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3SL Reference

Class Diagram (CD)

The Class Diagram (CD) is the heart of any UML modelling activity. CDs are used to identify classes, to characterise these classes, and to define the relationships between them.

Each CD is likely to start in a relatively superficial way, and acquires additional detail as its classes are fully elaborated in appropriate cycles of the development process.

A CD contains classes which have a name and are defined by specifications. The specification for a class is mandatory, because Cradle stores the set of class attributes and operations not within the CD(s) in which the class is shown, but within frames of the specifications, which are called:

  • ATTRIBUTES
  • OPERATIONS

On the CD, the ATTRIBUTES and OPERATIONS frames are displayed in the class symbols if the CD is shown in Definitions display mode in the diagram.

Each instance of a class on a CD may have a different symbolic representation, in that a per-instance filter is used by the user to specify which (if any) of the attributes and operations are to be shown, and how much of each is to be shown.

A class may appear on one or more CDs, and potentially more than once on any such CD if desired. A class is said to be defined in the package that contains it, but may be declared within (i.e. used within) any other package that the designer requires. It is usual for the definition of the class to show its attributes and operations in full details, whereas uses of the class in other packages do not show the full detail.

A class may be an instance of a stereotype such as:

  • Entity
  • Boundary
  • Control
  • Template
  • Utility

The CDs will initially show only terse details of classes, but as the iterative development progresses, the contents of the CDs will become increasingly detailed including, for example, full signatures (including data types and default values) of each class attribute, and full signatures (similar to procedure/function call interfaces) of all class operations. Because Cradle allows CDs in both its analysis and design models, it is expected that users will exploit this flexibility, and will not elaborate Essential Domain CDs to a greater extent than is consistent with the scope of analysis work.

Classes exhibit relationships with other classes. These relationships range from a simple association, through a part-of aggregation or composition relationships, to kind-of inheritance relationships. These relationships are shown in CDs by appropriate types of connection symbol, linking the classes together. However, unlike other connection symbols, these class relationships have 5 parts to their text notation:

  • A name for the relationship
  • A source label, which contains the cardinality of that class in the relationship, as an expression of the form:

quantity […quantity]

where:

quantity - is either a number, such as 0, 1 or 26. or * for any number

…quantity - can optionally be used to specify a range and an upper bound

For example:

  • 1 - means exactly 1
  • 6 - means exactly 6
  • 0..1 - means zero or 1
  • 1..* - means any number but at least 1
  • 0..* - means any number but excluding 0
  • A destination label, which contains the cardinality of the destination class in the relationship, as an expression of the form:

quantity […quantity]

The destination label (destination class cardinality) is drawn next to the destination class and either below, or to the left of, the relationship

  • A source role, which identifies the role that the source class plays in the relationship with a meaningful name. It is drawn next to the source class and either above, or to the right of, the relationship
  • A destination role, which identifies the role that the destination class plays in the relationship with a meaningful name. It is drawn next to the destination class and either above, or to the right of, the relationship.

As the elaboration of CDs evolves, navigation can be added to the relationships, by replacing them by corresponding symbols that include arrowheads. The navigation for a relationship indicates the direction in which it can be queried. By default, relationships are bidirectional.

CDs are not hierarchical. Their connectivity is:

Diagram showing the connectivity of a Class Diagram

CDs are available in models in both the Essential and Implementation Domains.