Structural Models
Structural models display the organization of a system in terms of its component parts and their relationships. They are fundamental representations used when discussing and designing the system architecture.
Static vs. Dynamic Models
Structural models can be categorized into two main types:
Static Models: Show the organization of the system design, focusing on components like object classes and their relationships.
Dynamic Models: Show the organization of the system executing, reflecting structures such as interacting threads.
Architectural Context
Although UML has component, package, and deployment diagrams for architectural modeling, the core of this section focuses on Class Diagrams to detail the static structure of objects.
Class Diagrams
Class diagrams show the classes in a system and the associations (links) between them.
Object Class: A general definition of one kind of system object. In early stages, these represent real-world entities (e.g., patient, doctor).
Association: A link indicating a relationship, often requiring one class to have knowledge of the other.
Class Structure Detail
Classes are represented by a rectangle divided into three sections:
Top Section: The name of the object class.
Middle Section: The class attributes (data fields).
Lower Section: The operations (methods) associated with the class.
Multiplicity (Associations)
Multiplicity indicates how many objects are involved in an association.
| Symbol | Meaning | Example |
|---|---|---|
1 | Exactly one object | A patient has exactly one record. |
1..* | One or more objects | A patient attends one or more consultations. |
* | An indefinite number | General indefinite relationship. |
1:1 means one-to-one relationship, a patient has one record and a record belongs to one patient.
1:Many (1..*) means one-to-many relationship, a patient can attend many consultations, but each consultation is for one patient.
Many:Many (..) means many-to-many relationship, a patient can have many conditions, and each condition can be diagnosed in many patients.
Example: Associations and Multiplicity
The following diagram represents classes and associations in the Mentcare system, showing how a Patient interacts with various entities, such as Consultation, Condition, and Hospital Doctor.
Mentcare Class Diagram
Example: Attributes and Operations
This diagram details the Consultation class, showing the specific attributes and operations.
Consultation Class Diagram
Generalization
Generalization (often called inheritance) allows the system to define hierarchies of classes. Attributes and operations in the higher-level classes (superclasses) are automatically associated with lower-level classes (subclasses). Lower-level classes can also have their own specific attributes and operations.
- Notation: An arrowhead pointing up toward the more general class.
A General practitioner and Hospital doctor are specialized types of Doctor. Furthermore, Hospital doctor is specialized into Consultant, Team doctor. A Trainee doctor and Qualified doctor are specialized types of Team doctor.
Doctor Generalization Diagram
All doctors share common attributes and operations defined in the Doctor class, while each subclass adds its own specific details. hospital doctor has additional attributes like Staff_# and Pager_#, while General practitioner has Practice and Address.
Aggregation
Aggregation is a type of association used to illustrate when objects are made up of different parts. Examples include a PatientRecord composed of Patient and multiple Consultation objects.
Definition: One object (the whole) is composed of other objects (the parts).
Notation: A diamond shape is added to the link next to the class representing the whole.
This shows that a PatientRecord contains one Patient and one or more Consultation objects.
Practice Questions
- What are structural models? Explain class diagrams with an example.
- Describe multiplicity, generalization, aggregation, and associations in class diagrams.
- Give an example class diagram for an ATM or a language-processing system.