PA103 - Object-oriented Methods for Design of Information Systems
OCL – Object Constraint Language
© Radek Ošlejšek
Fakulta informatiky MU
oslejsek@fi.muni.cz
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 2Spring 2015
Literature
 The Object Constraint Language
(Second Edition)
 Author:J. Warner, A, Kleppe
 Publisher: Addison-Wesley Professional
 Copyright: 2003
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 3Spring 2015
Lecture 3 / Part 1:
Introduction to OCL
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 4Spring 2015
History
 First developed in 1995 as IBEL by IBM's Insurance division
for business modeling.
 IBM proposed it to OMG's call for an object-oriented
analysis and design standard. OCL was then merged into
UML 1.1.
 OCL was used to define UML 1.2 itself (constraints in UML
meta-models)
 UML specification:
http://www.omg.org/spec/UML/2.4.1/Infrastructure/PDF
 Example: p. 125
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 5Spring 2015
UML Metamodels – UC Diagram
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 6Spring 2015
UML Metamodels – UC Diagram (cont.)
Generalization
discriminator
Relationship
Include
Relationship
Actor
Classifier
UseCase
Classifier
Extend
Relationship
Open file by
typing name
Open file by
browsing
Open file
System
Administrator
Browse for file
Ordinary User
Attempt to open file
that does not exist
«extend» «include»
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 7Spring 2015
Why use OCL?
 UML diagrams are not enough. We need a language to help
with specification and semantics of UML models.
 We look for some “add-on” instead a brand new language
with full specification capability.
 Q: Why not first order logic? A: Not object-oriented.
 OCL is not the only one, but is the only one that is
standardized (OMG standard).
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 8Spring 2015
Advantages of Formal Constraints
Better documentation
 Constrains add information about model elements and their
relationships to the visual models used in UML.
 It is a way of documenting UML models.
More precise
 OCL constrains have formal semantics. Can be used to reduce the
ambiguity in the UML models.
 No side effects.
 Evaluation of OCL cannot affect state of the running system.
 It is not possible to assign values to attributes via OCL expression.
Communication without misunderstanding
 UML models are used to communicate between developers. Using
OCL constraints modelers communicate unambiguously.
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 9Spring 2015
Where use OCL?
 To specify invariants for classes and types.
 Constraint that must be always met by all instances of the class.
 To specify pre- and post-conditions of an operation.
 Constraint that must be always true before/after the execution of
the operation
 As a navigation language.
 Syntax constructs enabling to navigate through object links.
 Test requirements and specifications.
 OCL expressions can be bound to any model element in UML.
 Constrains may be denoted within the UML model or in a separate
document
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 10Spring 2015
Example 1
Company * 1..*
employer employee
Person
boss
0..1
worker *
is in charge of
{Person.employer =
Person.boss.employer}
invariantinvariant
Company * 1..*
employer employee
Person
boss
0..1
worker *
is in charge of
Boss cannot be in charge of employees from other company.
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 11Spring 2015
Example 2 (cont.)
1. A person may have a mortgage only on a house he/she owns.
context Mortgage inv: security.owner = borrower
2. The start date of any mortgage must be before the end date.
context Mortgage inv: startDate < endDate
3. The social security number of all persons must be unique.
4. A new mortgage will be allowed only when the person's income is sufficient
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 12Spring 2015
OCL is strongly typed language
 Well-defined OCL expressions have to satisfy type rules
 e.g. it is not allowed to compare Integer and String
 Every classifier from UML model becomes OCL type
 e.g. all classes from class diagram
 OCL predefines several basic types and collections
 Note: OCL is declarative language
 Note: text staring with two dashes „--“ is a comment
 -- this is uninterpreted comment in OCL
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 13Spring 2015
Reference Model
Airport
Flight
Passenger
Airline
*
*
*
*
$minAge: Integer
age: Integer
needsAssistance: Boolean
departTime: Time
/arrivalTime: Time
duration : Interval
maxNrPassengers: Integer
origin
desti-
nation
name: String
name: String
{ordered}
arriving
Flights
departing
Flights
CEO
0..1
flights
passengers
book(f : Flight)
0..1
airline
airline
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 14Spring 2015
Lecture 3 / Part 2:
Constraints (invariants)
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 15Spring 2015
Constrains, Contexts and Self
 Constraint (invariant) is a boolean OCL expression, evaluates
to true/false.
 Context links OCL constraint to specific type (class, association
class, interface, etc.) in the UML model.
 Context object may be denoted within the expression using the
keyword 'self'.
