1 The Object Constraint Language Jos Warmer and Anneke Kleppe. OCL: The Constraint Language of the UML, Journal of Object-Oriented Programming, 2(2):10-13, May Jos Warmer and Anneke Kleppe. The Object Constraint Language, second edition, Addison-Wesley,

2 Outline Motivation Basics of OCL OCL types OCL collections Other features 2

3 Motivation---Problem UML diagrams don’t tell everything –Can’t express a rich semantics of and all relevant information about an application –Lack expressiveness Q: What does the following class diagram tell? parents Person * children

4 Motivation---Problem Is this a valid object diagram? –Q: What’s wrong with it? –Q: How to fix it using Z? parents joe: Person children parents Person * children

5 Motivation---Solution Specify constraints explicitly A person can’t be one’s own parents or children, e.g., this  parents inv: not parents->includes(self) parents Person * children

6 Exercise Formulate constraints for a derived association. parents Person * children 0..* /family tree ancestors descendants

7 What Is OCL? Standard “add-on” to UML –OCL expressions dependent on types from UML diagrams Language for expressing additional information (e.g., constraints and business rules) about UML models

8 Characteristics of OCL Constraint and query languages –UML 1.1: Constraints –UML 2.0: Queries, reference values, conditions, business rules Math foundation but no math symbols –Set theory and predicate logic Strongly typed language –Checking possible Declarative language –What vs. how (algorithmic or procedural) –No side effect –High level of abstraction (platform independence)

9 Outline Motivation Basics of OCL OCL types OCL collections Other features 9

10 Basics of OCL Associating OCL expressions to UML models –Directly to diagrams as notes –Separate accompanying texts, e.g., context Person inv: age >= 0 Person -age inv: age >= 0

11 Basic: Specifying Invariants Invariants: state conditions that must be always be met by all instances of context types (classes or interfaces) context Company inv: self.numberOfEmployees > 50 context c: Company inv: c.numberOfEmployees > 50 context c: Company inv enoughEmployees: c.numberOfEmployees > 50 self: contextual instance, an instance to which the OCL expression is attached An explicit specification of contextual instance, c an optional label

12 Basic: Pre and Post-conditions Pre and post-conditions: conditions that must be true at the moment when an operation begins and ends its execution. context Account::deposit(amt: Integer): void pre: amt > 0 post: balance = + amt context Account::deposit(amt: Integer): void pre argumentOk: amt > 0 post balanceIncreased: balance = + amt optional label pre-value, referring to previous value

13 Referring to Pre-value and denotes the value of a property at the start of an operations result: denotes the result of an operation context Account::payInterest(rate: Real): void post: balance = + context Account::getBalance(): Integer post: result = balance

14 Navigating in OCL Expressions Use dot notation to navigate through associations –Direction and multiplicity matter –Use role names or class names context Account inv: self.owner … -- evaluate to a single Customer self.customer … context Customer inv: self.accounts->size() … -- evaluate to a collection self.account … -- of accounts Customer Account 0..* 1 accountsowner Arrow notation for collection operations single line (--) or multiple lines (/* … */)

15 Exercise Write pre and post-conditions Pair (5 minutes) Person - sex: Sex + marry(p: Person): void 0..1 spouse

16 Exercise Formulate constraints or business rules for the following class diagram. 16

17 Exercise Model the following software by drawing a class diagram and formulating the constraints the class diagram has to satisfy. The software system automates test taking by allowing an instructor to prepare test questions and students to take tests. 1.The system shall allow an instructor to add test questions, Each question is a multiple choice question consisting of a stem---that presents the problem to be solved or the question to be answered---and a set of options---that are possible answers. Test questions are classified by topics, difficulty levels, and similarity. 2.The system shall allow an instructor to create a test on specific topics by suggesting a set of questions that meets the instructor's request (e.g., number of questions and their distributions among different topics and difficulty levels). 3.The system shall allow students to take tests prepared by the instructor. 4.The system shall grade tests taken by students to calculate test scores. 5.The system shall allow both the instructor and the students view their scores. However, students are allowed to view only their tests. 17

18 Outline Motivation Basics of OCL OCL types OCL collections Other features 18

19 Types in OCL Two different kinds –Predefined types (as defined in standard library) Basic types: Integer, Real, String, Boolean Collection types: Set, OrderedSet, Bag, Sequence –User-defined types: classes, interfaces, and enumerations. Value vs. object types –Immutable vs. mutable types –All predefined types are value types, i.e., there is no mutation operation defined. 19

