Common Mechanisms of UML

Common Mechanisms 

The four common mechanisms that apply throughout UML are: 

Specifications Adornments Common Divisions Extensibility Mechanisms

Here, the use of adornments and extensibility mechanisms are discussed 

• Node is an important adornment 
• Extensibility mechanisms are used to extend the language in controlled ways 
• These mechanisms contain stereotypes, tagged values and constraints 

Node  Stereotype  Tagged Value  Constraint

Node 

• Used for adding constraints or comments to an element/group of elements 
• Use to attach information to a model such as observations, requirements and explanations 
• Has no semantic impact, i.e., its content does not change the meaning of the model to which it is attached 
• Graphically represented as rectangle with a dog-eared corner 

Stereotype

• Used to extend the vocabulary of the UML by creating new model elements derived from existing ones that have specific properties suitable for a domain/problem 
• Each stereotype defines a set of properties that are received by elements of that stereotype 
• Graphically represented as a name enclosed by guillemots and placed above the name of another element («name») 

Tagged Value 

• Extend the properties of a UML building block by allowing to create new information in that element's specification 
• Can be defined for existing elements of the UML 
• Can also define tags that apply to individual stereotypes.  Then everything within that stereotype will have that tagged value 
• A tagged value is a kind of metadata, since its value applies to the element itself and not its instances
• Graphically represented as a string enclosed by brackets and placed below the name of another element

Constraint 

• Extend the semantics of a UML building block that allows to add new rules, or modify existing ones
• Graphically represented as a string enclosed by brackets and placed near the associated elements or connected to that elements by dependency relationships

Modeling New Properties 

• To model new properties:
• To model new properties, If there is no way to express what is needed by using UML and if there is no other way to describe the semantics, define a stereotype and assign the new properties to it 
• The rules of generalization are applied 
• The tagged values, which are defined for a kind of stereotype, apply to its children too

Modeling New Semantics 

To model new semantics: 

• If there is no way to express what is needed by using UML, and 
• If there is no way to convey the semantics, write new semantics in a constraint and place it near the element to which it refers 
• Depict a more precise relationship by connecting a constraint to its elements by using a dependency relationship 
• State semantics more accurately and formally; then write new semantics using Object Constraint Language (OCL)