Class Diagram

A class diagram represents a system's structure in terms of its classes and their relationships — describing the objects, attributes, methods, and relationships within a system to give a visual model of its static structure, independent of runtime behavior or timing.

Key elements

• Class — a template for creating objects, defining their attributes and methods; drawn as a rectangle with the class name at the top.
• Object — an instance of a class (shown when illustrating a class diagram alongside a specific example); see Object Diagram for the dedicated instance-level diagram type.
• Attributes — the properties describing an object's state; commonly listed inside the class rectangle (a compartment below the class name) rather than as separate attached ovals in most modern UML tooling, though both notations exist.
• Methods — the operations an object can perform, defining its behavior; listed in a further compartment below attributes.
• Relationships — association, aggregation, and composition (see Object Diagram for the concrete distinction between the latter two, which applies identically at the class level).

Association, more specifically

An association between classes expresses a real structural relationship — "is composed of," "contains," "has a" — drawn as a line connecting the two classes, often annotated with multiplicity (e.g. "1" to "many") to show how many instances of one class relate to instances of the other.

When a class diagram is the right choice

The standard tool for documenting or designing a system's object-oriented structure — which classes exist, what data and behavior each owns, and how they relate — used both for reverse-engineering an existing codebase's structure and for designing a new one before writing code.

Drawing one

See PlantUML Diagram for the @startuml class-diagram syntax (class blocks with {field}/{method} members, and relationship arrows like <|--o for aggregation, <|--* for composition) to generate one from version-controlled text.

Common pitfalls

• A class diagram with no relationships shown — a set of isolated class boxes with no connecting lines misses the point; the relationships are often the most important information a class diagram conveys.
• Confusing a class diagram with an object diagram — see Object Diagram; use a class diagram for the general, type-level structure, an object diagram for one concrete instance snapshot.
• Overcrowding one diagram with every class in a large system — a class diagram covering an entire large codebase becomes unreadable; scope diagrams to one bounded area of the system at a time, similar to Flowchart's advice against overcrowding a single process diagram.

Learn more

• PlantUML Diagram for the text-based syntax to draw this diagram type.
• Object Diagram for the instance-level snapshot counterpart.
• Package Diagram for organizing many classes into higher-level groups.