20 Basic Types TypeValuesOperations Booleanfalse, trueor, and, xor, not, =, <>, implies Integer-10, 0, 10, … =, <>,, =, +, -, *, /, mod(), div(), abs(), max(), min(), round(), floor() Real-1.5, 3.14, … String‘Carmen’ =, <>, concat(), size(), toLower(), toUpper(), substring() Several basic types with operations

21 Integer and Real No restriction on size and precision Operations: =, <>,, =, +, -, *, /, mod, div, abs, max, min, round, floor Prefix vs. infix operations, e.g., 5.max(10) 5.+(10) Axioms of associativity and commutitivity hold: a + (b + c) = (a + b) + c a + b = b + a

22 String Operations: concat, size, toLower, toUpper, substring, =, <> Constants: 'cs 5381' 1-based index for substring, substring(l,u) Q: Pre- and post-conditions of substring?

23 Boolean Operations: or, and, xor, not, =, <>, implies, if- then-else-endif Order of evaluation and "undefined" value –Undefined represented by OclVoid, conforming to all types (cf. isUndefined() from OclAny) – Mostly strict interpretation except for boolean operators –Q: Truth table for: and, or, xor, implies, not?

24 OclAny Supertype of all types Operations useful for every type of OCL instance, e.g., –= (o2: OclAny): Boolean –<> (o2: OclAny): Boolean –oclIsUndefined(): Boolean –oclIsTypeOf(t: OclType): Boolean –oclIsKindOf(t: OclType): Boolean –oclIsNew(): Boolean –oclAsType(t: OclType): OclType –T::allInstances(): Set(T) *Both = and <> are mostly redefined by subtypes Q: Use of oclAsType? Accessing subtype's features and supertype's overridden features Q: Difference between oclIsTypeOf and oclIsKindOf?

25 Using and Navigating User-defined Types Can use (class) attributes and query operations of user- defined types Can use associations and aggregations –Use role names or, if unambiguous, type names –If multiplicity > 1, collections based on properties Set: {unique} (default) OrderedSet: {unique, ordered} Bag: {notUnique} Sequence: {notUnique, ordered} context Account inv: self.owner.name <> '' context Customer inv: self.accounts->size() > 0 Customer Account 0..* 1 accountsowner

26 Navigating User-defined Types Navigating more than one association with multiplicity "many" –Bag –Sequence if at least one is marked {ordered} context Bank inv: self.customers.accounts->forAll(balance > 0) Customer Account 0..* accounts Bank 0..* customers

27 Using Enumeration Use “::” notation context Account inv: type = AccountType::gold implies minBalance > and type = AccountType::silder implies minBalance > 5000 > AccountType gold silver Account 1 type

28 Using Association Class Use class name (with a lowercase character) and role names context Course inv: self.outcome->forAll(score > 90) context Outcome inv: score > 90 Inv: students.enrolled -- the student associated with this outcome Course 0..* Outcome score: Integer Student students 0..*

29 Outline Motivation Basics of OCL OCL types OCL collections Other features 29

30 Collections in OCL Why? –Multiple objects produced by navigating associations Standard collection types –Parameterized with elements types, e.g., Set(Account) –Value/immutable types, not reference types –One abstract and four concrete types Collection Set, OrderedSet, Bag, Sequence Determined based on properties of associations, e.g., unique, ordered, and sorted.

31 Collection Types Properties TypeDuplicate?Ordered? Set OrderedSet Bag Sequence NNYYNNYY NYNYNYNY *Ordered doesn’t mean sorted. Constants –Set{10, 100} –OrderedSet{'apple', 'orange'} –Bag{10, 10, 100} –Sequence{10, 10, 100}, Sequence{1..10}, Sequence{1..(5 + 5)} –Set{Set{1}, Set{10}}

32 Collection Operations Large number of predefined operations Arrow notation, e.g., c->size() –Rationale: allow same-named, user-defined operations, e.g., c.size() Customer Account 0..* 1 accountsowner context Accountcontext Account inv: not owner->isEmpty() inv: not owner.isEmpty()

33 Collection Operations OperationDescription count(o) excludes(o) excludesAll(c) includes(o) includesAll(c) isEmpty() notEmpty() size() sum() Number of occurrences of o in the collection (self) Is o not an element of the collection? Are all the elements of c not present in the collection? Is o an element of the collection? Are all the elements of c contained in the collection? Does the collection contain no element? Does the collection contain one or more elements? Number of elements in the collection Addition of all elements in the collection Defined on all collection types Type-specific operations append, including, excluding, first, last, insertAt, etc.

