Finding all members of the B-gang

Content HISTORY COLLECTION TYPES AND QUERING IN OCL FORMAL LANGUAGE - STATEMENT EXAMPLES CONSTRAINTS AND OCL EXAMPLES

Logical implication – p implies q p  q If p is true then also q must be true. “p  q” is called a predicate A predicate is a statement that may be true or false depending on the values of its variables.

Universal Quantification:  In predicate logic, universal quantification formalizes the notion that a logical predicate is true for all the values that can be bound to the variable. E.g.: a  Integer | (a > 10)  (a + 1 > 10) Can be read as: For all a of type Integer the following is true: if a is greater then 10 then a +1 is also greater then 10 Also: a  Integer | a-1 < a

Example: Transitivity In logic and mathematics, a binary relation R is transitive if xRy and yRz together imply xRz In predicate logic: a,b,c  X | a R b  b R c  a R c Less than (<) is a transitive relation: a,b,c  Integer | a < b  b < c  a < c In english: For all Integer a, b, c the following is true if a < b and b < c then also a < c (e.g., 1<2  2<4  1<4) Is mother of a transitive relation? quantifier  is called the universal quantifier and it stands for “all” or “for all”.  is the existential quantifier and it stands for exists.

Content HISTORY COLLECTION TYPES AND QUERING IN OCL FORMAL LANGUAGE - STATEMENT EXAMPLES CONSTRAINTS AND OCL EXAMPLES

Constraint [1]: … condition or restriction represented as an expression…. can be attached to any UML model element…. indicates a restriction that must be enforced by correct design of the system… assertion uttales: assersjen A constraint is an assertion, not an executable mechanism.

Why OCL? The power of the graphical part of UML is limited! OCL gives you power to improve the documentation in a precise and unambiguous way. OCL parsers (/ evaluators) ensure that the constraints are meaningful and well formed within the model. To do MDD the models have to be unambiguous (e.g., when doing code generation, transformations). OCL give you power to improve the documentation in a precise (OCL is a formal language) and unambiguous way (in contrast to natural language).

Where To Use OCL? Invariants on classes and types Precondition and postconditions on operations (methods) Constraints on operations: operation=expression (the return value) …

Content HISTORY COLLECTION TYPES AND QUERING IN OCL FORMAL LANGUAGE - STATEMENT EXAMPLES CONSTRAINTS AND OCL EXAMPLES

An Invariant Example: Use of Size An invariant is predicate that is always true (i.e., when the system is at “rest”.) Gang name : String GangMember +member isArmy : Boolean 1..* 1..* 0..* 0..* register(newMember : GangMember) -- The gang is an army if there are more than 100 members. context Gang inv oneSimpleConstraint: self.isArmy = (member->size() > 100)

Use of pre –and post-condition Gang name : String GangMember +member isArmy : Boolean 1..* 1..* 0..* register(newMember : GangMember) -- The gang has grown with one when a new member has been added. -- NB! @pre accesses the value before executing the operation context Gang :: register(newMember : GangMember) pre: not member->includes(newMember) post : member = member@pre->including(newMember)

Comments to Previous Slide collection->includes(object: T): Boolean is True if object is an element of collection. E.g.: Set{@m1,@m2}->includes(@m2) = true p@pre refers to the value of p before the operation was executed. E.g.: member@pre is the set member before the operation was executed. collection->including(object: T): Bag(T) is the bag with all elements of collection plus object. E.g.: Set{@m1,@m2}->including(@m3) = Set{@m1,@m2,@m3}.

model xor class Account end class Person class Corporation association personOwner between Account[0..*] role account Person[0..1] role personOwner association cooperationOwner between Corporation[0..1] role cooperationOwner constraints context Account inv : personOwner->size() = 1 implies cooperationOwner->size() = 0 Using OCL in USE What we want: An account has either a person as owner or a cooperation as owner { The Constraint is part of the specification

Create Some Objects -- Create objects: !create a2801: Account !create Harald: Person !create UiA: Corporation -- Create link: !insert (a2801, Harald) into personOwner

What is this? This should not be allowed – this is not xor!

Modify the Constraint: context Account inv: (personOwner->size() = 1 implies cooperationOwner->size() = 0) and (personOwner->size() = 0 implies cooperationOwner->size() = 1)

forAll Variations (math. notation: ) employee Person Company * * age : Integer { context Company inv: self.employee->forAll( age <= 70 ) self.employee->forAll( p | p.age <= 70 ) self.employee->forAll( p : Person | p.age <= 70 ) Equivalent Constraints

Example: exists (math. notation: ) collection  exists(exp : OclExpression) : Boolean True if exp is true for one elements of the collection -- Every Gang must have a member with a car contex Gang inv: member->exists(car->size()>0)

It is possible to use the Evaluate OCL Expression dialog box to express invariants But you must transform it to an expression…