types of UML diagrams

Types of UML Diagrams

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Posted by: Lucid Content Team

If you’ve had any sort of formal code training, you were probably introduced to UML diagrams in school. But how much have you used them after that point? Because UML diagrams take some time to build and become out of date fairly quickly in an Agile environment, many software developers have forgotten about them.

However, visual documentation can help engineers in the form of UML diagrams in a number of ways, from onboarding new employees to giving stakeholders a quick overview of the system so developers don’t waste time in meetings.

While Lucidchart is an essential UML diagram tool, this article will introduce (or give you a refresher) on the different UML diagram types, as well as provide use case template examples that you can use in Lucidchart.

What are UML diagrams?

UML, which stands for Unified Modeling Language, is a way to visually represent the architecture, design, and implementation of complex software systems. When you’re writing code, there are thousands of lines in an application, and it’s difficult to keep track of the relationships and hierarchies within a software system. UML diagrams divide that software system into components and subcomponents.

Why should you use UML diagrams?

UML is a standardized modeling language that can be used across different programming languages and development processes, so the majority of software developers will understand it and be able to apply it to their work.

Though many engineers dread diagrams, they’re useful in an Agile development environment: they keep development productive and focused. Instead of thinking them as just a “nice to have,” treat your UML diagrams as core aspects of documentation. UML diagrams can help engineering teams:

  • Bring new team members or developers switching teams up to speed quickly.
  • Navigate source code.
  • Plan out new features before any programming takes place.
  • Communicate with technical and non-technical audiences more easily.

However, diagrams that don’t evolve with a project are useless, so it’s necessary to have constantly evolving diagrams. Lucidchart, a cloud-based diagramming solution, makes this process easier. Lucidchart can generate UML sequence diagrams from text markup, which makes diagramming automatic and elastic.

What are the types of UML diagrams?

To the uninitiated, it may seem as though there is an infinite number of UML diagrams, but in actuality, UML standards identify 13 types of diagrams that are divided into two groups, defined below.

Structural UML diagrams

Structural UML diagrams, as the name would suggest, show how the system is structured, including the classes, objects, packages, components, etc. in the system and the relationships between those elements.

Class diagram

Because a lot of software is based on object-oriented programming, where developers define types of functions that can be used, class diagrams are the most commonly used type of UML diagram. Class diagrams show the static structure of a system, including classes, their attributes and behaviors, and the relationships between each class.

A class is represented by a rectangle that contains three compartments stacked vertically—the top compartment contains the class’s name and is mandatory, but the bottom two compartments give details about the class attributes and class operations or behaviors.

domain model UML class diagram template
UML Class Diagram for Domain Models (Click on image to modify online)

Component diagram

A component diagram is essentially a more specialized version of the class diagram—the same notation rules apply for both. A component diagram breaks a complex system down into smaller components and visualizes the relationship between those components.

library management system UML component diagram template
UML Component Diagram for Library Management System (Click on image to modify online)

Deployment diagram

Deployment diagrams show how software is deployed on hardware components in a system. These diagrams are most useful for systems engineers, and they usually show performance, scalability, maintainability, and portability. When hardware components are displayed in relation to each other, it’s easier to keep track of your entire hardware mesh and make sure that all elements are accounted for in a deployment.

higher-level UML deployment diagram
UML Deployment Diagram - Higher-Level (Click on image to modify online)
lower-level UML deployment diagram
UML Deployment Diagram - Lower-Level (Click on image to modify online)

Composite structure diagram

These types of diagrams are essentially the blueprints for the internal structure of a classifier. They can also be used to show the behavior of a collaboration or the classifier interactions with their environment through ports. They can easily depict the internal components of any hardware to more thoroughly understand the inner workings.

UML composite structure diagram
UML Composite Structure Diagram (Click on image to modify online)

Object diagram

Object diagrams show examples of data structures at a specific time. You could use a class diagram to show a structure and then use object diagrams as test cases to verify the completeness of your class diagram. Or you could create an object diagram to discover information about model elements and their links.

ATM object diagram template
UML Object Diagram for ATM (Click on image to modify online)

Package diagram

Package diagrams are used to show dependencies between different packages in a system. A package, depicted as a file folder, organizes model elements, such as use cases or classes, into groups.

UML package diagram
UML Package Diagram (Click on image to modify online)

Behavioral UML diagrams

These UML diagrams visualize how the system behaves and interacts with itself and with users, other systems, and other entities.

Timing diagram

Often described as an inverted sequence diagram, a timing diagram shows how objects interact with each other in a given timeframe. Use these diagrams to see how long each step of a process takes and find areas for improvement.

Interaction overview diagram

This diagram overviews the flow of control between interacting nodes. They include initial nodes, flow final nodes, activity final nodes, decision nodes, merge nodes, fork nodes, and join nodes.

UML interaction overview diagram
UML Interaction Overview Diagram (Click on image to modify online)

Communication diagram

Communication diagrams, which used to be called collaboration diagrams, show how objects relate to each other. They model the way objects associate and connect through messages within an architectural design of a system. They can also show alternative scenarios within use cases or operations that require the collaboration of different objects and interactions.

UML communication diagram
UML Communication Diagram (Click on image to modify online)

State diagram

State diagrams, simply put, depict states and transitions. A state refers to the different combinations of information that an object can hold, and this UML diagram can visualize all possible states and the way the object transitions from one state to the next.

UML state diagram example
UML State Diagram Example (Click on image to modify online)

Use case diagram

Use case diagrams model how users, displayed as stick figures called “actors,” interact with the system. This type of UML diagram should be a high-level overview of the relationships between actors and systems, so it can be a great tool for explaining your system to a non-technical audience.

online shopping use case diagram example
UML Use Case Diagram for Online Shopping (Click on image to modify online)

Sequence diagram

A sequence diagram, sometimes referred to as an event diagram or an event scenario, shows the order in which objects interact. This way, you can visually represent simple runtime scenarios.

online shopping UML sequence diagram template
UML Sequence Diagram for Online Shopping (Click on image to modify online)

Activity diagram

Activity diagrams visualize the steps performed in a use case—the activities can be sequential, branched, or concurrent. This type of UML diagram is used to show the dynamic behavior of a system, but it can also be useful in business process modeling.

ATM UML activity diagram template
UML Activity Diagram for ATM (Click on image to modify online)

If you’d like additional inspiration, check out our full post of UML diagram template examples.

These UML diagram templates should give you a starting point and drive home our central point: that UML diagrams aren’t just an optional part of the development process, but a vital part of documentation that should be integrated into any Agile development team.

How to make a UML diagram

UML diagrams follow a specific set of rules and shapes, and you could spend a significant amount of time learning how to correctly build each type. Luckily, we’ve made it easy for you with easy tutorials, starting with class diagrams, that walk you through the process step by step.

Become a UML evangelist

Sometimes it’s not enough for you to be on board the UML diagramming train: After all, as a software developer, you’re usually working with teams, and it’s important to get everyone else along for the ride.

If your team is reluctant to integrate UML diagrams into the development process, propose using them for just one project to start. Once your team sees what a boon UML diagrams are to documentation, they’ll be more willing to start making them a necessary step.

Plus, with Lucidchart, creating UML diagrams aren’t a chore: They’re an asset. Get started with the UML templates and shape libraries in Lucidchart.

Start diagramming with Lucidchart today—try it for free!

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