34 Exercise Write pre- and post-conditions for: –includes –excludes –includesAll –excludesAll –isEmpty –notEmpty Hint: use forAll operations for includesAll and excludesAll.

35 Exercise Write pre and postconditions for: Set union(s: Set(T)): Set(T) =(s: Set(T)): Boolean including(o: T): Set(T) OrderedSet append(o: T): OrderedSet(T) insertAt(i: Integer, o: T): OrderedSet(T) Bag excluding(o: T): Bag(T) Sequence subSequence(l: Integer, u: Integer): Sequence(T)

36 Iteration Operations Loop over elements by taking one element at a time Iterator variables –Optional variable declared and used within body –Indicate the element being iterated –Always of the element type, thus, type declaration is optional context Customer inv: self.accounts->forAll(a: Account | a.owner = self) inv: accounts->forAll(a | a.owner = self) inv: accounts->forAll(owner = self) Customer Account 0..* 1 accountsowner

37 Iteration Operations OperationDescription any(expr) collect(expr) exists(expr) forAll(expr) isUnique(expr) iterate(x: S; y: T| expr) one(expr) reject(expr) select(expr) sortedBy(expr) Returns any element for which expr is true Returns a collection that results from evaluating expr for each element of self Has at least one element for which expr is true? Is expr true for all elements? Does expr has unique value for all elements? Iterates over all elements Has only one element for which expr is true? Returns a collection containing all elements for which expr is false Returns a collection containing all elements for which expr is true Returns a collection containing all elements ordered by expr

38 Examples accounts->any(a: Account | a.balance > 1000) accounts->collect(name) -- all the names accounts->exists(balance > 5000) accounts->forAll(balance >= 0) accounts->isUnique(name) accounts->iterate(a: Account; sum: Integer = 0 | sum + a.balance) accounts->one(name = “Carmen”) accounts->reject(balance > 1000) accounts->select(balance <= 1000) accounts->sortedBy(balance)

39 Select vs. Collect Q: Difference between select and collect? Note that the dot notation is short for collect, e.g., context Bank inv: self.customers.accounts->forAll(balance > 0) inv: self.customers->collect(accounts)->forAll(balance > 0) Customer 0..* accounts customers Account balance: Integer Bank 0..* Note that results are flattened for “collect” and not for “collectNested”.

40 The Iterate Operation Most fundamental and generic loop operation All other loop operations are special cases iterate(elem: T1; result: T2 = expr | expr-elem-result) Example Set{1, 2, 3}->sum() Set{1, 2, 3}->iterate(i: Integer; r: Integer = 0 | r + i)

41 Exercise Pair (3 minutes) Write the pre- and post-conditions of the getBelowAverage operation that returns all the accounts of a customer of which balances are below the average balance of the customer’s accounts. 0..* 1 accounts Account + getBalance(): Real Customer + getBelowAverage(): Set(Account)

42 Exercise Write pre- and post-conditions of the following operations in terms of the iterate operation: size, count, sum, and excluding(o: T): Sequence(T) Define the following operations in terms of the iterate operation: forAll, any, one, select, reject E.g., source->exists(vars|body) = source->iterate(vars; result: Boolean = false| result or body)

43 Initial Values and Derivation Rules Specify initial values for attributes and association ends context Account::balance init: 0 Specify derivation rules for derived attributes and associations context Person::printedName derive: lastName.concat(', ').concat(firtName) Q: How to indicate derived attributes and associations in UML? Q: Difference between initial values and derivation rules?

44 Outline Motivation Basics of OCL OCL types OCL collections Other features 44

45 Introducing New Attributes and Operations Can add new attributes and operations to UML models context Account def: isJointlyOwned: Boolean = owners->size() > 1 context Account def: numOfOwners(): Integer = owners->size() Customer 0..* owners Account balance: Integer

46 Message Sending The isSent (^) operator in post-conditions specifies communication (e.g., method call or signal) has taken place. context Subject::hasChanged() post: observer^update(10, 20) context Subject::hasChanged() post: observer^update(?: Integer, ?: Integer)