The Easy Guide to UML Class Diagrams | Class Diagram Tutorial

At the heart of any object-oriented system is the step of designing the class structure – therefore the saying goes that class diagrams are the most popular out of the UML diagram types.

In this easy class diagram tutorial, we’ve covered the key areas you need to know to draw class diagrams without a struggle. Scroll down to find out

• Class Diagram Definition
• Class Diagram Notations with Examples
• How to Draw a Class Diagram
• Class Diagram Best Practices
• Class Diagram Examples/Templates

Class Diagram Definition | What is a Class Diagram?

A class diagram is a UML diagram type that describes a system by visualizing the different types of objects within a system and the kinds of static relationships that exist among them. It also illustrates the operations and attributes of the classes.

They are usually used to explore domain concepts, understand software requirements and describe detailed designs.

Class Diagram Notations with Examples

There are several class diagram notations that are used when drawing UML class diagrams. We’ve listed below the most common class diagram notations.

Class

Classes represent the central objects in a system. It is represented by a rectangle with up to 3 compartments.

The first one shows the class’s name, while the middle one shows the class’s attributes which are the characteristics of the objects. The bottom one lists the class’s operations, which represents the behavior of the class.

The last two compartments are optional. The class notation without the last two compartments is called a simple class and it only contains the name of the class.

Interface

The interface symbol in class diagrams indicates a set of operations that would detail the responsibility of a class.

Package

The package symbol is used to group classes or interfaces that are either similar in nature or related. Grouping these design elements using the package symbols improves the readability of the diagram

Class Diagram Relationships

To learn about the class diagram connector types and the different relationships between classes in detail, refer to our handy guide on class diagram relationships.

For a full list of class diagram notations/ class diagram symbols refer to this post.

How to Draw a Class Diagram

Class diagrams go hand in hand with object-oriented design. So knowing its basics is a key part of being able to draw good class diagrams.

When required to describe the static view of a system or its functionalities, you’d be required to draw a class diagram. Here are the steps you need to follow to create a class diagram.

Step 1: Identify the class names

The first step is to identify the primary objects of the system.

Step 2: Distinguish relationships

Next step is to determine how each of the classes or objects are related to one another. Look out for commonalities and abstractions among them; this will help you when grouping them when drawing the class diagram.

Step 3: Create the Structure

First, add the class names and link them with the appropriate connectors. You can add attributes and functions/ methods/ operations later.

Class Diagram Best Practices

• Class diagrams may tend to get incoherent as they expand and grow. It’s best to avoid creating large diagrams and breaking them down into smaller ones that you can link to each other later. You can very easily do this with Creately. It helps you improve the readability of your diagrams.

• Using the simple class notation, you can quickly create a high-level overview of your system. A detailed diagram can be created separately as required, and even linked to the first one for easy reference.

• The more lines overlap on your class diagrams, the more cluttered it becomes. The reader will only get confused trying to find the path. Make sure that no two lines cross each other.

• Use colors to group common modules. Different colors on different classes help the reader differentiate between the various groups.

Class Diagram Examples / Templates

Class Diagram Example 1

Class Diagram Example 2

Class Diagram Example 3

Other Class Diagram Resources

• Guidelines for UML Class Diagrams – Part 1
• Guidelines for UML Class Diagrams – Part 2
• Tools, Templates and Resources to Draw Class Diagrams

Share Your Thoughts on the Class Diagram Tutorial

In this class diagram tutorial, we’ve covered what a class diagram is, class diagram notations, how to draw a class diagram and best practices you can follow when creating class diagrams. In addition, we’ve added a few class diagram examples that you can instantly edit online.

Interested in learning about other UML diagram types?