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1
The Object Constraint Language: Expressing Constraints in the UML
(Most slides created by Robert B. France, ProfessorDepartment of Computer Science, Colorado State University. Other material from: (Source: http://www.cse.dmu.ac.uk/~aoc/teaching-notes/Contents/CSCI3007/CSCI3007OCLtutorial.pdf )
2
What is OCL?
The Object Constraint Language (OCL) is a declarative language for describing rules that apply to UML models
OCL can be used to describe constraints
A constraint is a restriction on one or more values of a model or system.
A constraint is an expression that evaluates to true or false as a query language
Queries are expressions that evaluate to a value (true, false and other values)
Can be used to define new attributes and operations OCL expressions are always associated with a UML
model OCL expressions can be associated with any model element
in UML
3
Specifying Constraints
4
Different kinds of constraints Class invariant
a constraint that must always be met by all instances of the class
Precondition of an operation a constraint that must always be true BEFORE the
execution of the operation Postcondition of an operation
a constraint that must always be true AFTER the execution of the operation
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Specifying Constraints: Invariants
6
Note: self can be omitted
context Flightinv capacity: maxNrPassengers <= 1000
Example: Expressing Invariants Flight capacity constraint: The maximum number of passengers
that can be on a flight must be less than or equal to 1,000.
context Flightinv capacity: self.maxNrPassengers <= 1000
Airport
Flight
*
*
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
arrivingFlights
departingFlights
1
1
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Constraint context and self
Every OCL expression is bound to a specific context. The context is often the element that the
constraint restricts The context may be denoted within the
expression using the keyword ‘self’. ‘self’ is implicit in all OCL expressions Similar to ‘this’ in C++
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Notation Constraints may be denoted within the
UML model or in a separate document. the expression:
context Flight inv durationLimit: self.duration < 4
is identical to:context Flight inv: duration < 4
Flight
duration: Integerinv: duration < 4
is identical to:
9
Example model
Airport
Flight
Passenger
Airline
*
**
*
minAge: Integerage: IntegerneedsAssistance: Boolean
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
name: String
{ordered}
arrivingFlights
departingFlights
CEO
0..1
flights
passengers
book(f : Flight)
0..1
airline
airline
Time
difference(t:Time):Intervalbefore(t: Time): Booleanplus(d : Interval) : Time
$midnight: Timemonth : Stringday : Integeryear : Integerhour : Integerminute : Integer
Interval
equals(i:Interval):Boolean$Interval(d,h,m : Integer) : Interval
nrOfDays : IntegernrOfHours : IntegernrOfMinutes : Integer
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1
1
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Airport
Flight
Passenger
Airline
*
**
*
minAge: Integerage: IntegerneedsAssistance: Boolean
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
name: String
{ordered}
arrivingFlights
departingFlights
CEO
0..1
flights
passengers
book(f : Flight)
0..1
airline
airline
Time
difference(t:Time):Intervalbefore(t: Time): Booleanplus(d : Interval) : Time
$midnight: Timemonth : Stringday : Integeryear : Integerhour : Integerminute : Integer
Interval
equals(i:Interval):Boolean$Interval(d,h,m : Integer) : Interval
nrOfDays : IntegernrOfHours : IntegernrOfMinutes : Integer
11
1
1
• Constraint: The difference between the depart and arrival time for a flight must be the same as its duration.
context Flightinv duration:
self.departTime.difference(self.arrivalTime).equals(self.duration)
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Elements of an OCL expression In an OCL expression these elements may
be used: basic types: String, Boolean, Integer, Real. classifiers from the UML model and their features
attributes, and class attributes associations from the UML model
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OCL types
13
Precedence Rules
14
Some OCL Reserved Words
AND ATTR ELSE ENDIF IF IMPLIES INV LET NOT OR POST PRE THEN XOR
15
OCL Comments
Comments in OCL are written following two successive dashes “--”
-- this is a comment
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Airport
Flight
Passenger
Airline
*
**
*
minAge: Integerage: IntegerneedsAssistance: Boolean
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
name: String
{ordered}
arrivingFlights
departingFlights
CEO
0..1
flights
passengers
book(f : Flight)
0..1
airline
airline
Time
difference(t:Time):Intervalbefore(t: Time): Booleanplus(d : Interval) : Time
$midnight: Timemonth : Stringday : Integeryear : Integerhour : Integerminute : Integer
Interval
equals(i:Interval):Boolean$Interval(d,h,m : Integer) : Interval
nrOfDays : IntegernrOfHours : IntegernrOfMinutes : Integer
11
1
1
• context Airline inv: name.toLower = ‘usair’
• context Passenger inv: age >= ((9.6 - 3.5)* 3.1).floor implies mature = true
• NB: A B = ~A v B
Example: OCL basic types
17
Associations and navigations
Every association in the model is a navigation path.