 'self' is implicit in all OCL expressions
 Similar to 'this' in C++ or Java
 Constraint should have a name followed by the 'invariant' or
'inv:' keyword
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 16Spring 2015
Context Notation
 Constraint may be denoted within the UML model or in a separate
document.
 Expression:
 context Flight inv: self.duration < 4
 is identical to:
 context Flight inv: duration < 4
 is identical to:
Flight
duration: Integer{ duration < 4 }
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 17Spring 2015
Elements of an OCL expression
 Basic types:
 Boolean (true, false),
 ops: and, or, xor, not, implies, if … then … else … endif
 Integer (1, -5, 2, 34, 26524, …),
 ops: *, +, -, /, abs
 Real (1.5, 3.14, …),
 ops: *, +, -, /, floor
 String ('To be or not to be...’),
 ops: toUpper, concat, …
 Classifiers from UML models and their features
 Attributes
 Query operations
 Associations from UML models
 Including role names at either end of an association
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 18Spring 2015
Airport
Flight
Passenger
Airline
*
*
*
*
$minAge: Integer
age: Integer
needsAssistance: Boolean
departTime: Time
/arrivalTime: Time
duration : Interval
maxNrPassengers: Integer
origin
desti-
nation
name: String
name: String
{ordered}
arriving
Flights
departing
Flights
CEO
0..1
flights
passengers
book(f : Flight)
0..1
airline
airline
Invariants with Basic Types
context Airline inv:
name.toLower = ‘klm’
context Passenger inv:
age >= ((9.6-3.5)*3.1).floor implies mature = true
Some crazy invariants:
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 19Spring 2015
Airport
Flight
Passenger
Airline
*
*
*
*
$minAge: Integer
age: Integer
needsAssistance: Boolean
departTime: Time
/arrivalTime: Time
duration : Interval
maxNrPassengers: Integer
origin
desti-
nation
name: String
name: String
{ordered}
arriving
Flights
departing
Flights
CEO
0..1
flights
passengers
book(f : Flight)
0..1
airline
airline
Invariants on Attributes
context Flight inv:
self.maxNrPassengers <= 1000
context Passenger inv:
age >= Passenger.minAge
Normal attributes: Class (static) attributes:
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 20Spring 2015
Invariants with Query Operations
context Flight inv:
self.departTime.difference(self.arrivalTime).
equals(self.duration)
-- Flight duration is just the difference between
-- arrival and departure time.
-- Invariant have to be boolean.
Time
difference(t:Time):Interval
before(t: Time): Boolean
plus(d : Interval) : Time
midnight: Time
month : String
day : Integer
year : Integer
hour : Integer
minute : Integer
Interval
equals(i:Interval):Boolean
Interval(d, h, m : Integer) :
Interval
nrOfDays : Integer
nrOfHours : Integer
nrOfMinutes : Integer
Flight
departTime: Time
/arrivalTime: Time
duration : Interval
maxNrPassengers: Integer
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 21Spring 2015
Navigation over Association Ends
 Navigation over associations is used to refer to associated objects, starting
from the context object:
context Flight
inv: origin <> destination
inv: origin.name = 'Amsterdam'
context Flight
inv: airline.name = 'KLM'
If the role name is missing use class
name at the other end of the association,
starting with a lowercase letter.
Preferred: Always give role names.
Airport
Flight
*
*
departTime: Time
/arrivalTime: Time
duration : Interval
maxNrPassengers: Integer
origin
desti-
nation
name: String
arriving
Flights
departing
Flights
10..* Airline
name: String
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 22Spring 2015
Navigation to Association Classes
 Association classes have no role names. OCL expression therefore has to use
class name, starting a lowercase letter:
context Person inv:
if self.name = ‘Ivan Hrozny’ then
job.type = #trainer
else
job.type = #programmer
endif
-- Ivan Hrozny is trainer, other employees are programmers
Company
Job
* 1
employee employer
type : {trainer, programmer}
name : String
Person
name : String
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 23Spring 2015
Lecture 3 / Part 3:
Collections
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 24Spring 2015
OCL Collections
 Most navigations return collections rather than single elements
0..*Passenger
passengers
10..*Flight Airplane
flights
context Flight inv: airplane. --> single airplane
context Airplane inv: flights. --> collection of flights
context Airplane inv: flights.passengers --> ???
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 25Spring 2015
OCL Collections (cont.)
