Ontologies Reasoning Components Agents Simulations Agent Internal Structural Modeling with UML2 and OCL2 Jacques Robin Outline Object-orientation for software and knowledge engineering Short history of object-oriented languages UML2 as a knowledge representation language OCL2 UML2 Components Agent and UML2 Components Review of Key Object-Orientation Concepts Class (or concept, or category): abstract representation of a set of individuals with common structural and/or behavioral properties A class defines a complex type Object (or individual, or instance): individual instance of a given class An object conforms to the complex type defined by its class An object is created by instantiating its class (constructor method) Each object has a unique identifier (oid) that distinguishes it from other instances of the same class sharing the same properties The structural properties of a class are a set of attributes (also called fields or slots), which value is constrained to be of a certain subset of types (primitive types or classes) The structural properties of an object are specific values for these attributes within the ranges defined by its class The behavioral properties of a class are a set of operations (also called methods, procedures, deamons or functions) that its instances can execute The signature of a class is the set of type constraints on its attributes and on the parameters and return value of its operations The properties of a class have various visibilities such as public, protected and private allowing their encapsulation Classes are organized in a generalization (specialization) hierarchy Properties are inherited down the hierarchy from a class to its subclasses and its objects Inheritance Allows concise knowledge representation through reuse of specifications and implementations among classes and objects down a specialization hierarchy Types of inheritance: Structural inheritance Attribute signature inheritance (constraint inheritance) Value inheritance Behavioral inheritance Operation signature inheritance (constraint inheritance) Operation code inheritance Inheritance multiplicity Simple inheritance (each class restricted to having a single super-class, and each object restricted to belong to a single class) Multiple inheritance of different properties from different sources Multiple inheritance of same property from different sources Inheritance monotonicity Monotonic inheritance: simple without overriding Non-monotonic inheritance: with overriding, logically equivalent to default reasoning, semantics beyond Classicial First-Order Logic Software EngineeringDistributed Systems History of Object-Oriented Languages ProgrammingKnowledge Representation Databases Simula Sketchpad Java C# Semantic Networks Description Logics Frame Logics SQL99 Frames Smalltalk C++ OQL UML1 OCL1 MOF1 OCL2 UML2 MOF2 Semantic Web OWL SWSL CHORD Motivation for OO in Software Engineering Improved productivity, quality, legibility and maintainability in developing software artifacts Software reuse instead of rewriting or cut and paste More intuitive Divide software in abstract entities and relations that directly match common cognitive abstraction of modeled domain Easy to learn Unifying notation Single representation paradigm for all software process stages Single, unified modeling language (UML) Initial Motivation for OO in Knowledge Representation Reasoning at the level of categories Inheritance as reasoning task Representing structural knowledge with a notation that is more intuitive than formal logic Easier to acquire, understand, maintain, etc. Reasoning about classifying instances into categories and inheritance can internally reuse a logic-based theorem prover, but in a way that is transparent, hidden from the domain expert Benefits of software engineering carrying over to knowledge (base) engineering UML as KR Language Class diagram: Modern, well-founded version of semantic networks Activity diagram Modern, well-founded version of flow charts Graphical syntax for procedures Class diagrams + Activity diagrams : Graphical syntax of expressive power approximately equivalent to that of Frames Strengths: Universal standard, well-thought, well-known and well-tooled (CASE) Facilitates convergence between software and knowledge engineering Limitations: Lack of full UML compilers to executable languages Lack of inference engine to automatically reasoning with knowlege represented only as UML models No mathematically defined formal semantics yet Thus: Only useful at the knowledge level Need to be used in conjunction with other language(s) that provide the formalization and/or implementation level UML Class Diagram Categories represented as classes (nodes) Classes encapsulates: Primitive type properties, attributes Behaviors, operations Relationships between classes represented as associations (edges) Special associations for: Specialization-Generalization relationship partOf relationship (aggregation and compositions) Reified relationships represented as association classes Role names and cardinality constraints on associations Many other logical constraints built-in class diagram syntax Arbitrary logical constraints relating any part of the class diagram using Object Constraint Language (OCL) Classes: Attributes Common characteristics of the class members Fields (slots): Base or derived Visibility (public, protected, private) Name Type (Primitive Built-In or Used-Defined Enumerations) Initial default value Property Object attributes: different value for each object Class attributes: same value for all objects Attributes for KR: as many fields as possible! Classes: Operations Common signature of services provided by the class members Fields: Visibility Name Input parameter Direction Name Type Multiplicity Default value Property Return type Property Object methods: called on objects