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In UML there are two related concepts:
What is the differences between instance specification and object?
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An instance is a specific representation of an object. An object is a generic thing while an instance is a single object that has been created in memory. Usually an instance will have values assigned to it's properties that differentiates it from other instances of the type of object.
An instance specification specifies the existence of an entity in a modeled system and completely or partially describes the entity. The description may include: The classification of an entity by one or more classifiers of which the entity is an instance.
Object instance is a specific occurrence of an object. For example, a specific mail message document is an instance.
In most of the cases, think of instance and object are the same thing. If you really interested with the difference between them, then...
We need to firstly know the relationship between "class" and classifier, the former is concrete, the latter is abstract.
UML2.5 / 11 Structured Classifiers / Classes Class is the concrete realization of EncapsulatedClassifier and BehavioredClassifier. The purpose of a Class is to specify a classification of objects and to specify the Features that characterize the structure and behavior of those objects.
I would say, an object is an instance of class; an InstanceSpecification is the instance of a classifier. Quoted from UML 2.5
The InstanceSpecification may represent: Classification of the instance by one or more Classifiers. If the only Classifier specified is abstract, then the InstanceSpecification only partially describes the instance.
"object" is actually not an concept of UML, the InstanceSpecification is a more abstract concept and all its semantics applies to "object".
Hope that helps.
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Instance Specification is a concept used for modeling the internals of UML.
An user of UML should not worry about this. Implementors of CASE tools in the other hand are aware of this concept since they need it to make the sintatic rules of the language be applied in their tools.
Object is a concept that is more OO than UML itself. An object is an instance of a class.
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