What is the difference between coupling and cohesion?

Question

Accepted Answer

**Coupling** — dependency between modules. **Cohesion** — internal strength of a module.

**Coupling (inter-module):**
- How modules depend on each other.
- Goal: Low coupling (independent).
- High coupling = hard to change.

```
Low Coupling:
  Module A → API → Module B
  (communicate via interface)

High Coupling:
  Module A directly accesses Module B internals
  (tight dependency)
```

**Cohesion (intra-module):**
- How elements within module belong together.
- Goal: High cohesion (focused).
- Low cohesion = module does too many things.

```
High Cohesion:
  UserService: login, logout, register
  (all auth-related)

Low Cohesion:
  UserService: login, sendEmail, calculateTax
  (unrelated responsibilities)
```

**Goal:** Low coupling, high cohesion.

Systems Analyst: What is the difference between coupling and cohesion?