What is a UML diagram?

UML diagrams explained

What is a UML diagram? A UML diagram is a way to visualize systems and software using Unified Modeling Language (UML). Software engineers create UML diagrams to understand the designs, code architecture, and proposed implementation of complex software systems. UML diagrams are also used to model workflows and business processes. Coding can be a complicated process with many interrelated elements. There are often thousands of lines of programming language that can be difficult to understand at first glance. A UML diagram simplifies this information into a visual reference that’s easier to digest. It uses a standardized method for writing a system model and capturing conceptual ideas.

UML diagrams help keep track of relationships and hierarchies between important lines of code. While these diagrams look similar to some decision trees or flowcharts, they have unique attributes. Simplifying something as complex as a barrage of code is extremely useful for engineers and non-technical stakeholders. It allows them to stay on top of the projects they are working on and avoid getting lost in the endless complexities inherent in software programming. UML diagrams also divide the components and subcomponents that are essential to building a piece of software.

Types of UML diagrams

There are two subcategories of UML diagrams: structural diagrams and behavioral diagrams.

Structural diagrams depict the components that make up a system and the relationship between those components. These diagrams show the static aspects of a system.

Behavioral diagrams represent what happens within a system. They show how all the components interact with each other and with other systems or users.

Structural diagrams

Let’s take a look at a few specific types of structural UML diagrams.

Class diagram

A UML class diagram is a fundamental building block of any object-oriented solution. It depicts a static, object-oriented system, defining projects by classes, attributes, and functions. In other words, it shows the classes within a system and the operations of each one. Both software engineers and business managers use this interaction diagram to model different connections involved within a process.

In the diagram, the class is represented by a rectangle. Each rectangle is split vertically into three sections. The top section has the name of the class, while the second and third sections provide details about class operations, behaviors, and attributes.

You can easily create these diagrams using a UML class diagram tool.

Package diagram

As the name suggests, a package diagram shows the dependencies and relationships between packages in a system. A package refers to anything that organizes any model element. In the diagram itself, it’s presented as a file folder.

The package diagram allows the user to group items into folders. Items that can be grouped include classes, use cases, documents, diagrams, and even other packages. Once each item is allocated to a package, all packages are arranged and ranked hierarchically in the diagram.

Because they easily portray the top-level structure, package diagrams are mostly useful for visualizing software systems.

Object diagram

Object diagrams show the attributes of different objects within a system. And like class diagrams, they show the relationships between the objects in a piece of software. The difference is that object diagrams use real-world examples. Object diagrams are also called instance diagrams because they display what the system looks like at a specific moment in time.

Component diagram

A UML component diagram breaks down a complex system into smaller components, making it easier to visualize. Unlike other UML diagrams, it specifically outlines the relationship between these components.

UML component diagrams give developers an overarching understanding of a system’s physical objects. This type of UML diagram shows the structural relationship between each physical component and subcomponent in a complex software system. This helps stakeholders understand how the components are organized and wired together.

Creating these diagrams is easy with a user-friendly UML component diagram tool.

Deployment diagram

A deployment diagram shows the configuration of processing nodes and all the components that live on them. It depicts the physical arrangement of the nodes in a distributed system. These diagrams are especially useful when you’re building software that will operate on different hardware systems.

System engineers are most likely to use these diagrams as they get to keep track of their entire hardware mesh and prepare the system for launch without any issues. It also allows them to see performance, maintainability, and scalability.

Composite structure diagram

Composite structure diagrams visualize a class’s internal structure. These diagrams break down the network of classes, interfaces, and components. This type of UML diagram also shows how these elements interact with each other and why they are essential to the overarching software structure.

Behavioral diagrams

Let’s take a look at some specific examples of behavioral UML diagrams.

Activity diagram

A UML activity diagram is a flowchart that outlines all of a system's activities. It shows everything from start to finish, defining the various decision paths and steps that need to happen to move from one activity to the next. The steps can be chronological, branched, or simultaneous.

You can use a UML activity diagram to hone in on any one component and to get a high-level overview of the dynamic aspects of a system. Using an intuitive UML activity diagram maker is the best way to create these diagrams.

Sequence diagram

UML sequence diagrams — sometimes known as event diagrams — show the order in which your objects interact. This includes the lifelines of your objects, the processes that interact with your objects, and the messages exchanged between the objects to perform a function.

Developers and business professionals often use these diagrams to understand how to structure a new system or improve an existing process. You can easily create them using an online UML sequence diagram.

Communication diagram

UML communication diagrams are also called collaboration diagrams. They are relatively similar to sequence diagrams, focusing on the messages passed between different objects. While sequence diagrams look at processing over time, a communication diagram creates a complete, big-picture map of your product. A communication diagram tool makes it easy to create these visualizations.

Interaction overview diagram

Like activity diagrams, interaction overview diagrams visualize the flow of activity and the sequence of those activities. The difference is that each activity in an interaction overview diagram is shown as a frame. The nodes in an interaction overview diagram represent the interactions in a system.