 Set (non-ordered, no duplicities)
context Airport inv: self.arrivingFlights
 Bag (non-ordered, duplicities)
context Airport inv: self.arrivingFlights.passengers.name
 Sequence (ordered, duplicities)
context Flight inv: self.passengers
Airport
Flight
Passenger
*
*
*
departTime: Time
/arrivalTime: Time
duration : Interval
maxNrPassengers: Integer
origin
desti-
nation
name: String
{ordered}
arriving
Flights
departing
Flights
passengers
{unique}
{unique}
name: String
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 26Spring 2015
The collect Operation
 Syntax:
 collection->collect(elem : T | expr)
 collection->collect(elem | expr)
 collection->collect(expr)
 collection.expr -- abbreviated syntax
 The collect operation returns a bag containing the value of the
expression expr for each of the items in the collection.
 For instance, it is used to get all values for certain attribute of all
objects in a collection.
 Similar to projection in relational algebra (SQL).
Arrow ”->” is used for
predefined operations
instead
of the dot ”.” operator
Here can be an arbitrary
name of the collection
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 27Spring 2015
Example: collect Operation
Airport Flight Airline
*
*
*
arriving Flights
departing Flights
flights airline
context Airport inv:
self.arrivingFlights -> collect(airline) -> notEmpty
airp1
airp2
f1
f2
f3
f4
f5
airline1
airline2
airline3
departing flights
arriving flights
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 28Spring 2015
The select and reject Operations
 Syntax:
 collection->select(elem : T | expr)
 collection->select(elem | expr)
 collection->select(expr)
 The select operation results in the subset of elements for which expr is
true.
 Similar to selection in relational algebra (SQL).
 reject is the complementary operation to select.
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 29Spring 2015
Example: select Operation
Airport Flight Airline
*
*
*
arriving Flights
departing Flights
flights airline
duration: Interval
departTime: Time
context Airport inv:
self.departingFlights->select(duration<4)->notEmpty
departing flightsarriving flights
airp1
airp2
airline1
airline2
airline3
f5
duration = 2
f1
duration = 2
f4
duration = 5
f2
duration = 5
f3
duration = 3
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 30Spring 2015
The forAll Operation
 Syntax:
 collection->forAll(elem : T | expr)
 collection->forAll(elem | expr)
 collection->forAll(expr)
 The forAll operation results in true if expr is true for all elements of the
collection.
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 31Spring 2015
Example: forAll Operation
context Airport inv:
self.departingFlights->forAll(departTime.hour>6)
departing flights arriving flights
airp1
airp2
airline1
airline2
airline3
f5
depart = 8
f1
depart = 7
f4
depart = 9
f2
depart = 5
f3
depart = 8
Airport Flight Airline
*
*
*
arriving Flights
departing Flights
flights airline
duration: Interval
departTime: Time
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 32Spring 2015
forAll Operation with two variables
context Airport inv:
self.departingFlights->forAll(f1, f2 |
f1.departTime <> f2.departTime)
-- all flights differ in their departure time
Airport Flight Airline
*
*
*
arriving Flights
departing Flights
flights airline
duration: Interval
departTime: Time
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 33Spring 2015
The exists Operation
 Syntax:
 collection->exists(elem : T | expr)
 collection->exists(elem | expr)
 collection->exists(expr)
 The exists operation results in true if there is at least one element in the
collection for which the expr is true.
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 34Spring 2015
Example: exists Operation
context Airport inv:
self.departingFlights->exists(departTime.hour<6)
departing flights arriving flights
airp1
airp2
airline1
airline2
airline3
f5
depart = 8
f1
depart = 7
f4
depart = 9
f2
depart = 5
f3
depart = 8
Airport Flight Airline
*
*
*
arriving Flights
departing Flights
flights airline
duration: Interval
departTime: Time
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 35Spring 2015
The iterate Operation
 Syntax:
 collection->iterate(elem : Type; answer : Type = <value> |
<expression-with-elem-and-answer>)
 Example:
 is identical to:
context Airline inv:
flights->iterate (f : Flight;
answer : Set(Flight) = Set{ } |
if f.maxNrPassengers > 150 then
answer->including(f)
else
answer
endif)->notEmpty
context Airline inv:
flights->select(maxNrPassengers > 150)->notEmpty
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 36Spring 2015
Other collection Operations
 c->isEmpty: true if collection has no elements.
 c->notEmpty: true if collection has at least one element.
 c->size: number of elements in collection.
 c->sum: summation of numerical elements in collection.
 c->count(elem): number of occurrences of elem in collection.
 c->includes(elem): true if elem is in collection.
 c->excludes(elem): true if elem is not in collection.
 c->includesAll(coll): true if every element of coll is found in c.
 c->excludesAll(coll): true if no element of coll is found in c.
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 37Spring 2015
Other collection Operations (cont.)
For bags and sequences:
 c-> asSet: transforms bag or sequence collection to set, i.e. removes
duplicities.
