Class Diagram: A Complete Guide

Class Diagram is widely used in object-oriented programming to visualize and design the structure of software systems before writing any code.

What is a Class Diagram?

A Class Diagram is a type of static structure diagram in the Unified Modeling Language (UML) that describes the structure of a system by showing its classes, attributes, methods, and the relationships among objects.

Key Components of a Class Diagram

Class

Represented by a rectangle divided into three parts:

• Top: Class name
• Middle: Attributes (variables/properties)
• Bottom: Methods (functions/operations)

-------------------
|   Person         |
-------------------
| - name: String   |
| - age: int       |
-------------------
| + speak(): void  |
| + walk(): void   |
-------------------

Attributes

• Represent class properties or fields.
• Format: visibility name: type
• Example: – name: String

Methods

• Represent operations the class can perform.
• Format: visibility name(parameter: type): returnType
• Example: + walk(): void

Visibility

• + Public
• – Private
• # Protected

Relationships

• Association (→) – “has a” relationship
• Inheritance/Generalization (⇨) – “is a” relationship
• Aggregation (◇) – whole-part with independent lifecycle
• Composition (◆) – whole-part with dependent lifecycle
• Dependency (—>) – one class uses another

Types of Relationships in Detail

Association: Shows that one class is connected to another.

• A basic connection between classes (bidirectional or unidirectional).
• Example: A Person has an Address.
• The arrow (if shown) usually points from the “user” class to the “used” class.

Person → Address

Inheritance (Generalization): Indicates one class is a subclass of another.

• Example: A Dog is a Animal.
• Arrow points to the parent (superclass).

Animal ⇨ Dog

Aggregation: A weak “whole-part” relationship.

• Example: A Team has Players.
• Diamond is on the “whole” side (Team), and arrow points to the “part” (Player).

Team ◇────→ Player

Composition: A strong “whole-part” relationship.

• Example: A House contains Rooms.
• Filled diamond on the whole (House), pointing to the part (Room).

House ◆────→ Room

Dependency: Represents a temporary relationship (e.g., method parameters).

• Example: Order uses Payment.
• Dashed arrow pointing to the dependent class (Payment).

Order ────→ Payment

Summary

Conclusion

Class diagrams are essential in software design—they help model the static structure of systems, understand object relationships, and communicate design clearly to stakeholders. Whether you’re designing a small module or a complex system, class diagrams give you a bird’s-eye view of your application’s architecture.