Class methods: called to manipulate class attributes Operations for KR: as many fields as possible! Associations Association: Generic relation between N classifiers Fields: One or two Names Navigation direction Two Ends, each with: One Multiplicity Range (default = 1) Zero to One role Zero to one Qualifier Navigation: Role if present Otherwise destination class name Associations for KR: as many fields as possible! N-ary Associations Single association between N classes Different from N-1 binary associations Different from one binary association class Example: Ca has objects A1, A2 Cb has objects B1, B2 Cc has objects C1, C2 No link in the ternary association Ca-Cb-Cc corresponding to pair of links A1-B1, B2-C1 Association Classes Class connected to an association and not to any of its ends Allows associating properties and behaviors to an association One object of the association class for each link of the connected association A one-to-many or many-to-many association class cannot be substituted by a simple class and a pair of simple associations Example: Ca has objects A1, A2, A3, A4 Cb has objects B1, B2, B3, B4 Extent of association class Cc between Ca and Cb with * multiplicity at both ends has necessarily 16 instances Class Cc associated to Ca through association Aca and to Cb through association Acb could have only 4 instances Elevator control QueueElevator Difference with: ? 4 Aggregations and Compositions Aggregation: Association with part-whole semantics Associate composite class to its building blocks Static, definitional characteristic of the whole class Composition: Special case of one-to-one or one- to-many aggregation where part(s) cannot exist(s) without the unique whole Deletion of the whole must therefore always be followed by automatic deletion of the parts Class generalizations Taxonomic relation between a class and one of its more general direct super-class Special case of generalization between any two classifiers Several generalizations form a taxonomic tree free of generalization cycles Sub-classifier inherits the features from all its direct super-classifiers Private attributes and operations not accessible from sub-classes Protected attributes and operations accessible from sub-classes but not from associated classes UML generalizations allow multiple inheritance and overriding Instances of a sub-class must satisfy all the constraints on all its super-classes (principle of substitutability) Abstract Classes Class that cannot be instantiated Only purpose: factor gradual refinements of common and distinct structures and behaviors down a taxonomic hierarchy Abstract operation: common signatures of distinct implementations specified in subclasses Supports polymorphism: generic call signature to distinct operations, with automatic dispatch to the implementation appropriate to each specific call instance Generalization Sets Subclass set that can be labeled as: complete or incomplete overlapping or disjoint Complete and disjoint generalization sets form a partition of the super-class Sub-subclass can specialize members of two overlapping generalization sets Power Types Generalization set of a super-class defined in terms of a class associated to it Subclasses of each power type inherits features from the associated class of the super-class that defines the power type Allows separation of orthogonal concerns Useful for MDA as a rich modeling element UML Object Diagrams Object Diagram contains: Specific (named) or generic (named after role, unnamed) instances of classes Possibly several instances of the same class Specific instances of associations (links) among objects Possibly several instances of the same association Illustrates specific instantiation patterns of associated class diagram UML2 Active x Passive Objects Active objects Instances of active classes Possess their own, continuous execution thread Concurrent to other active objects Exchange data with other active objects asynchronously through message passing Does not wait for the other active object target of the message to respond to pursue its own processing Can be pro-active: execute behavior on its own initiative without waiting to receive a request from another object Passive (regular) objects Instances of passive (regular) classes Share a single thread with the other passive objects constituting a sequential application Exchange data with other passive objects synchronously through method invocation Interrupts its processing, waiting for an answer of the other passive object before pursuing its own processing Purely reactive: execute behavior only as response to a method invocation request from another object UML2 Active Classes and Objects UML2 classes can encapsulate other classes Thus, UML2 objects can encapsulate other objects What is OCL? Definition and Role A textual specification language to adorn UML and MOF diagrams and make them far more semantically precise and detailed OCL2 integral part of the UML2 standard OCL complements UML2 diagrams to make UML2: A domain ontology language that is self-sufficient at the knowledge level to completely specify both structure and behaviors A complete input for the automated generation of a formal specification at the formalization level to be verified by theorem provers A complete input for the automated generation of source code at the implementation level to be executed by a deployment platform OCL forms the basis of model transformation languages such as Atlas Transformation Language (ATL) or Query-View-Transform (QVT) which declaratively specify through rewrite transformation rules the automated generation of formal specifications and implementations from a knowledge level ontology OCL expressions are used in the left-hand and right-hand sides of such rules To specify objects to match in the source ontology of the transformation