Class diagram tutorial

Class diagrams are one of the most useful types of diagrams in UML as they clearly map out the structure of a particular system by modeling its classes, attributes, operations, and relationships between objects. With our UML diagramming software, creating these diagrams is not as overwhelming as it might appear. This guide will show you how to understand, plan, and create your own class diagrams.

What is a class diagram in UML?

The Unified Modeling Language (UML) can help you model systems in various ways. One of the more popular types in UML is the class diagram. In UML’s broader taxonomy, class diagrams sit within the structural (static) family of diagrams, which focus on what a system is made of rather than how it behaves over time. 

UML was set up as a standardized model to describe an object-oriented programming approach. Since classes are the building blocks of objects, class diagrams are the building blocks of UML. The various components in a class diagram can represent the classes that will actually be programmed, the main objects, or the interactions between classes and objects. Because they align closely with object-oriented programming, class diagrams can often be translated directly into code constructs in common OOP languages, such as Python, Java, and C++.

The class shape itself consists of a rectangle with three rows. The top row contains the name of the class, the middle row contains the attributes of the class, and the bottom section expresses the methods or operations that the class may use. Classes and subclasses are grouped together to show the static relationship between each object.

No matter your level of familiarity with UML or class diagrams, our UML software is designed to be simple and easy to use. The UML shape library in Lucidchart can help you create nearly any custom class diagram using our UML diagram tool. And with our diagram as code feature, it's easier and faster than ever. You can create a fully custom class diagram and add it to your Lucidchart document using Mermaid coding.

Benefits of class diagrams

Class diagrams offer a number of benefits for any organization. Use UML class diagrams to:

• Help developers to think about class structure and interactions before writing any code. This can lead to cleaner and better-functioning software.
• Draw detailed charts and document requirements that need to be included in the code.
• Share information with stakeholders more easily with visual presentations without diving into the code.
• Illustrate data models for any system, no matter how complex.
• Easily share information throughout your business.
• Have a better understanding of the general schematics of an application.
• Document requirements and other aspects of a system without complicated, text-heavy explanations.
• Create models from a business perspective as a business analyst.

They’re also useful early in the lifecycle as a thinking tool—helping teams reason about structure and relationships before implementation—and as a communication artifact for both technical and non-technical stakeholders.

Basic components of a class diagram

Depending on the context, classes in a class diagram can represent the main objects, interactions in the application, or classes to be programmed. To answer the question "What is a class diagram in UML?" you should first understand its basic makeup.

• Classes: A template for creating objects and implementing behavior in a system. In UML, a class represents an object or a set of objects that share a common structure and behavior. They're represented by a rectangle that includes rows of the class name, its attributes, and its operations. When you draw a class in a class diagram, you're only required to fill out the top row—the others are optional if you'd like to provide more detail.
• Signals: Symbols that represent one-way, asynchronous communications between active objects.
• Data types: Classifiers that define data values. Data types can model both primitive types and enumerations.
• Packages: Shapes designed to organize related classifiers in a diagram. They are symbolized with a large tabbed rectangle shape.
• Interfaces: A collection of operation signatures and/or attribute definitions that define a cohesive set of behaviors. Interfaces are similar to classes, except that a class can have an instance of its type, and an interface must have at least one class to implement it.
• Enumerations: Representations of user-defined data types. An enumeration includes groups of identifiers that represent values of the enumeration.
• Objects: Instances of a class or classes. Objects can be added to a class diagram to represent either concrete or prototypical instances.
• Artifacts: Model elements that represent the concrete entities in a software system, such as documents, databases, executable files, software components, etc.

Sections within a class

The standard class is composed of three sections:

Upper section

Contains the name of the class. This section is always required, whether you are talking about the classifier or an object. The class name should:

• Start with a capital letter.
• Be formatted with bold typeface.
• Be centered in the upper section.
• Be descriptive.

Middle section

Contains the attributes of the class. Use this section to describe the qualities of the class. This is only required when describing a specific instance of a class.

Bottom section

Includes class operations (methods). Displayed in list format, each operation takes up its own line. The operations describe how a class interacts with data. The text in the bottom two sections is usually aligned to the left and typed with lowercase first letters.

Member access modifiers

All classes have different access levels depending on the access modifier (visibility). Here are the access levels with their corresponding symbols:

• Public (+)
• Private (-)
• Protected (#)
• Package (~)
• Derived (/)
• Static (underlined)

Member scopes

There are two scopes for members: classifiers and instances.

Classifiers are often recognized as static, meaning that attribute values remain the same in all instances, and method invocation doesn't affect the instance's state. In comparison, instances contain methods that can change the instance's state and attribute values that may vary between instances. To indicate a classifier, just underline its name; leave the text to indicate an instance.

Interactions

The term "interactions" refers to the various relationships and links that can exist in class and object diagrams. Some of the most common interactions include:

Inheritance: The process of a child or subclass taking on the functionality of a parent or superclass, also known as generalization. It's symbolized with a straight connected line with a closed arrowhead pointing towards the superclass.

In this example, the object "Car" would inherit all of the attributes (speed, numbers of passengers, fuel) and methods (go(), stop(), changeDirection()) of the parent class ("Vehicle") in addition to the specific attributes (model type, number of doors, auto maker) and methods of its own class (Radio(), windshieldWiper(), ac/heat()). Inheritance is shown in a class diagram by using a solid line with a closed, hollow arrow.

Bidirectional association: The default relationship between two classes. Both classes are aware of each other and their relationship with the other. This association is represented by a straight line between two classes.

