A Quick Guide to UML Class Diagrams

1. Class Block

A class is represented as a rectangle with three compartments: the first for the name, the second for the attributes, and the third for the methods.

2. Attributes and Operations

Inside the class block, list data as field: Type and behavior as method(params): ReturnType.

3. Visibility and Modifiers

Use + for public, ~ for package (default), # for protected, — for private, underline static members, and italicize abstract ones.

4. Association

A solid line shows a persistent link between instances, and add role names if helpful.

5. Multiplicity

Quotes on the line ends express “how many.”

Here, the class diagram indicates that a customer can place zero or more orders.

6. Inheritance (Generalization)

Open arrow means is-a. Subclasses inherit all attributes and operations.

7. Interface Realization

Mark a contract with <<interface>>, and a class realizes it via a dashed open arrow <|..

8. Aggregation vs. Composition

• Aggregation is represented by o — and it means parts may live on
• Composition is represented by * — and it means parts die with the whole

9. Dependency

A dashed arrow ..> marks a temporary collaboration: one class needs another only while a method runs, never storing it afterward

Keep diagrams small, focus on relationships, and update them during code reviews. With these nine patterns, you can sketch most designs.