Timing diagram

Timing diagrams are useful for deployments, as they depict the behavior of specific objects within an explicit time frame. These diagrams are usually fairly straightforward, but when dealing with more than one object, they show the interactions between many different and important sequences within that time frame.

Use case diagram

Use case diagrams provide a graphic overview of the actors involved in a software system. By illustrating system functionality and outlining a system’s expected behavior, they help developers analyze the relationships between use cases and personas. You can easily visualize these interactions using a use case diagram tool.

State machine diagram

Also known as state chart diagrams, UML state machine diagrams show the behaviors of different components in a system. You can visualize how elements act differently according to the state of the program being developed. Create a state chart easily using Miro’s state diagram tool.

Benefits of UML diagrams

UML diagrams are incredibly beneficial to any software development team. Here are some of the main advantages of using a UML diagram:

Makes it easier to understand complex ideas and systems

UML diagrams make abstract ideas and software systems easier to understand through visualization. This is beneficial for software engineers who need to collaborate when building software. UML diagrams also make it easier for software engineering teams to communicate with external stakeholders. Software engineers can use the UML model to explain how the system works to non-tech-savvy people.

It turns complex code into a visual diagram

Building a piece of software often requires thousands of lines of complex code, with relationships and hierarchies within them. Interpreting this code can be difficult and time-consuming. Using UML simplifies this process by turning coding environments into easy-to-understand visual diagrams.

It keeps everyone on the same page

UML works across different programming languages and development processes. It is a standardized way to visualize a system’s design. Using this universal language improves communication and understanding between a team of software engineers. If anyone on the team questions how a system works, the visual diagram acts as a point of reference. This aligns team members on the progress and task priorities within the software development process.

It allows developers to see the big picture of a system

During the software development process, it’s important to understand and remember what you’re trying to build. Developers can get bogged down when writing massive libraries of code. This can cause them to get lost in the process rather than see the project as a whole. A UML diagram helps to create an overarching, big picture of the overall system. This helps developers stay focused on the end goal.

It helps non-programmers understand the processes and functionalities of software

Software engineers are the only ones who can truly understand the functionalities, relationships, and hierarchies within a library of code. But other people in the same organization can also play an important role in the project. Product owners, managers, and stakeholders often can’t decipher the programming language used to build a piece of software. A UML diagram helps everyone outside the development team understand the programming process and how the system is meant to work.

Using a common notation means any programmer can understand it

Not all programmers understand and specialize in the same type of code and programming languages. There are hundreds of different ways to develop the same library of code. A UML diagram ensures that programmers with different skills and efficiencies can work together.

When to use a UML diagram

UML diagrams are important during the software development process and when analyzing existing software. That said, they’re useful for visualizing any kind of complex system. Let’s look at how UML diagrams add value to two different scenarios.

Software development

Software development teams use UML diagrams to build new pieces of software and analyze and understand existing software. Before any coding or development begins, UML diagrams help the software development team visualize and plan the project. This, in turn, helps the team with prioritization and resource allocation. UML diagrams also ensure that everyone speaks the same versatile visual “language,” even when the coding used in the project isn’t universal. Having a diagram that everyone understands makes cross-company communication easier. Even those without a coding background will understand the nuances and flow of a project.

Business management

UML diagrams have become increasingly popular in business management as they help leaders organize and visualize their workforce’s workflow. These diagrams are also useful for business owners who want to improve their understanding of certain business operations’ progress. Teams can use UML to visualize the trajectory of a process and how the elements within that process will interact with each other. UML diagrams also help teams understand what’s required for a system to function properly and how it can integrate with other systems and software. Because visual information is absorbed more easily than non-visual, diagrams ensure that employees and management are on the same page, regardless of their role or experience.

UML diagram best practices

Creating clear and effective UML diagrams is crucial for communicating and documenting the design of a system. Here are some best practices to help you create meaningful and useful diagrams:

Start with a template

Creating a UML diagram from scratch can seem daunting, especially if it’s your first time. Using pre-made diagramming templates can save you time and ensure you aren’t missing any important diagram components.

Templates also give you an excellent starting point that you can build on. Over time, you can adapt the template to suit your specific needs and requirements.

Here are some ready-made UML templates to help you get started:

• UML diagram template

• UML activity diagram template

• UML state machine diagram template

• UML class diagram template

• UML component diagram template

• UML communication diagram template

• UML use case diagram template

Understand the reason behind creating the UML diagram

A UML diagram serves a specific purpose — which is why there are so many different types. You should sit down with your development team and understand why you’re creating the UML diagram. This will help you choose the most optimal type of diagram, which will, in turn, help you achieve the best possible results.

Share and collaborate with your team

The primary purpose of a UML diagram is to increase a team’s understanding through visualization. However, visualizing isn’t the only way to improve understanding. You can also share your UML diagrams with your team to promote collaboration and ensure that everyone is on the same page. This gives all members of the team a chance to contribute, thus increasing collaboration. It also ensures that nothing gets left out and that everyone understands the information in the diagram.