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An abstract operation is shown in italics.
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A property of a set of values, such as the set of objects related to an object across anassociation, stating whether the set is ordered or unordered.
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The ordering property declares whether the set is ordered or unordered.
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If it is ordered, the elements in the set have an explicitly imposed order.
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Theelements can be obtained in that order.
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When a new link is added to the associa-tion, its position in the sequence must be specified by the operation adding it.
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Theposition may be an argument of the operation or it may be implicit.
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For example, agiven operation may place a new link at the end of the existing list of links, but thelocation of the new link must be specified somehow.
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A sorting is totally determined by the val-ues of the objects in the set.
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Therefore, it adds no information, although it maycertainly be useful for access purposes.
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The information in an ordered association,on the other hand, is additional to the information in the elements themselves.
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The ordering property applies to any element that takes a multiplicity, such as
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An ordered relationship may be implemented in various ways, but the imple-mentation is usually stated as a language-specified code generation property.
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A sorted set requires a separate specification of the sorting rule itself, which isbest given as a constraint.
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Ordering is specified by a keyword in braces near the end of the path to which itapplies (Figure 13-138).
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The absence of a keyword indicates unordered.
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The key-word {ordered} indicates an ordered set.
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For design purposes, the keyword{sorted} may be used to indicate a set arranged by internal values.
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For an attribute with multiplicity greater than one, one of the ordering key-words may be placed after the attribute string, in braces, as part of a propertystring.
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If an ordering keyword is omitted, then the set is unordered.
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An ordered set has information in the ordering, information that is additional tothe entities in the set itself.
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And just because aset is ordered does not mean that any ordering of entities will be allowed.
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These aredecisions that the modeler must make.
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In general, the position of the new entitywithin the list is a parameter of the creation operation.
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Note that the ordering of a binary association must be specified independentlyfor each direction.
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Ordering is meaningless unless the multiplicity in a direction isgreater than one.
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An association can be completely unordered, it can be ordered inone direction and not the other, or it can be ordered in both directions.
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Then, usually, a new link will be added as an operation on an A object, speci-fying a B object and a position in the list of existing B objects for the new link.
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Frequently, an operation on an A object creates a new B object and also creates alink between A and B.
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The list must be added to the list of links maintained by A.
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Of course, a programmer can implement morecomplicated situations if needed.
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An association that is ordered in both directions is somewhat unusual, becauseit can be awkward to specify the insertion point in both directions.
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But it is possi-ble, especially if the new links are added at default locations in either direction.
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Note that a sorted set does not contain any extra information beyond the infor-mation in the set of entities.
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Sorting saves time in an algorithm, but it does notadd information.
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It may be regarded as a design optimization and need not be in-cluded in an analysis model.
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The location of the new entity must be determined automatically by themethod by examining the attributes on which the list is sorted.
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The set of reservations is unordered.
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One of a set of states that partition a composite state into substates, all of which areconcurrently active.
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An indication of whether the feature applies to an individual object or is shared byan entire class.
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Owner scope indicates whether there is a distinct attribute slot for each instance ofa class, or if there is one slot for the entire class itself.
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For an operator, owner scopeindicates whether an operation applies to an instance or to the class itself (such as acreation operator).
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Possible values areinstance Each classifier instance has its own distinct copy of anattribute slot.
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Values in one slot are independent of valuesin other slots.
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For an operator, the operator applies to an individualobject.
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Allthe instances of the classifier share access to the one slot.
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For an operator, the operator applies to the entire class,such as a creation operator or an operator that returnsstatistics about the entire set of instances.
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A class-scope attribute or operator is underlined (Figure 13-139).
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An instance-scope attribute or operator is not underlined.
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For an association, this would say whether the source position of a link holds in-stances or classifiers.
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A general-purpose mechanism for organizing elements into groups.
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Packages maybe nested within other packages.
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A system may correspond to a single high-levelpackage, with everything else in the model contained in it recursively.
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Both modelelements and diagrams may appear in a package.
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See also access, dependency, import, model, namespace, subsystem.
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A package is a grouping of model elements and diagrams.
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Every model elementthat is not part of another model element must be declared within exactly onenamespace; the namespace containing the declaration of an element is said to ownthe element.
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A package is a general-purpose namespace that can own any kind ofmodel element that is not restricted to one kind of owner.
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Each diagram must beowned by exactly one package, which may be nested within (and therefore ownedby) another package.
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A package may contain subordinate packages and ordinarymodel elements.
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Some packages may be subsystems or models.
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The entire systemdescription can be thought of as a single high-level subsystem package with every-thing else in it.
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All kinds of UML model elements and diagrams can be organizedinto packages.
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Packages own model elements, subsets of the model, and diagrams.
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Packages arethe basis for configuration control, storage, and access control.
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Each element canbe directly owned by another model element or by a single package, so the owner-ship hierarchy is a strict tree.
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However, model elements (including packages) canreference other elements in other packages, so the usage network is a graph.
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The special kinds of package are model, subsystem, and system.
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It is the only model el-ement not owned by some other model element.
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It indirectly includes everythingin the model.
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There are several predefined stereotypes of model and subsystem.
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See Chapter 14, Standard Elements, for details.
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Packages may have dependency relationships to other packages.
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In most casesthese summarize dependencies among the contents of the packages.
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The access dependency is particular to packages themselves and is not a sum-marization of dependencies on their elements.
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It indicates that elements in the cli-ent package are granted permission to have relationships to elements in thesupplier package.
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The relationships are also subject to visibility specifications.
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The accessdependency variation import is like an Ada uses statement.
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It adds the namesfrom the supplier namespace to the client namespace (they must not conflict).
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Butthe access dependency does not alter the client namespace.
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It is mainly an accesscontrol mechanism in larger development projects, rather than a fundamental se-mantic relationship.
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A nested package has access to any elements directly contained in outer pack-ages (to any degree of nesting), without needing either import dependencies orvisibility.
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A package must import its contained packages to see inside them, how-ever.
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A contained package is, in general, an encapsulation boundary.
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A package defines the visibility of its contained elements as private, protected,or public.
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Private elements are not available at all outside the containing package(regardless of imports).
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Protected elements are available only to packages withgeneralizations to the containing package, and public elements are available to im-porting packages and to descendants of the package.
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See access for a full description of the visibility rules for elements in variouspackages.
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A package is shown as a large rectangle with a small rectangle (a “tab”) attached onone corner (usually, the left side of the upper side of the large rectangle).
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If contents of the package are not shown, then the name of the package is placedwithin the large rectangle.
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If contents of the package are shown, then the name ofthe package may be placed within the tab.
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A keyword string may be placed above the package name.
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Keywords may in-clude subsystem, system, and model.
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User-defined stereotypes are also notatedwith keywords, but they must not conflict with the predefined keywords.
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A list of properties may be placed in braces after or below the package name.
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The visibility of a package element outside the package may be indicated by pre-ceding the name of the element by a visibility symbol (‘+’ for public, ‘-’ for private,‘#’ for protected).
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Relationships may be drawn between package symbols to show relationshipsamong at least some of the elements in the packages.
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In particular, dependencyamong packages (other than permission dependencies, such as access and import)implies that there exist one or more dependencies among the elements.
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A tool may also show visibility by selectively displaying those elements that meet achosen visibility level, for instance, all the public elements only.
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A tool may show visibility by a graphic marker, such as color or font.
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