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These identifiers can beused as arguments to subsequent messages.
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In other cases, it may be the name of an event that israised on the receiving object.
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In normal practice withprocedural overloading, both the message name and theargument list types are required to identify an operation.
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The parentheses can be used even if the list isempty.
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The following are samples of control message label syntax.
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Instead of text expressions for arguments and return values, data tokens may beshown near a message (Figure 13-123).
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A token is a small circle labeled with theargument expression or return value name.
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It has a small arrow on it that pointsalong the message (for an argument) or opposite the message (for a return value).
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Tokens represent arguments and return values.
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The choice of text syntax or tokensis a presentation option, but text is more compact and is recommended for mostpurposes.
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The syntax of messages may be expressed in the syntax of a programming lan-guage, such as C++ or Smalltalk.
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All the expressions on a single diagram shoulduse the same syntax, however.
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A class whose instances are classes.
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Metaclasses are typically used to constructmetamodels.
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A model that defines the language for expressing a metamodel.
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The relationshipbetween a meta-metamodel and a metamodel is analogous to the relationship be-tween a metamodel and a model.
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This level of indirection is usually relevant onlyto tool builders, database builders, and the like.
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UML is defined in terms of a meta-metamodel, called the Meta-Object Facility (MOF).
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A model that defines the language for expressing a model; an instance of a meta-metamodel.
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The UML metamodel defines the structure of UML models.
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A generic term for all entities in a metamodeling language.
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A term grouping relationships that connect descriptors to their instances.
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Theseinclude the instance relationship and the powertype relationship.
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A method is an implementation of an operation.
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The language must be matched to the purpose,of course.
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A human language, for instance, may be adequate for early analysis butnot suitable for code generation.
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An operation declaration implies the presence of a method unless the operationis declared as abstract.
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In a generalization hierarchy, each repeated declaration ofthe operation implies a new method that overrides any inherited method of thesame operation.
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Two declarations represent the same operation if their signaturesmatch.
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Note that a method is an executable procedure—an algorithm—not simply aspecification of results.
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A before-and-after specification is not a method, for exam-ple.
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A method is a commitment to implementation and addresses issues of algo-rithm, computational complexity, and encapsulation.
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In some respects, a method may have stricter properties than its operation.
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But ifthe operation is a query, then the method must be a query.
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Similarly, a methodmay strengthen the concurrency property.
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A sequential operation may be imple-mented as a guarded or concurrent method.
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In these cases, the method is consis-tent with the declarations of its operation.
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If the operation is inherited, the method can beshown by repeating the operation declaration in normal (nonitalic) text to show aconcrete operation.
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Method on nonabstract operationShapedraw ( )
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A semantically complete abstraction of a system.
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A model is a more or less complete abstraction of a system from a particular view-point.
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It is complete in the sense that it fully describes the system or entity, at thechosen level of precision and viewpoint.
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Different models provide mostly inde-pendent viewpoints that can be manipulated separately.
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A model may comprise a containment hierarchy of packages in which the top-level package corresponds to the entire system.
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The contents of a model are thetransitive closure of its containment (ownership) relationships from top-levelpackages to model elements.
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A model may also include relevant parts of the system’s environment, repre-sented, for example, by actors and their interfaces.
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In particular, the relationshipof the environment to the system elements may be modeled.
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A system and its envi-ronment form a larger system at a higher level of scope.
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Therefore, it is possible torelate elements at various levels of detail in a smooth way.
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Elements in different models do not directly affect each other, but they oftenrepresent the same concepts at different levels of detail or stages of development.
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Therefore, relationships among them, such as trace and refinement, are importantto the development process itself and often capture important design decisions.
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A model can be shown as a package with the stereotype «model».
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There is little no-tational detail to show about models, however.
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Tools can show lists of models, butmodels have few relationships among themselves.
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Most useful is the ability totraverse from a model name to its top package or to a map of its overall contents.
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No one view of a system, or indeed no system itself, is ever complete in and of it-self.
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There are always connections to the wider world, and a model always fallsshort of reality.
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Therefore, the concept of a closed model is always an approxima-tion in which arbitrary lines must be drawn for practical work.
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A UML model is represented as a package hierarchy that emphasizes one view ofa system.
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An element that is an abstraction drawn from the system being modeled.
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Contrastwith presentation element, which is a (generally visual) presentation of one ormore modeling elements for human interaction.
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All elements that have semantics are model elements, including real-world con-cepts and computer-system implementation concepts.
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Graphic elements whosepurpose is to visualize a model are presentation elements.
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They are not model ele-ments, as they do not add semantics to the model.
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Model elements may have names, but the use and constraints on names vary bykind of model element and are discussed with each kind.
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Each model element be-longs to a namespace appropriate to the kind of element.
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All model elements mayhave the following attached properties.
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The tag is aname that identifies the meaning of the value.
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Constraints are restrictions that are expressed as
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The stereotype does not alter the struc-ture of the base class, but it may add constraints andtagged values that apply to the model elements bearingthe stereotype.
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In addition, model elements may participate in dependency relationships.
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See Chapter 14, Standard Elements, for a list of predefined tags, constraints, andstereotypes.
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That aspect of a model dealing with the organization of the model itself into struc-tured parts—namely, packages, subsystems, and models.
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Refers to something that occurs during a modeling activity of the software devel-opment process.
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Usage note: When discussingobject systems, it is often important to distinguish between modeling-time andrun-time concerns.
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See also development process, stages of modeling.
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A software unit of storage and manipulation.
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Modules include source code mod-ules, binary code modules, and executable code modules.
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The word does notcorrespond to a single UML construct, but rather includes several constructs.
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A classifier role that denotes a set of objects rather than a single object.
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See also classifier role, collaboration, message.
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A multiobject is a classifier role that denotes a set of objects, usually the set of ob-jects on the many side of an association.
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A multiobject is used within a collabora-tion to show operations that address the entire set of objects as a unit rather than asingle object in it.
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For example, an operation to find an object within a set operateson the entire set, not on an individual object.
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The underlying static model is unaf-fected by this grouping.
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A multiobject is shown as two rectangles in which the top rectangle is shiftedslightly vertically and horizontally to suggest a stack of rectangles (Figure 13-125).
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A message arrow to the multiobject symbol indicates a message to the set of ob-jects—for example, a selection operation to find an individual object.
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To perform an operation on each object in a set of associated objects requirestwo messages: an iteration to the multiobject to extract links to the individual ob-jects, then a message sent to each object using the (temporary) link.
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This may beelided on a diagram by combining the messages into one that includes an iterationand an application to each object.
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The target rolename takes a many indicator (*)to show that many links are implied.
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An object from the set is shown as a normal object symbol, but it may be at-tached to the multiobject symbol using a composition link to indicate that it ispart of the set.
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A message arrow to the simple object symbol indicates a message toan individual object.
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Typically, a selection message to a multiobject returns a reference to an individ-ual object, to which the original sender then sends a message.
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A semantic variation of generalization in which an object may belong directly tomore than one class.
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This is a semantic variation point under which an object may be a direct instanceof more than one class.
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When used with dynamic classification, objects may ac-quire and lose classes during run time.
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This allows classes to be used to representtemporary roles an object may play.
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Although multiple classification matches logic and everyday discourse well, itcomplicates implementation of a programming language and is not supported bythe popular programming languages.
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A semantic variation point of generalization in which an element may have morethan one parent.
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This is the default assumption within UML and is necessary forproper modeling of many situations, although modelers may choose to restrict itsuse for certain kinds of elements.
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