UML Class Diagram Maker

A UML class diagram describes classes, attributes, operations, and relationships to make an object-oriented design easier to review. Open a finished example below, or describe your own UML class diagram. Free, no signup; hourly fair-use limits apply.

Start with the responsibilities of the objects in your domain. Name the classes, then add the attributes and operations that explain those responsibilities. For a small model, a few meaningful members are more useful than copying every field from a codebase. State attribute types and method return types where they clarify the design, and use visibility markers when public and internal members matter.

Choose relationships deliberately. Inheritance describes an is-a relationship, while an association describes objects that know about or work with one another. Composition implies a stronger whole-part lifecycle relationship and should not replace every ordinary association. Add multiplicities when a relationship can be one-to-many or optional; without them, two reviewers may imagine different object ownership rules from the same line.

The examples show a shape hierarchy with an interface and abstract class, a generic repository with operations, and namespace frames around related services. Open a rendered example to inspect the notation before describing your own classes. Check that inheritance arrows point toward the general type, that method names express behavior, and that package boundaries reflect real responsibilities. The generated diagram is documentation of a proposed model, not an automatic reverse-engineering pass over your repository. It does not compile the design or generate implementation code. Use an ER diagram for relational tables and database keys, and use a sequence diagram when the question is how these objects interact over time.