What is a class diagram in UML?
The Unified Modeling Language can help you model various subsets of diagrams, one of the more popular types in UML is the class diagram. Popular among software engineers to document software architecture, class diagrams are a type of structure diagram because they describe what must be present in the system being modeled.
No matter your level of familiarity, class diagrams can be easy to understand and our UML software is designed to be simple and easy to use. As the foundation element of UML, class diagrams represents the core purposes of UML because they separate the design elements from the coding of the system. UML was set up as a standardized model to describe an object-oriented programming approach. Since classes are the building block 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 interaction between class and object.
The class shape itself consists of a rectangle with three rows. The top row contains the name of the class, the middle row has the attributes of the class, and the bottom section expresses the methods or operations that the class may utilize. In a diagram, classes and subclasses are grouped together to show the static relationship between each object. The UML shape library in Lucidchart can help you create nearly any custom class diagram using our UML diagram tool.
Benefits of class diagrams
Class diagrams have a number of benefits for any organization. Try using them to:
- Illustrate data models for information systems, no matter how simple or complex.
- Better understand the general overview of the schematics of an application.
- Visually express any specific needs of a system and disseminate that information throughout the business.
- Create detailed charts that highlight any specific code needed to be programmed and implemented to the described structure.
- Provide an implementation-independent description of types used in a system that are then later passed between its components.
Basic components of a class diagram
The standard class diagram is composed of three sections:
The upper section contains the name of the class - This section is always required whether you are talking about the classifier or an object.
The middle Section contains the attributes of the class - The attributes describe the variables that describe the qualities of the class. This is only required when describing a specific instance of a class.
The bottom section includes class operations (methods) - Displayed in list format, each operation takes up its own line. The operations describe how a class can interact with data.
Member Access Modifiers
All classes have different access levels depending on the access modifier (visibility). Here are the following access levels with their corresponding symbols:
- Public (+)
- Private (-)
- Protected (#)
- Package (~)
- Derived (/)
- Static (underlined)
There are two scopes for members: classifiers and instances.
Classifiers are static members while instances are the specific instances of the class. If you are familiar with basic OO theory, this isn't anything groundbreaking.
Additional class diagram components
Depending on the context, classes in a class diagram can represent the main objects, interactions in the application, and classes to be programmed.To answer the question of "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 drawing a class diagram on a class diagram, only the top row must be filled out—the others are optional if you'd like to provide more detail.
Name - the first row in a class shape.
Attributes - the second row in a class shape. Each attribute of the class is displayed on a separate line.
Methods - the third row in a class shape. Also known as operations, these are displayed in list format with each operation on its own line.
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 - this containing shape is designed to organize related classifiers in a diagram. It is symbolized with a large tabbed rectangle shape.
Interfaces - similar to a class, 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 when representing 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, and so on.
Interactions is a term used for the various relationships and links that can exist in class and object diagrams. Some of the most common interactions include:
Inheritance - also known as generalization, this is the process of a child or sub-class taking on the functionality of a parent or superclass. It's symbolized by a straight connected line with a closed arrowhead pointing towards the superclass.
For example, if we had the object vehicle, a child class Go-go mobile would inherit all the attributes (speed, numbers of passengers, fuel) and methods (go(), stop(), changeDirection()) of the parent class in addition to the specific attributes(modelType, # of doors, autoMaker) 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 Go-go mobile class and RoadTrip class are interrelated. At one end of the line the Go-go mobile takes on the association of "assignedCar" with the multiplicity value of 0..1 which means that when the instance of RoadTrip exists, it can either have one instance of Go-go mobile associated with it or no Go-go mobiles 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 Go-go mobiles associated with it. Since one Go-go mobile instance could have multiple "getRoadTrip" associations-- in other words, one go-go mobile 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
Utilizing a class diagram to map out process flows is easy and can be especially useful. Consider the two examples below when it comes to creating class diagrams in UML.
Class diagram for a hotel management system
Hotel management systems need to reflect the relationships between a variety of hotel-related objects. These objects can be sorted into classes via a class diagram, which shows the relationships between each object in the system or application. Many factors of hotel management must be accounted for, including the guest info, staff responsibilities, and room occupancy. A class diagram can map them out and provide a useful overview of the system. Get started on a class diagram by clicking the template below.
Class diagram for an ATM system
ATMs are deceptively simple: 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.
How to make a class diagram
In Lucidchart, creating a class diagram from scratch is surprisingly simple.
Open a blank document or start with a template and enable the UML shape library.
Select the shape you want and drag out symbols from the toolbox to the canvas
Then model the process flow by drawing lines between shapes while adding text.
Dive deeper into this guide on how to draw a class diagram in UML for additional insight. It's easy to resize and style any element. You can even import SVG shapes and Visio files for a custom solution. If you'd like to learn more about UML, check out our What is UML tutorial.