For sets:
 s1->intersection(s2): returns set of those elements found in s1 and also in s2.
 s1->union(s2): returns set of those elements found in either s1 or s2.
 s1->excluding(x): returns s1 with object x omitted.
For sequences:
 s->first()
Predefined set-theoretic predicates:
 size(set), sum(set), max(set), average(set), ...
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 38Spring 2015
Exercise
What returns self.capacity for context Course?
What returns self.teacher for context Course?
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 39Spring 2015
Exercise (cont.)
What returns self.courses for context Teacher?
What returns self.courses->first().capacity for context Teacher?
What returns self.courses.capacity for context Teacher?
What is the result of:
context Teacher inv: self.courses->size() > 0
What is the result of:
context Teacher inv: max(self.courses.capacity) < 150
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 40Spring 2015
Lecture 3 / Part 4:
Advanced OCL Concepts
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 41Spring 2015
Pre- and Post-conditions
Airport
Flight
*
*
departTime: Time
/arrivalTime: Time
duration : Interval
maxNrPassengers: Integer
origin
desti-
nation
name: String
arriving
Flights
departing
Flights
servedAirports()
context Flight::servedAirports() : Set(Airport)
pre : -- none
post: result = flights.destination->asSet
Context of a method
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 42Spring 2015
@pre in Post-conditions
context Passenger::book(f : Flight)
pre : not flights->include(f)
post: flights->include(f) and
getTotalPrice() = getTotalPrice@pre() + f.ticketPrice()
-- getTotalPrice() include the price of booked flight
Flight
Passenger
*
age: Integer
needsAssistance: Boolean
departTime: Time
/arrivalTime: Time
duration : Interval
maxNrPassengers: Integer
{ordered}passengers
book(f : Flight)
getTotalPrice(): Float
* {ordered}flights
 @pre refer to the previous value
of some property
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 43Spring 2015
Reference to State
context Bottle inv:
self.oclInState(just_bought) implies filled = #full
empty half full
pour into pour into
pour out pour out
Bottle
filled : enum {empty, half, full}
 oclInState returns true if the object is in the specified state
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 44Spring 2015
Local Variables
context Airport inv:
let supportedAirlines : Set (Airline) =
self.arrivingFlights->collect(airline)
in (supportedAirlines->notEmpty)
and (supportedAirlines->size < 500)
 let construct defines variables local to one constraint:
 Let var : Type = <expression1> in <expression2>
Airport
Flight
Airline
*
*
*
departTime: Time
/arrivalTime: Time
duration : Interval
maxrPassengers: Integer
origin
desti-
nation
name: String
name: String
arriving
Flights
departing
Flights
flights
airline
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 45Spring 2015
Local Variables – Advanced Example
context Person inv:
let income : Integer = self.job.salary->sum
let hasTitle(t:String) : Boolean = self.job->exists(title=t) in
if self.hasTitle(‘manager’) then
self.income >= 1000
else
self.income >= 100
endif
Company * 1..*
employer employee
Person
Job
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 46Spring 2015
Inheritance of Constrains
Inheritance principle:
 Whenever an instance of a class is expected, one can always substitute an
instance of any of its subclasses
Consequences for invariants:
 An invariant is always inherited by each subclass.
 Subclasses may strengthen the invariant.
Consequences for pre- and post-conditions:
 A precondition may be weakened (contravariance) in subclass
 context SuperClass::foo(i : Integer) pre: i > 1000
 context SubClass::foo(i : Integer) pre: i > 0
 => ok, because the sub-class is able to process the same input values as
its super-class.
 A postcondition may be strengthened (covariance) in subclass
 context SuperClass::foo() : Integer post: result > 0
 context SubClass::foo() : Integer post: result > 1000
 => ok, because a caller gets always number > 0, even from the sub-class
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 47Spring 2015
Type Checking and Casting Operations
 oclType represents the type of „self“ object
 oclIsTypeOf(t : OCLType) returns true if „self“ and „t“ are of the same type.
context AirBus inv: self.oclIsTypeOf(Airplane) -- is false
 oclIsKindOf(t : OCLType) returns true if „self“ and “t” are of the same type or if „t“ is supertype
of „self“.
context AirBus inv: self.oclIsKindOf(Airplane) -- is true
context Airline inv:
self.airplane->select(oclType = Airbus)->notEmpty
-- Every airline has at least one Airbus in its airplane fleet.
-- Hide this constraint from EU representatives as they could
adopt this idea ;)
Airline Airplane
*
airline airplane
*
AirBus
Boing
PA103: OO Methods for Design of Information Systems © R. Ošlejšek, FI MU 48Spring 2015
Questions?