The context of the expression is the starting point.
Role names are used to identify the navigated association.
18
Example: navigations
The name of the airline for a flight is Delta
context Flight
inv: airline.name = ‘Delta’
Airport
Flight
*
*
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
arrivingFlights
departingFlights
Airline
name: String
airline1 The origin of a flight must be different than its destination
context Flight
inv: origin <> destination
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Example: Invariant involving Enumerated Type
• The type of a flight must be the same as the type of the airplane.
{context Flightinv ftype = Ftype::cargo implies Airplane.atype = Ftype::cargoinv ftype = Ftype::passenger implies Airplane.atype = Ftype:: passenger}
1*Flight Airplane
ftype : Ftype atype : Ftypeflights
<<enumeration>>Ftype
cargopassenger
20
Association classes Persons working with IBM have the job type “trainer”
otherwise their job type is “programmer”.context Person inv:if employer.name = ‘IBM’ then
Job.type = JobType::trainerelse Job.type = JobType::programmerendif
Person Company
Job
* 1employee employer
type : JobType
name : String
JobType
trainerprogrammer
21
Specifying Constraints: Operation Specifications
22
Syntax for specifying operationscontext NameOfClass::operationName():returnTypepre : -- some expressionbody : -- some expressionpost : some expression
23
Pre- and PostCondition Example
A class named Account has an attribute balance and an operation overdraft() that returns true if the balance is less than 0 and false otherwise.
context Account::overdraft():Booleanpre : -- nonepost : result = (balance < 0)
24
More complex operation specifications
The operation birthdayOccurs() adds 1 to the customer age.
context Customer::birthdayOccurs()pre : -- nonepost : age = age@pre + 1
What does this describe?
context Account::safeWithdraw(amt:Integer)pre : balance > amtpost : balance = balance@pre - amt
25
Specifying Queries
26
Example model
Airport
Flight
Passenger
Airline
*
**
*
$minAge: Integerage: IntegerneedsAssistance: Boolean
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
name: String
{ordered}
arrivingFlights
departingFlights
CEO
0..1
flights
passengers
book(f : Flight)
0..1
airline
airline
Time
difference(t:Time):Intervalbefore(t: Time): Booleanplus(d : Interval) : Time
$midnight: Timemonth : Stringday : Integeryear : Integerhour : Integerminute : Integer
Interval
equals(i:Interval):Boolean$Interval(d,h,m : Integer) : Interval
nrOfDays : IntegernrOfHours : IntegernrOfMinutes : Integer
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1
1
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Derived Attribute & Initial Value Example
Defining derived attributes
context Flight::arrivalTime:Timederive:departTime.plus(duration)
Defining initial attribute value
context Flight::maxNrPassengers:Integerinit: 100
Defining initial association end value
context Flight::passengers:Set(Passenger)init: Set{}
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Query operation examplesReturn all the departing flights from a given airport
context Airport::departures():Set(Flight)body: result=departingFlights
Return all the airports served by an airline
context Airline::served():Set(Airport)body: result=flights.destination->asSet()
Airline
name: StringAirport
Flight
*
* *
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
arrivingFlights
departingFlights
flights
airline1
1
1
1
29
More Advanced OCL Expressions
30
Significance of collections in OCL Most navigations return collections rather
than single elements
10..*Flight Airplane
type : enum of cargo, passenger
type : enum of cargo, passenger
flights
31
Subtypes of Collection
1. Set: each element occurs only once (Non-ordered, unique)
2. OrderedSet: a set with ordered elements.
3. Bag: elements may be present more than once(non-ordered, non-unique)
4. Sequence: a bag with ordered elements (ordered, non-unique)
32
Subtypes of CollectionAirline
name: StringAirport
Flight
*
* *
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
arrivingFlights
departingFlights
flights
airline1
1
1
1
Passenger*
$minAge: Integerage: IntegerneedsAssistance: Boolean
{ordered}passengers
book(f : Flight)
Set: arrivingFlights - from the context Airport Non-ordered, unique
Bag: arrivingFlights.duration - from the context Airport Non-ordered, non-unique
Sequence: passengers - from the context Flight Ordered, non-unique
33
Collection operations
OCL has a great number of predefined operations on the collection types.
