High coupling and low cohesion lead to difficult-to-maintain code. Strive for low coupling and high cohesion for more maintainable, understandable, and flexible software.
Coupling
Coupling refers to the degree of interdependence between software modules — how closely connected modules are and how much they rely on each other.
| Type | Description |
|---|
| High Coupling | Modules are tightly connected; changes in one module can significantly affect others. |
| Low Coupling | Modules are independent; changes in one module have minimal impact on others. |
Cohesion
Cohesion refers to the degree to which elements within a module belong together — how closely related and focused the responsibilities of a single module are.
| Type | Description |
|---|
| High Cohesion | A module has a single, well-defined responsibility and all its elements are closely related. |
| Low Cohesion | A module has multiple, unrelated responsibilities and its elements are not closely related. |
Differences Between Cohesion and Coupling
| Aspect | Coupling | Cohesion |
|---|
| Definition | Degree of interdependence between modules | Degree to which elements within a module belong together |
| Focus | Relationships between modules | Relationships within a module |
| Goal | Low coupling (loosely coupled) | High cohesion (highly cohesive) |
Real-World Example: Library System
Here’s a Library System designed with high cohesion and low coupling:
BookManager UserManager NotificationManager
─────────── ─────────── ───────────────────
Add Book Add User Send Email
Remove Book Remove User Send SMS
Search Book Search User
│ │
└────────────────────┴──────► NotificationManager
(loosely coupled via interface)
BookManager only manages books (add, remove, search).UserManager only manages users (add, remove, search).NotificationManager only handles notifications (email, SMS).
Low Coupling — modules work independently and interact only through clear interfaces:
BookManager, UserManager, and NotificationManager are loosely coupled.- Modules interact via well-defined interfaces rather than direct internal access.
Think of high cohesion as “each class does one thing well” and low coupling as “classes don’t need to know the internals of other classes”. Together, these principles make systems easier to test, extend, and maintain.