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A complete description ofthe OCL syntax and semantics can be found in [Warmer-99].
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The following se-lected summary contains the most useful OCL syntax for creating navigation ex-pressions and Boolean conditions.
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Syntax for some common navigation expressions is shown below.
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The left-most element must be an expression for an object ora set of objects.
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The expressions are meant to work on sets of values when applica-ble.
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For more details and syntax, see the OCL description.
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The result is the value of the attribute or the related object(s).
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The result is the related object selected by the qualifier.
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Note that this syntax is applicable to array indexing as a form of qualification.
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The result is the property of the set.
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set -> select ( boolean-expression )
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The result is the subset of objects in the set for which the Boolean expression is true.
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The set of pilots who have enough training hourscompany.
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An operation is a specification of a transformation or query that an object may becalled to execute.
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It has a name and a list of parameters.
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A method is a procedurethat implements an operation.
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It has an algorithm or procedure description.
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See also call, call event, method.
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An operation specifies a transformation on the state of the target object (and pos-sibly the state of the rest of the system reachable from the target object) or a querythat returns a value to the caller of the operation.
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An operation may be imple-mented as a a method or as a call event that causes a transition in the state ma-chine of an active object.
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An operation is invoked by a call, which suspends thecaller until the execution of the operation is complete, after which the caller re-sumes control beyond the point of the call, receiving a return value if one is sup-plied by the operation.
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An operation is declared in a class.
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The declaration is inherited by the descen-dants of the class.
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If another declaration has the same “matching signature,” it isthe same operation.
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An implementation may specify a rule for matching signa-tures to test for conflict, but by default, it includes the name of the operation andthe classes (but not the names or directions) of the parameters, not including re-turn parameters.
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The same operation can appear in a descendant class.
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In thatcase, it is treated as a repetition of the inherited declaration and ignored.
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The pur-pose is to permit an operation to be declared multiple times in classes that are de-veloped in different packages, using name matching.
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It represents the governing declaration of the operation that isinherited by the others.
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A method is the implementation of an operation (it may also be implementedby a call event).
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If an operation is declared in a class without the abstract property,then it has a method definition in the class.
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Otherwise, the operation may be ab-stract (and there is no method), or it may be concrete with an inherited method.
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An operation has the following main constituents.
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Possible values aresequential Callers must coordinate so that only one call to anobject (on any sequential operation) may execute atonce.
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If concurrent calls occur, then the semanticsand integrity of the system cannot be guaranteed.
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The others are blocked until the execution ofthe first operation is complete.
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It is the responsibil-ity of the modeler to ensure that deadlocks do notoccur because of simultaneous blocks.
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Guarded op-erations must perform correctly (or block them-selves) in the case of a simultaneous sequentialoperation, or guarded semantics cannot beclaimed.
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All of them may proceed concurrently withcorrect semantics.
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Concurrent operations must bedesigned so that they perform correctly in the caseof a concurrent, sequential, or guarded operationon the same object.
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Otherwise, concurrent seman-tics cannot be claimed.
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If false, the current implementation isinherited unchanged by all descendants.
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If false, it may alter the state of the system, but achange is not guaranteed.
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with the list of parameter types (not including parameternames or return types), is called the matching signatureof the operation.
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The matching signature must be uniquewithin the class and its ancestors.
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If there is a duplication,it is taken as a redeclaration of the operation, which mustmatch completely.
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If they match, all but the operationdeclaration in the highest ancestor are ignored.
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If they donot match, the model is ill formed.
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return type A list of the types of the values returned by a call of theoperation, if any.
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then this property has the value null.
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Note that many lan-guages do not support multiple return values, but itremains a valid modeling concept that can be imple-mented in various ways, such as by treating one or moreof the parameters as output values.
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Possible values areinstance The operation may be applied to individual objects.
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The format of the specification is not prescribedby UML and can take various forms.
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A method has the same constituents as an operation.
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In addition, it may have oneor more ofbehavior An optional state machine describing the implementationof the method.
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Usually, this would be expressed in a program-ming language, although a natural language expression ispossible for informal specifications.
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Generally, this valuewould not be supplied if the state machine is supplied.
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A call event has the same constituents as an operation.
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The implementation of theoperation must be specified by one or more transitions that have the call event as atrigger.
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An operation is shown as a text string that can be parsed into properties of the op-eration.
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The stereotype, visibility, return-type-expression, and property string are op-tional (together with their delimiters).
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The parameter list may be empty.
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Figure 13-137 shows some typical operations.
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A string that is the name of the operation (not including parameters).
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A string containing a comma-separated list of names of classifiers(classes, data types, or interfaces).
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The type string follows a colon (:) that followsthe parameter list of the operation.
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Some, but not all, pro-gramming languages support multiple return values.
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The visibility is shown as one of the punctuation marks ‘+’, ‘#’, or ‘-’,representing public, protected, or private.
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Alternately, visibility can be shown as akeyword within the property string (for example, {visibility=private}).
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This formmust be used for user-defined or language-dependent choices.
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An operation and a method are declared using the same syntax.
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The top-most appearance of an operation signature within a generalization hierarchy is thedeclaration of an operation.
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Identical signatures in descendant classes are redun-dant declarations of the operation, but these may be useful for declaring methodsor for declaring operations when the classes are developed separately.
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If an opera-tion declaration has the abstract property (notated by operation name in italics orthe keyword abstract), then there is no method corresponding to the declaration.
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Otherwise, the declaration represents both an operation declaration and a methodimplementing it.
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In matching operations and methods, the name of the operation and the or-dered list of parameter types are used, not including return parameters.
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If the re-maining properties are inconsistent (for example, an in-parameter is matched toan out-parameter), then there is a conflict and the model is ill formed.
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If two identical operation declarations have no common ancestor operationdeclaration, yet are inherited by a common class, then the model is ill formed.
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The body of a method may be shown as a string within a note at-tached to an operation declaration.
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Otherwise, it should be normal text if it is just a natural-language de-scription of the behavior (a comment).
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The connection of a method declarationand its state machine or collaboration has no visual representation, but it wouldgenerally be represented by a hyperlink within an editing tool.
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An expression describing the effects of performing the operation.
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This may be stated in various ways, including text, before-after conditions, and in-variants.
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In any case, the specification should be expressed in terms of the observ-able effects of the operation on the state of the system, not in terms of theexecution algorithm.
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The algorithm is the business of the method.
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The choice true may also be shown by the keyword query.
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Theabsence of an explicit choice indicates the choice false—that is, the operation mayalter the system state (but it does not guarantee to alter it).
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Theabsence of an explicit choice indicates the choice true—that is, overridable.
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An instance-scope operation is indicated by not underlining the operationstring.
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A class-scope operation is indicated by underlining the name string.
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The choice is shown by a property string of the form concur-rency=value, where the value is one of sequential, guarded, or concurrent.
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To indicate that a class accepts a signal, the keyword «signal» is placed infront of an operation declaration within the list of operations.
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The declaration may not have a return type.
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The re-sponse of the object to the reception of the signal is shown with a state machine.
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Among other uses, this notation can show the response of objects of a class to er-ror conditions and exceptions, which should be modeled as signals.
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The argument list and return type may be suppressed (together, not separately).
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A tool may show the visibility indication in a different way, such as by using a spe-cial icon or by sorting the elements by group.
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The syntax of the operation signature string can be that of a particular program-ming language, such as C++ or Smalltalk.
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Operation names typically begin with a lowercase letter.
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Operation names are shown in plain face.
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