Introduction to the Metamodel

Many model elements present an 'type/instance' dichotomy, in which a type represents the essence of an abstraction, and the instance forms a concrete sample. There is also a 'type/class' dichotomy, in which classes and primitive types implement types.

A type specifies a domain of values and a set of operations applicable to those values. A class implements a type: it provides the representation of attributes and the implementation of operations (methods). This distinction is propagated with subclasses, so that the specification given for a type is valid for all subclasses, and a subclass may implement several types.

The class diagram below represents both the type/instance dichotomy and the type/class dichotomy.

Simplified metamodel — an instance expresses a type; a class implements a type.

The

Type

• The primitive type, corresponding to a type that is not implemented by a class, such as integers or string types
  
  
• The class, which provides the implementation of a type
  
  
• The use case, which corresponds to a sequence of actions performed by an actor and a system

The following diagram represents the different kinds of types proposed by UML:

Simplified metamodel — representation of different kinds of types.

The

Class

• The active class, which substantiates one or more execution flows
  
  
• The signal, which corresponds to a named event
  
  
• The component, which corresponds to a reusable element that contains the physical constituents of the model elements
  
  
• The node, which is the physical device on which components may be deployed for execution

Simplified metamodel — representation of the various kinds of classes.

Relationships

UML defines five kinds of relationships. The most general is called a dependency relationship and applies uniformly to all the model elements. Association and generalization apply to all types, in particular classes and use cases. The last two relationships — transitions and links — apply to certain model elements pertaining to behavior.

This section is limited to the description of static relationships between classes, shown in the diagram below. Links and transitions are discussed later on in this chapter.

Simplified metamodel—representation of relationships between classes.

Association

An association specifies a bi-directional, semantic connection between types. An association has at least two roles, which describe the part played by the types that participate in the association.

Simplified metamodel—representation of the main characteristics of associations.

Each role comprises the following attributes:

• Multiplicity — specifies the number of instances that participate in the relationship
  
  
• Navigability — specifies if links (association instances) may be navigated along in the direction of the role being considered.
  
  
• Aggregation indicator — specifies if the instances of the type associated to the role correspond to the whole in a (whole, part) relationship. Only one of the roles of an association may have an aggregation indicator set to

  True

  . If the multiplicity is larger than

  1

  , several instances play the role of the whole and share the parts.
  
  
• Changeability — specifies if the semantics of the association are preserved when an instance of the type that participates in the role is replaced by another instance
  
  
• Ordering — applies when the multiplicity value is greater than 1, to mean that the instances are ordered

An association role may also include a set of attributes whose values implement a partition of the set of objects of the associated class.

Generalization

Generalization specifies a classification relationship in which an instance of a subtype may be substituted for an instance of a supertype. A supertype may have several subtypes, and a subtype may have several supertypes.

Simplified metamodel—representation of the generalization relationship and of elements prone to generalization.

Any element that may be generalized includes the following Boolean attributes:

• Abstract — the value

  True

  specifies that the element cannot be instantiated directly
  
  
• Leaf — the value

  True

  specifies that the element may not have subtypes
  
  
• Root — the value

  True

  specifies that the element may not have supertypes

UML defines three kinds of generalizable elements:

• Stereotypes allow the classification of a model element and, eventually, the definition of a particular graphical representation. A stereotype cannot have a subtype.
  
  
• Packages provide a general mechanism for grouping elements (both model and visual elements). A package cannot have supertypes.
  
  
• Types specify a definition domain and the operations applicable to that domain.

Dependency

Dependency is a unidirectional usage relationship between elements (both model and visual). The dependency relationship connects elements within the same model.

Simplified metamodel—representation of dependency relationships

UML also provides a 'trace' relationship between elements that belong to different models. Notably, the trace may be used to represent the history of current constructions in various models. This trace relationship is a stereotyped dependency relationship.

Simplified metamodel—the trace relationship is a stereotyped dependency relationship.