🧱 Class and Module Design
Class design is the discipline of defining how individual classes should be structured, what responsibilities they should have, and how they should collaborate with other classes. It sits at the code‑level of software design and is essential for building maintainable, flexible, and testable systems.
Class design — definition
Section titled “Class design — definition”Class design is the process of defining a class’s responsibilities, data, behavior, and collaborations so that it is cohesive, loosely coupled, and easy to maintain. It ensures each class has a clear purpose and interacts cleanly with other classes through well‑defined interfaces.
It answers:
- What does this class represent?
- What responsibilities does it own?
- What data does it manage?
- How does it collaborate with other classes?
Why class design matters
Section titled “Why class design matters”Good class design:
- Reduces complexity
- Improves readability
- Increases maintainability
- Supports testability
- Enables flexibility and extensibility
- Prevents tight coupling and duplicated logic
Poor class design leads to fragile, tangled code that is hard to evolve.
Core principles of class design
Section titled “Core principles of class design”- Single responsibility — each class should have one reason to change
- Encapsulation — hide internal details behind clear interfaces
- Cohesion — class responsibilities should be strongly related
- Low coupling — minimize dependencies on other classes
- Abstraction — expose only what is necessary
- Composition over inheritance — prefer combining objects instead of deep inheritance
- Clear interfaces — define stable contracts for collaboration
- Immutability where possible — reduce side effects and improve predictability
These principles ensure classes remain clean and evolvable.
Key elements of class design
Section titled “Key elements of class design”1. Responsibilities
Section titled “1. Responsibilities”Define what the class should and should not do.
Avoid “God classes” that do too much.
2. Attributes (state)
Section titled “2. Attributes (state)”Define the data the class owns.
State should be minimal and consistent.
3. Methods (behavior)
Section titled “3. Methods (behavior)”Define what the class can do.
Methods should operate on the class’s own data.
4. Collaborations
Section titled “4. Collaborations”Define how the class interacts with other classes.
Use interfaces to reduce coupling.
5. Visibility
Section titled “5. Visibility”Use private, protected, and public intentionally.
Expose only what is necessary.
6. Construction
Section titled “6. Construction”Use constructors, builders, or factories to create objects cleanly.
Class design workflow (practical)
Section titled “Class design workflow (practical)”- Identify the class’s purpose
- Define its responsibilities
- Define its attributes
- Define its methods
- Define its collaborators
- Apply SOLID principles
- Review cohesion and coupling
- Refactor if needed
Example
Section titled “Example”If you were designing classes for a Daily Scrum application:
DailySummaryGenerator
Section titled “DailySummaryGenerator”- Responsibility: generate daily summaries
- Attributes: rules, templates
- Methods:
GenerateSummary(session) - Collaborations:
TaskRepository,MemberRepository
PdfExporter
Section titled “PdfExporter”- Responsibility: export summaries to PDF
- Attributes: PDF settings
- Methods:
Export(summary) - Collaborations: none (pure function)
NotificationService
Section titled “NotificationService”- Responsibility: send notifications
- Attributes: channels, templates
- Methods:
SendReminder(member) - Collaborations:
EmailClient,MessageBus
Each class has a clear, focused responsibility.
Takeaway
Section titled “Takeaway”Class design defines how classes are structured and how they collaborate. Good class design uses SOLID principles, encapsulation, cohesion, and clear interfaces to produce maintainable, flexible, and testable code. It is the foundation of clean software design.