How to Create UML Class Diagrams Online Free — OOP Design Tool

What Is a UML Class Diagram?

A class diagram is the blueprint of an object-oriented system. It shows classes with their properties and methods, and the relationships between them — inheritance, composition, association, and dependency. It is the most-used UML diagram type and essential for software design, code reviews, and technical documentation.

How to Create a Class Diagram

1. Open the Class Diagram tool
2. Define classes with their attributes and methods using text syntax
3. Specify relationships (inheritance, composition, association)
4. The diagram renders in real-time
5. Export as PNG or SVG

Text-based input is dramatically faster than drag-and-drop tools. Define a class in 3 lines, not 30 clicks.

Relationship Types Explained

• Inheritance (--|>) — "Dog inherits from Animal." Subclass extends superclass.
• Composition (*--) — "Car has an Engine." Strong ownership — Engine doesn't exist without Car.
• Association (--) — "Student takes Course." Independent objects that reference each other.
• Dependency (..>) — "OrderService depends on PaymentGateway." Temporary usage relationship.

Common Use Cases

• System design — plan class hierarchies before writing code
• API contracts — visualize request/response models and their relationships
• Code reviews — attach diagrams to PRs that introduce new models
• Interviews — system design interviews frequently require class diagrams
• Documentation — keep living diagrams in your project wiki