In the example above, the Car class and RoadTrip class are interrelated. At one end of the line, the Car takes on the association of "assignedCar" with the multiplicity value of 0..1, so when the instance of RoadTrip exists, it can either have one instance of Car associated with it or no Cars associated with it. In this case, a separate Caravan class with a multiplicity value of 0..* is needed to demonstrate that a RoadTrip could have multiple instances of Cars associated with it. Since one Car instance could have multiple "getRoadTrip" associations—in other words, one car could go on multiple road trips—the multiplicity value is set to 0..*

Unidirectional association: A slightly less common relationship between two classes. One class is aware of the other and interacts with it. Unidirectional association is modeled with a straight connecting line that points an open arrowhead from the knowing class to the known class.

As an example, on your road trip through Arizona, you might run across a speed trap where a speed cam records your driving activity, but you won't know about it until you get a notification in the mail. It isn't drawn in the image, but in this case, the multiplicity value would be 0..* depending on how many times you drive by the speed cam.

Class diagram examples

Creating a class diagram to map out process flows is easy. Consider the two examples below as you build your own class diagrams in UML.

Class diagram for a hotel management system

A class diagram can show the relationships between each object in a hotel management system, including guest information, staff responsibilities, and room occupancy. The example below provides a useful overview of the hotel management system. Get started on a class diagram by clicking the template below.

Class diagram for an ATM system

ATMs are deceptively simple: Although customers only need to press a few buttons to receive cash, there are many layers of security that a safe and effective ATM must pass through to prevent fraud and provide value for banking customers. The various human and inanimate parts of an ATM system are illustrated by this easy-to-read diagram—every class has its title, and the attributes are listed beneath. You can edit, save, and share this chart by opening the document and signing up for a free Lucidchart account.

Tips for planning clear class diagrams

Before you start diagramming (or while iterating on an early draft), a few lightweight planning habits can make your class diagrams clearer and more useful to others.

• The class name should be descriptive and have meaning. For example, if you name a class UserAccount, it’s clear that the class will create a user account of some kind.
• Clearly identify what each class is responsible for.
• Identify each element in advance and understand how they relate to each other.
• Add only relevant properties to each class. Adding unnecessary properties can make your diagram look cluttered and overly complicated.
• Use brief notes when you need to describe a specific part of the diagram.
• Collaborate with team members and rework the diagram as needed to get it right.

These practices are especially helpful because class diagrams rarely live in isolation; they typically represent a set of classes that must work together as part of a larger system. While you might be diagramming only a single class, it’s likely that it will be used in a larger system. A system is represented in a class diagram as a collection of classes that need to work together. That’s why it’s important to identify elements in advance and understand how they relate to each other. When several classes are grouped into a single class diagram, it helps you understand the static structure and relationships among them. This can also help you to identify and fix potential problem areas before anything bad happens.

How to make a class diagram

In Lucidchart, creating a class diagram from scratch is surprisingly simple. Just follow these steps:

1. Get started with Lucidchart

If you’re brand new to Lucidchart, you’ll first need to create an account and open a document before you start dragging shapes onto the canvas. If you haven't registered with Lucidchart, you can sign up for free. Log in and select a template or open a new document.

2. Add or import shapes

Every class diagram must include the UML class shapes. In Lucidchart, you can add the entire UML shape library to your toolbar. Click Shapes on the left side of the editor and choose "UML." From there, you can expand beyond the default UML library if your diagram needs additional visuals or custom assets. If you still need more shapes, you can search icons using the image search feature, or you can import your own stencils and icons. When it comes to shapes, there are no limitations in Lucidchart. When you place a class shape, you can choose how much detail to show—some diagrams omit attributes and operations to stay high-level. The standard class symbol has three rows, but you can use a similar shape that omits the bottom two rows if you prefer.

3. Add texts and concepts to your class diagram

As you fill in each class, follow the standard row-by-row convention so readers can quickly interpret what they’re seeing. To add text in each row of the class shape, just click and type. The first row should include only the class name, while the second features the attributes of the class. The last section lists potential methods or operations the class could take. Continue to add class shapes until your static application is fully represented.

Indicate scope by underlining certain members (the term for attributes or methods in a class). Members can be classified as either instances or classifiers. Classifiers are often recognized as static, meaning that attribute values remain the same in all instances, and method invocation doesn't affect the instance's state. In comparison, instances contain methods that can change the instance's state and attribute values that may vary between instances. To indicate a classifier, just underline its name; leave the text to indicate an instance.

Demonstrate visibility by placing the correct symbol within the class shape and before the particular attribute or method. This is also called assessing visibility. Use the symbol associated with each access level: public (+), private (-), protected (#), package (~), and derived (/).

Show your audience the interactions with class and object diagrams, or the relationships and links that exist between objects. In Lucidchart, you can indicate relationships between objects by drawing a line and then changing the notation style as needed.

4. Format your class diagram

Once the content is in place, you can polish the diagram’s look and make it easier to consume with consistent styling. Easily format your class diagrams in Lucidchart by updating fonts, colors, and line settings.

5. Publish, implement, and share

Finally, class diagrams are most valuable when they’re shared—whether that’s embedded in documentation, distributed to stakeholders, or integrated into the tools your team already uses. Once you've created a class diagram in Lucidchart, you can easily share it to increase understanding throughout your entire organization. Publish and embed your chart in your company wiki, or send it to other current Lucidchart users or non-users with easy share features. You can also download your final class diagram in any file format. 

With so many Lucidchart integrations, you can include your class diagrams in the applications that you use daily. With our free Google Workspace integrations, you can add your class diagram with additional context in Google Docs, Sheets, or Slides. Our Microsoft integrations allow you to import your diagrams seamlessly after you collaborate in real time within the cloud.

Class diagrams are one of the most common types of diagrams in UML, and Lucidchart has made it easy to understand and create them. Jump right into one of our templates, import an existing class diagram and continue working on it within Lucidchart, or start from scratch. We have all the features and tools you need to get started.