Syntax: collection->operation
Use of the “->” (arrow) operator instead of the“.” (dot) operator
34
The collect operation
Syntax:collection->collect(elem : T | expr)collection->collect(elem | expr)collection->collect(expr)
The collect operation results in the collection of the values resulting from evaluating expr for all elements in the collection
Shorthand often trips people up. Be Careful!
35
Example: collect operation
context Airport inv:
self.arrivingFlights -> collect(airLine) ->notEmpty
airp1
airp2
f1
f2
f3
f4
f5
airline1
airline2
airline3
departing flights arriving flights
All arriving flights must be associated with an airline
36
The select operation
Syntax:collection->select(elem : T | expression)
collection->select(elem | expression)
collection->select(expression)
The select operation results in the subset of all elements for which expression is true
37
Example: select operation
context Airport inv:
self.departingFlights->select(duration<4)->notEmpty
departing flights
arriving flights
airp1
airp2
airline1
airline2
airline3
f5duration = 2
f1duration = 2
f4duration = 5
f2duration = 5
f3duration = 3
There must be at least one departing flight whose duration is less than 4
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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
39
Example: forAll operation
context Airport inv:
self.departingFlights->forAll(departTime.hour>6)
departing flights arriving flights
airp1
airp2
airline1
airline2
airline3
f5depart = 8
f1depart = 7
f4depart = 9
f2depart = 5
f3depart = 8
All departing flights must leave after 6
40
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 expression expr is true.
41
Example: exists operationcontext Airport inv:
self.departingFlights->exists(departTime.hour<6)
departing flights arriving flights
airp1
airp2
airline1
airline2
airline3
f5depart = 8
f1depart = 7
f4depart = 9
f2depart = 5
f3depart = 8
42
Other collection operations
isEmpty: true if collection has no elements notEmpty: true if collection has at least one element size: number of elements in collection count(elem): number of occurences of elem in
collection includes(elem): true if elem is in collection excludes(elem): true if elem is not in collection includesAll(coll): true if all elements of coll are in
collection
43
Examples
A vehicle owner must be at least 18 years old.:context Vehicle inv: self.owner. age >= 18
44
A car owner must be at least 18 years old.:context Car inv: self.owner. age >= 18
Examples
45
Nobody has more than 3 vehicles:context Person inv: self.fleet->size <= 3
Examples
46
All vehicles of a person are black:context Person inv: fleet->forAll(v | v.colour = #black)
Examples
47
Nobody has more than 3 red vehicles:context Person inv: fleet->select(v | v.colour = #red)->size <= 3
Examples
48
context Personinv: age<18 implies self.fleet->forAll(v | not v.oclIsKindOf(Car))
A person younger than 18 owns no cars..
Examples
49
There is a red car in somebody’s fleet:context Person inv: fleet->select(v | v.colour = #red)->notEmpty
context Car inv: Car.allInstances()->exists(c | c.colour=#red)
Examples
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Examples: pre-post conditions
If setAge(. . . ) is called with a non-negative argument then the argument becomes the new value of the attribute age.
context Person::setAge(newAge:int) pre: newAge >= 0 post: age = newAge
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Examples: pre-post conditions
Calling birthday() increments the age of a person by 1.
context Person::birthday() post: age = age@pre + 1
52
Examples: pre-post conditions
Calling getName() delivers the value of the attribute name.
context Person::getName() post: result = name
53
Local variables The let construct defines variables local to
one constraint:Let var : Type = <expression1> in <expression2>
Example:context Airport inv:Let supportedAirlines : Set (Airline) =
self.arrivingFlights -> collect(airLine) in (supportedAirlines ->notEmpty) and (supportedAirlines ->size < 500)
54
Iterate
The iterate operation for collections is the most generic and complex building block.
collection->iterate(elem : Type;
answer : Type = <value> |
<expression-with-elem-and-answer>)
55
Iterate example
Example iterate:context Airline inv:flights->select(maxNrPassengers > 150)->notEmpty
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
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Inheritance of constraints
Guiding principle Liskov’s Substitution Principle (LSP): “Whenever an instance of a class is expected,
one can always substitute an instance of any of its subclasses.”