To specify objects to create in the target formal specification or code of the transformation What is OCL? Characteristics Formal language with well-defined semantics based on set theory and first- order predicate logic, yet free of mathematical notation and thus friendly to mainstream programmers Object-oriented functional language: constructors syntactically combined using functional nesting and object-oriented navigation in expressions that take objects and/or object collections as parameters and evaluates to an object and/or an object collection as return value Strongly typed language where all expression and sub-expression has a well-defined type that can be an UML primitive data type, a UML model classifier or a collection of these Semantics of an expression defined by its type mapping Declarative language that specifies what properties the software under construction must satisfy, not how it shall satisfy them Side effect free language that cannot alter model elements, but only specify relations between them (some possibly new but not created by OCL expressions) Pure specification language that cannot alone execute nor program models but only describe them Both a constraint and query language for UML models and MOF meta- models What is OCL? How does it complement UML? Structural adornments: Specify complex invariant constraints (value, multiplicity, type, etc) between multiple attributes and associations Specify deductive rules to define derived attributes, associations and classes from primitive ones Disambiguates association cycles Behavioral adornments: Specify operation pre-conditions Specify write operation post-conditions Specify read/query operation bodies Specify read/query operation initial/default value OCL: Motivating Examples Diagram 1 allows Flight with unlimited number of passengers No way using UML only to express restriction that the number of passengers is limited to the number of seats of the Airplane used for the Flight Similarly, diagram 2 allows: A Person to Mortgage the house of another Person A Mortgage start date to be after its end date Two Persons to share same social security number A Person with insufficient income to Mortgage a house 1 2 OCL: Motivating Examples 1 2 context Flight inv: passengers -> size() isUnique(socSecNr) context Person::getMortgage(sum:Money,security:House) pre: self.mortgages.monthlyPayment -> sum() asSet() -> size() = 1 In some context self keyword is required: context Membership inv: participants.cards.Membership.includes(self) Specifying Default Attribute Values Initial values: context LoyaltyAccount::points : integer init: 0 context LoyaltyAccount::transactions : Set(Transaction) init: Set{} Specifying Attribute Derivation Rules context CustomerCard::printedName derive: owner.title.concat( ).concat(owner.name) context TransactionReportLine: String derive self.date = transaction.date ... context TransactionReport inv dates: lines.date -> forAll(d | d.isBefore(until) and d.isAfter(from)) ... Specifying Query Operation Bodies Query operations: context LoyaltyAccount::getCustomerName() : String body: Membership.card.owner.name context LoyaltyProgram::getServices(): Set(Services) body: partner.deliveredServices -> asSet() Specifying Operations Pre and Post Conditions context LoyaltyAccount::isEmpty(): Boolean pre: -- none post: result = (points = 0) used to refer in post-condition to the value of a property before the execution of the operation: context LoyaltyProgram::enroll(c:Customer) pre: c.name post: participants = -> including(c) Keyword oclIsNew used to specify creation of a new instance (objects or primitive data): context LoyaltyProgram:: enrollAndCreateCustomer(n:String,d:Date):Cust omer post: result.oclIsNew() and result.name = n and result.dateOfBirth = d and participant -> includes(result) oclIsNew only specifies that the operation created the new instance, but not how it did it which cannot be expressed in OCL Association Navigation Abbreviation of collect operator that creates new collection from existing one, for example result of navigating association with plural multiplicity: context LoyaltyAccount inv: transactions -> collect(points) -> exists(p:Integer | p=500) context LoyaltyAccount inv: transactions.points -> exists(p:Integer | p=500) Use target class name to navigate roleless association: context LoyaltyProgram inv: levels -> includesAll(Membership.currentLevel) Call UML model and OCL library operations Generalization Navigation OCL constraint to limit points earned from single service to 10,000 Cannot be correctly specified using association navigation: context ProgramPartner inv totalPoints: deliveredServices.transactions.points -> sum() < 10,000 adds both Earning and Burning points Operator oclIsTypeOf allows hybrid navigation following associations and specialization links context ProgramPartner inv totalPoints: deliveredServices.transactions -> select(oclIsTypeOf(Earning)).points -> sum() < 10,000 OCL Visibility and Inheritance By default, OCL expressions ignore attribute visibility i.e., an expression that access a private attribute from another class is not syntactically rejected OCL constraints are inherited down the classifier hierarchy OCL constraints redefined down the classifier hierarchy must follow substituability principle Invariants and post-condition can only become more restrictive Preconditions can only become less restrictive Examples violating substituability principle: context Stove inv: temperature forAll(s | s.oclIsType(plasticStrings)) context ElectricGuitar inv: strings -> forAll(type = StringType::metal) context ClassicGuitar inv: strings -> forAll(type = StringType::plastic) context Guitar inv: type = GuitarType::classic implies strings -> forAll(type = StringType::plastic inv: type = GuitarType::classic implies strings -> forAll(type = StringType::plastic