This article provides an overview of UML State Machine Diagrams (SMDs), explaining their use in modeling the dynamic behavior of systems in software engineering. It covers the key concepts of SMDs, including notation, construction steps, common errors, and their relationship to sequence diagrams.
Alexander S. Ricciardi
February 8, 2025
In software engineering, Unified Modeling Language (UML) state machine diagrams, also known as state diagrams or statecharts, are used to model the dynamic of systems both in the problem space (MOPS) and the solution space (MOSS) (Unhelkar, 2018). UML State Machine Diagrams are helpful for visualizing how objects change state over time. This post explores UML State Machine Diagrams, including their notation, construction steps, common errors, and relationship to UML sequence diagrams.
State Machine Diagrams Overview
UML State Machine Diagrams (SMDs) are behavior diagrams that show the discrete behavior of a system by depicting the state transitions of a part of that system (UML®, n.d.). Two kinds of state machines are defined in UML. 2.5, behavioral state machine and protocol state machine. A behavioral state machine focuses on event-driven transition of an object (Dwivedi, 2019). A protocol state machine is a behavioral state diagram that specializes in modeling the sequence of event-driven transition of an object. This post focuses on behavioral state diagrams as they are more widely used and the notations of the two types are very similar.
In software modeling, SMDs provide a dynamic illustration of the object within a system. They are used in MOPS (Model of Problem Space) to model the state states, conditions, and transitions of system entities, and in MOSS (Model of Solution Space), they are used to model the states, conditions, and transitions of objects including interface and control objects (Unhelkar, 2018). In other words, they are used to visualize the different states of objects within a system and how, in response to events, they transition between those states. Various components are utilized to create SMDs, the table below lists the main components that can be found in SMDs.
Table 1
UML SMD Basic Notation
Note: This table provides an illustration and a description of the basic notation elements that can be found within state machine diagrams. From several sources (Unhelkar, 2018; Dwivedi, 2019; GeeksForGeeks, 2025; Visual-Paradigm, n.d.; IBM, 2023, FastBitLab, 2022).
When building an SMD, it is generally a good practice to follow these steps:
Select an important/complex object.
Understand the different stereotypes present in the selected object (Entity, boundary, control, or table).
Identify states based on attribute values from the class documentation.
Figure out how the object changes state.
Draw transitions between states.
Specify conditions for transitions, aka guards and related events and triggers.
Group related states hierarchically using composite states.
Clarify the diagram by adding notes.
Consider modeling other objects/use cases.
(Unhelkar, 2018)
The table below lists the most common errors that can be made while building an SMD and how to rectify them.
Table 2
SMD Common Error and Rectifications
Note: The table lists the most common errors made while constructing an SMD and how to rectify them. From “Chapter 12 — Dynamic modeling With state Machine Diagrams. Software Engineering with UML” by Unhelkar (2028). Modified.
The diagram below is an example of an SMD illustrating an online shopping order’s states.
Figure 1
SMD Example of an Online Shopping Order
Note: The figure is an illustration of a UML State Machine Diagram (SMD) depicting the states of an online shopping order.
SMDs vs Sequence Diagrams
Another type of UML diagram used to represent the dynamic aspect of a system is the sequence diagram. Sequence diagrams “provide a dynamic illustration of object interactions within a system. In SE, they are usually based on UML class diagrams and are used primarily to show the interactions between objects (classes) in the chronological order in which those interactions occur” (Ricciardi, 2025, p.1). These diagrams are also used in the MOPS to model the behavior of systems from the actors’ (users) perspective and in the MOSS to model objects' interactions in detail; details such as the sequence of messages (methods) within a system, the parameter list within messages, and the return values of the messages. Both the SMDs and sequence diagrams are useful for modeling the dynamic aspects of a system and depending on the specific needs of analysis and design task one is more suited than the other one. For example, to model the internal behavior of a single object, a state machine diagram is best, whereas to model multiple objects and the sequence of operations within a system, a sequence diagram is the better choice.
To summarize, SMDs are used to understand and model the dynamic behavior of systems, particularly the state changes/transitions of individual objects. They help to model the states, transitions, events, and guard conditions of a system. Thus, software engineers need to understand the common errors that can occur when building SMDs and best practices, as well as the relationship to other UML diagrams like sequence diagrams. Ultimately, SMDs are a powerful tool for analyzing (in MOPS) and designing (in MOSS) the dynamic behavior of individual objects within systems.
References:
Dwivedi, N. (2019, September 9.). Software design: Modeling with UML. State machine diagram. Modeling with the Unified Modeling Language (UML). LinkedIn Learning. https://www.linkedin.com/learning/software-design-modeling-with-uml/state-machine-diagram?u=2245842
FastBitLab (2022, January 20). FSM Lecture 11- UML state machine types of transitions. FasBitLab. https://fastbitlab.com/fsm-lecture-11-uml-state-machine-types-of-transitions/
GeeksforGeeks (2025, January 3). State machine diagrams | Unified Modeling Language (UML). GeeksforGeeks. https://www.geeksforgeeks.org/unified-modeling-language-uml-state-diagrams/
IBM (2023, September 2018). Creating transitions between states. IBM DevOps Model Architect. IBM Documentation. https://www.ibm.com/docs/en/dma?topic=diagrams-creating-transitions-between-states
Ricciardi, A. (2025, January 27). A Guide to UML Sequence Diagrams: Notation, strengths, and Limitations. Code Chronicles - Omegapy. https://www.alexomegapy.com/post/a-guide-to-uml-sequence-diagrams-notation-strengths-and-limitations
UML® (n.d.) State machine diagrams. UML diagrams. UML Diagrams Org. https://www.uml-diagrams.org/state-machine-diagrams.html
Unhelkar, B. (2018). Chapter 12 — Dynamic modeling with state machine diagrams. Software engineering with UML. CRC Press. ISBN 9781138297432
Visual-Paradigm (n.d.). What is state machine diagram? Visual-Paradigm . https://www.visual-paradigm.com/guide/uml-unified-modeling-language/what-is-state-machine-diagram/