State Machine Diagrams in UML

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Contents

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• What is a State Machine Diagram?
• Key Components of a State Machine Diagram
• Notation Elements of a State Machine Diagram
• Sequence of actions
• How to Create a State Machine Diagram
• Examples of State Machine Diagrams
  • Simple Digital Watch
  • Back ATM
• Advantages of State Machine Diagrams

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When it comes to understanding the behavior of a system or an object within a system, State Machine Diagrams in Unified Modeling Language (UML) are an indispensable tool.

What is a State Machine Diagram?

• A state diagram is a graph in which nodes correspond to states and directed arcs correspond to transitions labelled with event names.
• A state diagram combines states and events in the form of a network to model all possible object states during its life cycle, helping to visualise how an object responds to different stimuli.

Key Components of a State Machine Diagram

1. States: These represent the different conditions or situations of an object. A state is depicted by a rounded rectangle. A state can be defined as the duration of time during which an object is doing an activity.

2. Transitions: These are arrows that connect states, representing the shift from one state to another.

3. Events: These are external triggers that cause the transition from one state to another. An event occurs at a time point and transmits information from one object to another.

4. Activity: An activity is an operation with certain duration that can be interrupted by another event. For example, a bulb in the “On” state is doing a continuous activity of “illumination” and this operation can be interrupted by another event like “switch off”.

5. Actions: These are operations or activities that result from a transition. An action occurs in response to an event and cannot be interrupted.

6. Initial and Final States: The initial state is where the object begins its lifecycle, represented by a filled circle. The final state, represented by a circle surrounding a smaller filled circle, shows where the lifecycle ends.

7. Guard: A guard is a logical condition placed before a transition that returns either a true or a false. A guarded transition occurs only if the return value is true.

Notation Elements of a State Machine Diagram

• Final state
  • A real state
  • Object can remain in that state forever
• Terminal state
  • Pseudo state
  • Object is destroyed
• Decision Node
  • Pseudo state
  • Used to model alternative transitions

Sequence of actions

• Guard is tested while in state (before exist action)
• Transition action after exit action

For example:

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1. Enter state S1, x=4
2. Event e occurs and x==4 then execute exit action x=5
3. Followed by the transition action x*2=10
4. S2 is entered and entry is x++ then x=11

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How to Create a State Machine Diagram

1. Identify the States: Start by listing the different states that the object can be in during its lifecycle.

2. Determine the Events and Transitions: Identify the events that trigger transitions between states.

3. Specify Actions: Define any actions that result from transitions between states.

4. Draw the Diagram: Use the UML notation to draw the diagram, connecting states with transitions and labeling them with the respective events and actions.

Examples of State Machine Diagrams

Simple Digital Watch

A simple digital watch has a display and two buttons to set it, the A button and the B button. The watch has two modes of operation, display time and set time. In the display time mode, hours and minutes are displayed, separated by a flashing colon. The set time mode has two sub-modes, set hours and set minutes. The A button is used to select modes. Each time A is pressed, the mode advances in sequence: display, set hours, set minutes, display etc. Within the sub-modes, the B button is used to advance the hours or minutes once each time it is pressed. Buttons must be released before they can generate another event. Prepare a state diagram of the watch.

State Machine Diagram of Digital Watch

Back ATM

• The ATM was initially turned off. After the power is turned on, ATM performs startup action and enters Self Test state.
• If the test fails, ATM goes into Out of Service state, otherwise there is triggerless transition to the Idle state. In this state ATM waits for customer interaction.
• The ATM state changes from Idle to Serving Customer when the customer inserts debit or credit card in the ATM’s card reader.
• On entering the Serving Customer state, the entry action readCard is performed. Note, that transition from Serving Customer state back to the Idle state could be triggered by cancel event as the customer could cancel transaction at any time.
• Serving Customer state is a composite state with sequential substates Customer Authentication, Selecting Transaction and Transaction.
• Customer Authentication and Transaction are composite states by themselves which is shown with hidden decomposition indicator icon.
• Serving Customer state has triggerless transition back to the Idle state after transaction is finished.
• The state also has exit action ejectCard which releases customer’s card on leaving the state, no matter what caused the transition out of the state.

State Machine Diagram of Back ATM

Advantages of State Machine Diagrams

1. Clarity: They provide a clear and comprehensive visualization of the different states and transitions.

2. Ease of Understanding: They simplify the understanding of complex behaviors.

3. Effective Communication: They act as a valuable communication tool within development teams.

4. Facilitates Testing: They clearly outline the different states and transitions, making it easier to test the system.

• State diagrams are mostly used in reactive/event driven systems/components
• State diagrams are also useful in modeling concurrent or parallel behavior by allowing multiple states to exist simultaneously.
• This is helpful in modeling systems with multiple threads or concurrent processes.