Goals of the Lecture Object Constraint Language

Transcription

1 Goals of the Lecture Object Costrait Laguage Object-Orieted Aalysis ad Desig - Fall 1998 Preset the Object Costrait Laguage Ð As best as possible, with the limited iformatio available from UML i a Nutshell ad the Ratioal Website The official referece for the OCL will be the forthcomig Addiso-Wesley book: Ð The Uified Modelig Laguage Referece Maual (expected mid-december) Additioal Referece Costraits: A review The Object Costrait Laguage Ð Precise Modelig with UML Jos Warmer Aeke Kleppe Addiso-Wesley: ISBN Just published! A costrait is a restrictio o oe or more values of (part of) a objectorieted model or system Ð Supports desig by cotract Operatios ca be provided pre- ad postcoditios These coditios ca be specified usig costraits

2 Costraits Review cotiued Advatages of Costraits A precoditio must be true at the momet that a operatio is executed A postcoditio must be true at the momet the operatio fiishes executig A differet type of costrait is a ivariat Ð A ivariat specifies a coditio that must always be true of its associated elemets Ð This cotrasts with pre- ad post-coditios that oly eed to hold before ad after the executio of a sigle operatio Better Documetatio Ð The sematics are kept close to the model they costrai Improved Precisio Ð Costraits caot be iterpreted differetly by differet people Commuicatio without Misuderstadig Declarative or Operatioal Advatages of Declarative style Two types of iterpretatios Declarative Ð States what must be true, ot what must be doe Operatioal Ð Breakig a costrait triggers a operatio For example, i Eiffel, if a costrait is broke a exceptio is throw UML adopts a declarative style of costraits Costraits have o side-effects Ð The state of a system does ot chage whe a costrait is evaluated It separates the specificatio of a costrait from the respose required if a costrait is broke Costraits should be stable; actios may chage over time Atomicity requiremets ca be avoided

3 Object Costrait Laguage OCL, cotiued. OCL is a pure expressio laguage Ð It ca ot modify the state of a model Ð It ca however specify a state that is required by a pre- or post-coditio Ð A OCL expressio simply states a requiremet that must be met i order to cosider a istatiatio of the model to be valid OCL is ot a programmig laguage Ð It ca ot be used to ivoke a process Ð It ca ot be used to code program logic Ð It ca ot be used to specify flow of cotrol OCL is a typed-laguage Ð Each expressio has a type Ð Ay UML Classifier ca fuctio as a OCL type: class, use case, actor, etc. OCL, cotiued Uses for the OCL OCL was developed at IBM i 1995 It was submitted ito the Object Maagemet GroupÕs stadards efforts i 1996 It was merged ito the UML stadard i 1997 ad became a official OMG stadard i November 1997 (UML 1.1) Note: the UML metamodelõs sematics are specified usig OCL To specify ivariats o classes To specify type ivariats for stereotypes. To describe pre- ad post-coditios o operatios ad methods To describe guards To avigate associatios To specify costraits o operatios

4 OCL Buildig Blocks OCL Types The basic buildig blocks of OCL are Ð Objects Ð Object properties Each object ca have a type Ð Predefied Types Basic types Collectios Ð User-defied model types Basic Types Ð Iteger, Real, Strig, ad Boolea Collectio Types Ð Collectio, Set, Bag, ad Sequece Model Types Ð Customer, Perspective, Drafter, etc. Ð Also eumeratio types Expressios ad Costraits Cotext of a Expressio Not all expressios are costraits For istace Ð Ò1+3Ó is a valid expressio Ð Its result is Ò4Ó ad its retur type is ÒItegerÓ OCL costrait Ð A OCL Expressio that returs a Boolea e.g. {perso.age > 1 + 3} Each OCL expressio must have a cotext Ð The cotext is the model elemet to which the costrait is attached Ð This elemet is referred to as the Òcotextual elemetó Perspective objects->otempty()

5 Cotext of a Ivariat The cotext of a ivariat is always a type (class, iterface, attribute, etc.) Customer Name = ÒEdwardÓ Specifies that all Customer objects must have the ame of Edward Not a very useful ivariat! Cotext of Pre- ad Post- Coditios The cotext of pre- ad post-coditios is always a operatio or method AddAchor(L : Lik, A : Achor) pre: ot L.achors->Icludes(A) post: L.achors->Icludes(A) Example i a Class Diagram Keywords for Post-Coditios Lik achors: Set Id: Iteger addachor( ) «ivariat» self.id > 0 {froze} «precoditio» ot achors.cotais(a) «postcoditio» Ð Attach this to a elemet to refer to the value of that elemet i the precoditio Ð {age = age@pre + 1} result Ð Use this keyword to place a costrait o the retur value of a operatio Ð This does ot defie the value, it just costrais it Ð {result < 100 * iput@pre} or {result = self.age}

