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Class Diagrams

Oct 30, 20242 min read

Class Diagrams

Ref

Overview

  • Definition: A static model that shows classes and their relationships to one another.

Example Class Diagram invert_B

Elements

  • Classes:
    • Objects within the system (e.g., person, place, thing).
    • Stores and manages information and contains:
      • Attributes: Characteristics of the class.
      • Operations: Activities the class can perform.
  • Relationships:
    • Associations between classes, depicted as lines.
    • Multiplicity indicates how many of one object is associated with another.

Attributes

  • Definition: Properties of a class.
  • Examples: Last name, first name, address, etc.
  • Derived Attributes:
    • Preceded with a slash (/), e.g., age derived from date of birth.
  • Visibility:
    • Public (+): Visible to all classes.
    • Private (-): Visible only to the class instance in which they are defined.
    • Protected (#): Visible to the class instance and its descendants.

Operations

  • Common operations are not shown (create/delete, return/set value).
  • Types of Operations:
    • Constructor: Creates an object.
    • Query: Provides information about the object’s state.
    • Update: Changes values of some or all attributes.
    • Destructor: Deletes or removes an object.

Relationships

  • Denotes associations between classes.
  • Depicted with a line labeled with the name of the relationship.
  • Directional Relationships: Depicted with a triangle (e.g., a patient schedules an appointment).
  • Self-Associations: Classes related to themselves (e.g., employees and managers in the same class).

Multiplicities

  • Department Boss: 1 to 1 (A department has one and only one boss).
  • Employee Child: 1 to 0..* (An employee has zero to many children).
  • Boss Employee: 1 to 1..* (A boss is responsible for one or more employees).

Association Classes

  • Common in many-to-many relationships.
  • Used to record attributes about the relationship between two classes.
    • Example: Students related to courses; a Grade class provides attributes describing this relationship.

Generalization & Aggregation

  • Generalization:
    • Denotes inheritance; properties and operations of the superclass are valid for the subclass.
    • Depicted as a solid line with a hollow arrow pointing at the superclass.
  • Aggregation:
    • Denotes a logical “a-part-of” relationship.
  • Composition:
    • Denotes a physical “a-part-of” relationship.

Simplifying Class Diagrams

  • Fully populated class diagrams can be difficult to understand.
  • Common simplifications:
    • Show only concrete classes.
    • The view mechanism shows a subset of classes.
    • Packages show aggregations of classes or any elements in UML.

Graph View

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