UML Diagrams: An Overview

The Unified Modeling Language (UML) provides a standard set of diagrams for describing a system from different perspectives. UML 2.5 defines 14 diagram types, classified into structure diagrams and behavior diagrams. Interaction diagrams form a subset of behavior diagrams.

Classification

UML diagrams
├── Structure diagrams
│   ├── Class diagram
│   ├── Object diagram
│   ├── Component diagram
│   ├── Composite structure diagram
│   ├── Package diagram
│   ├── Deployment diagram
│   └── Profile diagram
└── Behavior diagrams
    ├── Use case diagram
    ├── Activity diagram
    ├── State machine diagram
    └── Interaction diagrams
        ├── Sequence diagram
        ├── Communication diagram
        ├── Interaction overview diagram
        └── Timing diagram

The two main categories answer different kinds of questions:

• Structure diagrams describe what the system is made of and how its elements are organized.
• Behavior diagrams describe what the system does and how it changes over time.
• Interaction diagrams focus specifically on communication between system elements.

Structure Diagrams

Structure diagrams present the static aspects of a system.

Diagram	Overview
Class diagram	Shows classes, their features, and relationships.
Object diagram	Shows object instances and their links at a particular moment.
Component diagram	Shows software components and the dependencies between them.
Composite structure diagram	Shows the internal structure of a classifier and how its parts collaborate.
Package diagram	Shows how model elements are grouped into packages and how packages depend on one another.
Deployment diagram	Shows software artifacts deployed onto hardware or execution environments.
Profile diagram	Shows extensions that adapt UML to a particular domain or platform.

Behavior Diagrams

Behavior diagrams present the dynamic aspects of a system.

Diagram	Overview
Use case diagram	Shows system goals and the external actors that interact with the system.
Activity diagram	Shows workflows, actions, decisions, and concurrent flows.
State machine diagram	Shows the states of an element and the transitions between them.

Interaction Diagrams

Interaction diagrams are behavior diagrams that describe how participants exchange messages.

Diagram	Overview
Sequence diagram	Emphasizes the time-ordered exchange of messages.
Communication diagram	Emphasizes the links between participants that exchange messages.
Interaction overview diagram	Provides a high-level flow that coordinates other interactions.
Timing diagram	Emphasizes state or value changes along a time axis.

Choosing a Category

Start with the question the model should answer:

• Use a structure diagram to describe organization, composition, or deployment.
• Use a behavior diagram to describe goals, workflows, or lifecycle changes.
• Use an interaction diagram to describe communication and message flow.

A system rarely needs every UML diagram. Choose only the diagrams that clarify the design for their intended audience.

References

1. Object Management Group. OMG Unified Modeling Language (OMG UML), Version 2.5.1. 2017.
2. Martin Fowler. UML Distilled: A Brief Guide to the Standard Object Modeling Language. 3rd ed., Addison-Wesley, 2003.

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