What is a class diagram and how to read it?

Question

Accepted Answer

**Class Diagram** — UML diagram showing classes, attributes, methods, and relationships.

**How to read:**
```
┌─────────────────┐
|     Class       │
├─────────────────┤
| - id: int       │  ← private attribute
| + name: string  │  ← public attribute
| # email: string │  ← protected attribute
├─────────────────┤
| + login(): void │  ← public method
| - validate(): bool│ ← private method
└─────────────────┘
```

**Relationships:**
- **Association** — A → B (uses).
- **Aggregation** — A ◇→ B (has-a, weak).
- **Composition** — A ◆→ B (has-a, strong).
- **Inheritance** — A △→ B (is-a).
- **Dependency** - - -> A (uses temporarily).

**Cardinality:**
- `1` — exactly one.
- `0..1` — zero or one.
- `*` — many.
- `1..*` — one or many.

Systems Analyst: What is a class diagram and how to read it?