Free Online Class Diagram Editor

Design object-oriented systems with UML class diagrams. Define classes, attributes, methods, and relationships using simple text.

Create Class Diagrams

Key Features

  • Define classes with attributes and methods
  • Show inheritance and composition
  • Visualize relationships and multiplicity
  • Support for interfaces and abstract classes
  • Instant diagram preview
  • Export to SVG or PNG

Common Use Cases

  • Design software architecture
  • Document object-oriented systems
  • Plan database schemas
  • Visualize code structure
  • Create technical specifications
  • Teach object-oriented concepts

Example Class Diagram

classDiagram
  class Animal {
    +int age
    +eat()
  }
  class Dog {
    +bark()
  }
  Animal <|-- Dog
Try this example

Why Use This Class Diagram Editor?

Our free online class diagram editor makes it easy to create professional diagrams using simple text-based syntax. No installation required - just start typing and see your diagram render in real-time.

Built on Mermaid.js, this editor provides instant preview, SVG/PNG export, and URL sharing capabilities. Perfect for developers, technical writers, project managers, and anyone who needs to create clear, professional diagrams quickly.

How to Create a Class Diagram

  1. Start with 'classDiagram' keyword to initialize your UML class diagram
  2. Define classes using 'class ClassName' syntax with descriptive, noun-based names
  3. Add attributes (properties) to classes with data types using the format: +attributeName : Type
  4. Add methods (operations) to classes using format: +methodName(params) ReturnType
  5. Specify visibility with symbols: + for public, - for private, # for protected, ~ for package
  6. Define relationships using arrows: <|-- for inheritance, *-- for composition, o-- for aggregation
  7. Add multiplicity to relationships (1, 0..1, 1..*, *) to show cardinality
  8. Use <<interface>> or <<abstract>> stereotypes to denote special class types

Best Practices for Class Diagrams

  • Follow naming conventions: Use PascalCase for class names, camelCase for methods/attributes
  • Show only relevant details: Include attributes and methods that matter for your diagram's purpose
  • Use appropriate relationships: Understand the difference between inheritance, composition, and association
  • Specify multiplicity clearly: Always show cardinality on relationships (1, *, 0..1, etc.)
  • Group related classes: Position related classes near each other for better readability
  • Use interfaces wisely: Extract common behaviors into interfaces when multiple classes share functionality
  • Keep diagrams focused: Don't try to show every class - create separate diagrams for different subsystems
  • Document visibility correctly: Use proper access modifiers to communicate encapsulation

Advanced Class Diagram Features

Generic/Template Classes

Show parameterized types using angle brackets like List<T> or Map<K,V>. Essential for documenting collections, generics, and type-safe data structures.

Stereotypes and Labels

Use <<interface>>, <<abstract>>, <<enum>>, or custom stereotypes to classify classes. Add labels to relationships to describe their nature or constraints.

Bidirectional Associations

Show relationships that navigate in both directions using -- or .., and specify different multiplicity at each end to model complex object graphs.

Association Classes

Document attributes that belong to relationships rather than classes themselves, perfect for modeling many-to-many relationships with additional data.

Common Class Diagram Mistakes

  • ⚠️Confusing composition with aggregation - composition means 'owns' (filled diamond), aggregation means 'has' (empty diamond)
  • ⚠️Wrong inheritance direction - arrow points from subclass to superclass, not the other way around
  • ⚠️Missing multiplicity - always specify how many instances are involved in relationships
  • ⚠️Showing too much detail - don't include every getter/setter or private helper method
  • ⚠️Using wrong relationship types - association (line) vs inheritance (<|--) vs composition (*--) have specific meanings
  • ⚠️Incorrect visibility symbols - + is public, - is private, # is protected; don't mix them up
  • ⚠️Creating god classes - classes should have single, well-defined responsibilities
  • ⚠️Forgetting return types - methods should specify what they return (void if nothing)

When to Use Class Diagrams vs Other Diagrams

Class diagrams focus on the static structure of a system - what classes exist and how they relate to each other. Use them during design phases to plan your object-oriented architecture, document existing codebases, or communicate design patterns to team members.

Choose sequence diagrams when you need to show how objects interact over time (behavioral view). Use ER diagrams when modeling database schemas rather than object-oriented code. For showing the different states an object can be in, state diagrams are more appropriate.

Class diagrams are fundamental in object-oriented design and are particularly valuable in software engineering documentation, architecture design documents, and technical specifications. They're essential tools for planning before coding and for onboarding new developers to complex systems.

Frequently Asked Questions

Common questions about using the Mermaid Viewer and creating diagrams