3C Clear Clean Concise  UML
communityUML 2torial

Important relationships

There are several relationships that are important for modeling:

• The Representation relationship: The relationship between a model or model element and what it represents.
• The Type Conformance relationship: The relationship between a type and a model element that conforms to that type.
• The Instantiation relationship:  The relationship between a specification and something that is created using that specification.
• The Description relationship:  The relationship between an object and objects it describes.
• The Powertype relationship:  The relationship between a class and the classes it classifies.
• The Meta-linguistic relationship:  The relationship between a term in a meta-language and a term in the language.
• The Model-Metamodel relationship:  The relationship between a model and the metamodel of the language of that model.
• The Language Use relationship:  The relationship between a language and the use of that language.

In UML 1 (and sometimes elsewhere at OMG) the word, 'instance,' and the phrase, 'instance of,' are used with at least five or six different meanings. 

So we find in the UML specification and in writing about UML and MOF, that 'instance' is used with all these meanings, for example:

• This loaded Java class, Dog, is an instance of this UML class, Dog. (representation)
• This object, Fido, is an instance of this class, Animal. (type conformance)
• This object, Fido, is an instance of this class, ThingFactory.   (instantiation)
• This object, Chihuahua, is an instance of this object, Breed.  (description)
• This class, Chihuahua, is an instance of this class, Breed (powertype)
• This class, Dog, is an instance of this class, Class.  (meta-linguistic)
• This model, PetStoreSystem, is an instance of the UML metamodel.   (model-metamodel)
• This model, PetStoreSystem, is an instance of UML.  (language use)

It is possible that these are not all distinct meanings; perhaps in some pair of the usages above the meanings of 'instance of' are identical.  It is certain that there is more than one distinct meaning of 'instance of' in the examples above. Likely somewhere between six and eight distinct meanings.

Life will be, not only simpler, but more pleasant, if we give each distinct relationship a different name.  For example:

• represented by: The thing is represented by the model element.
• instance of: The model element conforms to or satisfies the type; it is an instance of the type.
• instantiation of:  The model element was created using the specification; it is an instantiation of the specification.
• description of: The model element is a description of another model element.
• example of:  The model element is an example of the term in the meta-language.  The model element is an example of the model element in the metamodel.
• language metamodel of: The metamodel is a model of the language used by the model; it is the language metamodel of the language used by the model.
• uses: The model uses the language.

A metaphor:

That cat is represented by 'cat'.  Whiskers is represented by 'Whiskers.' 
Whiskers is an instance of  Domestic Shorthair.
'Whiskers' is an example of a proper noun; 'cat' is an example of a common noun.
'Whiskers is my cat' uses English.

Another metaphor:

Whiskers is represented by .        
   is an example of an icon.

Three concrete examples:

The OMG web server is represented by the object, omgWebServer.
The object, omgWebServer is an instance of the type, WebServer.
The object, omgWebServer is an instantiation of the specification, Specification for an OMG web server.
The object, omgWebServer is an instantiation of the Java class, JWebServer.
The phrase, 'OMG web server,' is an example of a noun phrase.
The object, omgWebServer, is an example of a UML Object

Whiskers is represented by the object, Whiskers.
Whiskers is an instance of the type, Pet.
Whiskers is an instantiation of  the class, Cat.
Whiskers is an example of a UML object.

<work in progress:>

In these memory locations The instructions in these memory locations is code and this data here is the line of code is excuting the program

Back to the 2torial 
(If you came here from the 2torial, but this link does not work for you,
please use the Back button on your browser to return to the 2torial.)

The 2torial is at http://community-ml.org/2torial/