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
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
Inheritance Hierarchy Class Diagram Template
Quickly map OOP inheritance chains - from abstract base classes to concrete implementations - using this ready-to-edit UML class diagram template.
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Start with a ready-to-edit e-commerce class diagram and visualize your object-oriented system design in minutes.
Object Oriented Design Template: UML Class Diagram
Model class hierarchies, interfaces, and relationships in a clean, professional class diagram built for OOP design.
Data Classification Framework Class Diagram Template
A ready-to-use UML class diagram template that models data assets, classification levels, ownership, access policies, retention rules, and audit logging in one structured view.
Classroom Management System Class Diagram Template
A structured UML class diagram that maps every key entity in a classroom management system — from teachers and students to assignments, grades, and attendance.
View All Templates
Browse the full library to find more templates tailored to your chart.