6 The self keyword Navigatig associatios The self keyword is used to refer to the cotextual elemet of a OCL costrait It ca be omitted whe the cotext is clear A example where its eeded Cotaier ot Cotaier->cotais(self) If A is related to B ad A plays the role of x ad B plays the role of y Ð The, a costrait o A ca refer to B {self.y = É} or {self.b = É} If the multiplicity of B is greater tha oe Ð The the type of self.y or self.b is a collectio Navigatig Associatio Classes Example A associatio class ca referece either ed of its associatio via the same rules described o the previous slide However, the type of a referece is always a istace, ever a collectio, regardless of the multiplicities of the associated associatio This is because a istace of a associatio class is associated with oly a sigle istace of a associatio A x C y * B {self.y.ocltype = B} {self.y.ocliskidof(collectio) = true}

7 Boolea Operatios Sematics of Implies or, ad, xor, ot, =, <>, implies if a the b else bõ edif Examples Perso Self.age > 17 implies self.cavote() = true Customer title = (if ismale = true the ÒMr.Ó else ÒMs.Ó edif) a implies b Returs true Ð If a is false (a falsity ca imply aythig) Ð Or a is true ad b is true Returs false Ð If a is true ad b is false Iteger ad Real Operatios Strig Operatios =, <> <, >, <=, >= +, -, *, / mod, div, abs max, mi roud, floor cocat() size tolower() toupper() substrig() =, <> ÔKeÕ.size = 3

8 Eumeratios Collectios Eumerated Types ca be defied Ð eum { blue, red, gree, yellow } Ð operatios: =, <> I order to avoid ame coflicts Ð eumerated types are refereced like this: #blue Customer (geder = #male) implies (title = ÒMr.Ó) The OCL has oe collectio type ad three subclasses Ð Set - uordered set of items, o duplicates Ð Bag - set of items, duplicates allowed Ð Sequece - ordered Bag There are a amazig umber of operatios defied for these four types Ð Collectio Operatios Other Operatios size cout(object) icludes(object) icludesall(col.) isempty iterate(expressio) eum() exists(expressio) forall(expressio) otempty A collectio operatio is always ivoked usig the arrow operator: achors->isempty() Set Operatios Ð equals, mius Ð symmetricdifferece Ð uio() Ð itersectio Ð assequece() Ð asbag() Ð... Sequece Ops Ð apped() Ð preped() Ð subsequece() Ð at() Ð first Ð last Ð...

9 Iteratio fuctios Iteratio Fuctios cotiued select(expressio) - returs collectio Hyperweb lik->select(l: Lik L.achors->size() = 2) (Fids all liks with exactly two achors) reject(expressio) Ð Opposite of select fids all members where the expressio evaluates to false collect(expressio) Ð The expressio computes a value for each elemet of a collectio. The values are retured i a collectio forall(expressio) Ð Specifies that the expressio holds for each member of the collectio Exists(expressio) Ð The expressio is true of at least oe elemet of the collectio Example Istace Operatios Perso age employee employer Compay {employee->forall(p: Perso p.age >= 18 ad p.age <= 65) } Meaig: A compay s employees all must be betwee the ages of 18 ad 65. =, <> ocltype ocliskidof() oclistypeof() oclastype() Sytax for forall: forall(elemet: Type <expressio>) Note: Elemet ad Type ca be omitted

10 Operator Precedece Type Castig Dot (Ò.Ó) ad arrow (Ò->Ó) Uary ÒotÓ ad Uary mius (Ò-Ó) Multiplicatio (Ò*Ó) ad Divisio (Ò/Ó) Additio (Ò+Ó) ad Subtractio (Ò-Ó) Logical ÒadÓ, ÒorÓ, ad ÒxorÓ Logical ÒimpliesÓ Logical Òif the else edifó Logical compariso operators Ò<Ò, Ò>Ó, etc. A type ca be cast to oe of its subtypes usig the followig sytax object.oclastype(type2) -- object is ow cosidered of type Type2 Notes Ð If Type2 is ot a valid type for object the result of the expressio is udefied Ð Commets are started with two dashes Type Coformace Type Coformace Rules I costructig a expressio, all subexpressios ad operators must ÒfitÓ properly coformace Defiitio of Coformace Ð Type1 coforms to Type2 if a istace of Type1 ca be substituted at each place where a istace of Type2 is expected. Type1 coforms to Type2 whe they are idetical Type1 coforms to Type2 whe Type1 is a subtype of Type2 Each type is a subtype of OclAy Type coformace is trasitive Iteger is a subtype of Real There are separate rules for collectios o the ext slide...

11 Type Coformace for Collectios Udefied Every type Collectio(T) is a subtype of OclAy Set(T), Bag(T), ad Sequece(T) are subtypes of Collectio(T) Collectio(Type1) coforms to Collectio(Type2) if Type1 coforms to Type2 Set(T) does ot coform to Bag(T) or Sequece(T) Bag(T) does ot coform to Set(T) or Sequece(T) Sequece(T) does ot coform to Set(T) or Bag(T) A OCL expressio may result i Udefied Ð Example: referecig a o-existet attribute o a object Ay expressio that has a sub-expressio that is udefied evaluates to udefied Ð With two exceptiosé True or Udefied = True False ad Udefied = False The OCL Book is good! Chapter 4 talks about modelig with costraits Ð Lots of examples Chapter 5 talks about extedig OCL Appedix A documets all operatios Appedix B provides a formal grammar for the Object Costrait Laguage