The Language of Structure: Class Diagrams

If HLD describes the islands in your system, a Class Diagram describes the architecture of the buildings on those islands. It is a static diagram that describes the structure of a system by showing its classes, their attributes, operations, and the relationships among objects.

Anatomy of a Class

A class in UML is typically represented by a rectangle divided into three compartments:

1. Top: The Class Name.
2. Middle: Attributes (fields) with their types and visibility.
3. Bottom: Methods (operations) with their parameters and return types.

Visibility Notations:

• + Public
• - Private
• # Protected
• ~ Package/Internal

Mastering Relationships

The true power of a class diagram lies in showing how objects relate to one another. There are five primary relationship types you must master.

1. Generalization (Inheritance)

Indicates an "is-a" relationship. Represented by a solid line with a hollow arrowhead pointing to the parent.

2. Realization (Implementation)

Indicates a class implementing an interface. Represented by a dashed line with a hollow arrowhead.

3. Association

A basic "knows-a" relationship. Class A has a reference to Class B. Represented by a simple solid line.

4. Aggregation

A "part-of" relationship where the child can exist independently of the parent (e.g., a Department and its Professors). Represented by a hollow diamond.

5. Composition

A strong "part-of" relationship where the child cannot exist without the parent (e.g., a House and its Rooms). Represented by a filled diamond.

Visualizing Relationships with Mermaid

In the diagram above:

• A University is composed of Departments (if the uni closes, departments are gone).
• A Department aggregates Professors (if the department closes, professors still exist).
• A Professor has an association with MathCourses.
• MathCourse realizes the Course interface.