Class Diagram

Visualize class structure and relationships

Overview

Class diagrams are the most common UML diagrams used in LLD. They show the static structure of a system by displaying classes, their attributes, methods, and relationships between them.

Class diagrams are essential for designing and documenting object-oriented systems.

Key Concepts

Classes shown as rectangles with three compartments

Top: Class name, Middle: Attributes, Bottom: Methods

Access modifiers: + (public), - (private), # (protected)

Relationships: Association, Aggregation, Composition, Inheritance

Multiplicity: 1, 0..1, , 1.., etc.

Code Example

java

1/*
2Class Diagram Notation:
3┌─────────────────────────────┐
4│         <<interface>>       │  ← Stereotype (optional)
5│           Vehicle           │  ← Class name
6├─────────────────────────────┤
7│ - brand: String             │  ← Attributes (- = private)
8│ - model: String             │
9│ # year: int                 │  ← # = protected
10├─────────────────────────────┤
11│ + start(): void             │  ← Methods (+ = public)
12│ + stop(): void              │
13│ + getInfo(): String         │
14└─────────────────────────────┘
15
16Relationships:
17─────────────────> Association (simple line with arrow)
18◇─────────────────> Aggregation (hollow diamond)
19◆─────────────────> Composition (filled diamond)  
20─ ─ ─ ─ ─ ─ ─ ─ ─ ▷ Realization/Interface (dashed with hollow arrow)
21────────────────────▷ Inheritance (solid with hollow arrow)
22- - - - - - - - - -> Dependency (dashed arrow)
23
24Example: E-commerce System
25┌─────────────────┐       ┌─────────────────┐
26│     Customer    │       │     Product     │
27├─────────────────┤       ├─────────────────┤
28│ - id: String    │       │ - id: String    │
29│ - name: String  │       │ - name: String  │
30│ - email: String │       │ - price: double │
31├─────────────────┤       ├─────────────────┤
32│ + placeOrder()  │       │ + getDetails()  │
33└────────┬────────┘       └────────┬────────┘
34         │ 1                       │ *
35         │                         │
36         ▼ *                       │
37┌─────────────────┐                │
38│      Order      │◆───────────────┘
39├─────────────────┤    (Composition)
40│ - orderId       │
41│ - orderDate     │
42│ - totalAmount   │
43├─────────────────┤
44│ + addItem()     │
45│ + checkout()    │
46└─────────────────┘
47*/

This shows UML class diagram notation including classes, attributes, methods, and different types of relationships.

Best Practices

1.
Keep diagrams focused - don't include everything
2.
Use proper notation for relationships
3.
Include multiplicity on associations
4.
Show only relevant attributes and methods
5.
Group related classes visually

💡 Interview Tips

Draw class diagrams during LLD interviews
Know all relationship types and their notation
Start with main entities, then add relationships
Use proper access modifiers
Practice drawing common systems (parking lot, library)