57
Inheritance of constraints Consequences of LSP for invariants:
An invariant is always inherited by each subclass. Subclasses may strengthen the invariant.
Consequences of LSP for preconditions and postconditions: A precondition may be weakened (contravariance)
A method in a subclass can weaken the precondition of a method in the superclass. That means the subclass method can accept a wider range of values. So data that is valid in the subclass may be invalid in the superclass.
A postcondition may be strengthened (covariance) A subclass’s method can strengthen the postcondition (but it cannot
weaken it): a subclass‘s method can return a subset of the values returned by the method it overrides.
Weakening Preconditions
abstract class Payment { /** * @pre amt >= 0 */ public void setPaymentAmount(int amt) {…} }
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class CreditCardPayment extends Payment { /** * @pre amt >= 25 */ public void setPaymentAmount(int amt) {…} }
// @pre true class CashPayment extends Payment { … }
Payment
setPaymentAmount(amt:int)
CreditCardPayment
setPaymentAmount(amt:int)
CashPayment
setPaymentAmount(amt:int)
@pre: amt >= 0
@pre: amt >= 0
@pre: true
Why does it not make sense to strengthen the precondition?
59
Example from: http://www.ugrad.cs.ubc.ca/~cs211/notes/L07-InheritanceGoodBad-4up.pdf
Payment
setPaymentAmount(amt:int)
CreditCardPayment
setPaymentAmount(amt:int)
CashPayment
setPaymentAmount(amt:int)
• Client should be able to do:
Payment p;// substitute CashPayment for Payment
p = new CashPayment();
p.setPaymentAmount( 5 );// substitute CreditCardPayment for Payment
p = new CreditCardPayment();
p.setPaymentAmount( 5 ); // oops!
@pre: amt >= 0
@pre: amt >= 0
@pre: true
Why does it not make sense to weaken the postcondition? • Suppose the client writes code based on the postcondition of the
superclass. • That client code could break if we substitute a superclass object
with an instance of one of its subclasses if the subclass' method has a weaker postcondition. • client writes code assuming that a method returns a value that is positive • subclass overrides method to return *any* value (so postcondition is weakened) • client code is going to break if a negative value is returned.
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Exercise: The Stack Data Structure isFull() – returns true if stack has maximum number of elements isEmpty() – returns true if stack has no elements top() – returns element on top of stack previous(Element elm) – returns the element inserted on the
stack immediately before elm. push(Element elm) – add elm to top of stack pop() – removes and returns element on top of stack
Assuming isFull, previous and top are already specified, specify pre and post conditions for the other operations.
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Summary and OCL Tips
OCL invariants allow you to model more precisely remain implementation independent
OCL pre- and postconditions allow you to specify contracts (design by contract) specify interfaces of components more precisely
OCL usage tips keep constraints simple always combine natural language with OCL use a tool to check your OCL
Further Resources for OCL
The Object Constraint Language ISBN 0-201-37940-6
64
ConclusionFinally!
!
The End
CSC550, Devon M. Simmonds, Computer Science Department, University of North Carolina Wilmington
???????????????
…CSC550 …
Q u e s t i o n s ?
66
Why OCL?
Required age of car owners? Requirement that a person may own at most one red car
- because UML is not enough!
67
Constraints vs. Queries
Examples of constraints: Duration of a flight is the same as the difference between the arrival and
departure times The maximum number of passengers on a flight must be less than
1,001 The origin of a flight must be different than its destination
Examples of queries: Return all the departing flights from a given airport Return all the flights departing from a given airport with a departure time
after 4p.m. Derive the arrival time by adding the duration of the flight to the
departure time.
Airport
Flight
*
*
departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer
origin
desti-nation
name: String
arrivingFlights
departingFlights
1
1