Class Diagram

Free Online Class Diagram Maker

What is a Class Diagram?

A class diagram is a UML (Unified Modeling Language) diagram that maps out the structure of an object-oriented system by showing its classes, attributes, methods, and the relationships between them. Software engineers and architects use class diagrams to design, document, and communicate how a codebase is organized before or during development. They make abstract OOP concepts — like inheritance, composition, and aggregation — immediately visible and easy to discuss. Use a class diagram any time you need to plan a system, review an existing architecture, or explain your code structure to a team.

Key Features

Full UML Relationship Support

Model inheritance, composition, aggregation, association, dependency, and realization — all eight standard UML relationship types are supported.

Class Member Editor

Define attributes and methods with visibility modifiers (public, private, protected, package), data types, return types, and static or abstract flags.

Stereotypes

Tag classes as Interface, Abstract, Enumeration, or Service to reflect accurate OOP semantics in your diagram.

Cardinality Labels

Annotate relationships with cardinality notation (1, 0..1, 1..*, *) to precisely describe how many instances participate in each relationship.

Zoom and Pan

Navigate large class diagrams with built-in zoom and pan controls so complex multi-class systems remain readable at any scale.

AI-Powered Generation

Describe your system in plain text and the AI builds the class structure for you — classes, members, and relationships included.

Best For

Object-oriented software design and planning

Documenting existing codebases for onboarding

Database schema visualization in ORM terms

Teaching or learning OOP concepts and patterns

Design pattern illustrations (MVC, Observer, Factory)

API contract and domain model reviews

When to Use

You are designing a new system and need to align the team on class responsibilities before writing code
You want to document how existing classes relate so new developers can get up to speed quickly
You are explaining an OOP pattern (e.g. inheritance hierarchy or composition chain) to a non-technical stakeholder
You need to visualize a database schema in terms of entities and their relationships
You are reviewing a pull request that adds or restructures classes and want a visual diff
You are preparing technical documentation or a software architecture proposal

Common Mistakes

Putting every method and attribute on every class — only include members that are relevant to the relationship or design decision you are communicating
Confusing composition and aggregation — composition means the child cannot exist without the parent; aggregation means it can
Using inheritance when composition is more appropriate — prefer composition over inheritance unless a true 'is-a' relationship exists
Omitting cardinality labels — leaving out 1..* or 0..1 annotations forces readers to guess multiplicity
Making one giant diagram instead of splitting into domain-focused sub-diagrams — large diagrams become unreadable above ~15 classes
Skipping stereotypes on interfaces and abstract classes — readers will assume everything is a concrete class without them

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Templates

Start with professionally designed templates

Classroom Management System Class Diagram Templateclassdiagram

Ready-to-use UML class diagram template for classroom management systems. Map teachers, students, courses, grades, and attendance visually. Free to edit.

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Object Oriented Design Template: UML Class Diagramclassdiagram

Free OOP class diagram template with inheritance, interfaces, and relationships for a library management system. Customize and export instantly.

UML Class Diagram Templateclassdiagram

Design clear UML class diagrams with this ready-to-use template. Visualize classes, attributes, methods, and relationships instantly. Free to use online.

Inheritance Hierarchy Class Diagram Templateclassdiagram

Visualize OOP inheritance with this UML class diagram template. Map abstract classes, subclasses, and overridden methods in minutes. Free and ready to use.

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Describe your system or paste class names — our AI generates a UML class diagram with members and relationships in seconds.

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