state diagram (state machine diagram or statechart diagram)

What is a state diagram (state machine diagram or statechart diagram)?

A state diagram (also known as a state machine or statechart diagram) is an illustration of all the possible behavioral states a software system component may exhibit and the various state changes it's predicted to undergo over the course of its operations.

Using standard Unified Modeling Language (UML) notation, a state diagram somewhat resembles an operational flowchart that outlines the various state-altering processes that occur within a system. However, state diagrams are specifically designed to focus on the state of an object rather than the change-inducing processes associated with them. Instead, any underlying events that trigger state changes are identified as transition elements.

Symbols and components of state diagrams

While the exact structure depends heavily on the specific system being modeled, a state diagram typically includes the following basic denotations:

  • Initial state. A solid black circle that represents the initial state of a system or a class.
  • States. Individual states are portrayed as boxes with rounded corners that contain the name of the state (usually written in CamelCase).
  • State actions. Any actions that a particular state is expected to invoke are designated by a box with two sections: An upper section containing the name of the state and a lower section containing the actions it will initiate (as well as any relevant variable names).
  • Transitions. Straight lines each with an arrow at one end connect various pairs of state boxes, providing an ordered designation of the state transitions that will occur.
  • Final state. Final state is portrayed as a large black dot with a circle around it.

In addition to these basic components, state diagrams can also indicate forked, joined, self-transitioning, composite and historical states.

Benefits and uses of state diagrams

State diagrams can be useful in a variety of application development scenarios -- particularly in cases of object-oriented programming (OOP) -- for a number of reasons. Some of its most prominent benefits include its ability to do the following:

  • List the events responsible for altering system states.
  • Model dynamic behavior and activity of a system.
  • Demonstrate the response of a system to different types of stimuli.
  • Represent finite state machines visually.
  • Illustrate the entire lifecycle of an object.

We can use the processes associated with an ATM as an example of how to apply a state diagram. The initial state of an ATM system is often a ready state that waits for a bank customer to initiate an action. The middle states of this process might include things like verifying the user identity, calculating dollar amounts or even experiencing a malfunction. The actual change-inducing events are dictated primarily by the user, who may request to check on their bank balance, withdraw cash or deposit a check. The final state occurs once the machine successfully executes and ends a session.

This was last updated in May 2023

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