State Machines and Transitions Quiz

State Machines Overview

• Lectures 13 and 14 cover state machines, a crucial concept in software engineering, particularly for reactive systems.
• State machines model the dynamic behavior of reactive objects.
• They are used when object behavior is complex and heavily influenced by external events.
• Avoid using state machines when numerous objects are involved, as interaction diagrams are more suitable.

State Machine Diagrams

• State machines utilize UML (Unified Modeling Language) diagrams.
• State machine diagrams illustrate states, transitions between states, and events that trigger these transitions.
• Key elements comprise states, events, and transitions.

Key Elements in State Machine Diagrams

• States represent a condition or situation during an object's life cycle.
• Events denote noteworthy occurrences in time and space.
• Transitions show the movement between states in response to events.

Reactive Objects and State Machines

• Reactive objects respond to external and internal events.
• Lifecycle is modeled as a sequence of states, transitions, and events.
• Current behavior is influenced by past events.

State Machine Classifiers

• State machines model the behavior of classes, use cases, subsystems, and entire systems.

Behavioral vs. Protocol State Machines

• Behavioral state machines specify implementation details.
• They are suited for describing the behavior of classifiers (with or without implementation).
• Protocol state machines don't specify implementation details, only the order and results of operations.

Examples/UML diagrams

• Illustrative examples using UML diagrams demonstrate the practical application of state machines.
• Diagrams show state transitions, events, actions, guard conditions.
• The different kinds and categories of state machines are showcased.

Types of States

• Real states are the standard, defined states of a state machine.
• Pseudo-states are transient states, not actual states but rather control structures that affect the state machine's transitions.
  • Initial states
  • Decision nodes
  • Parallelization/Synchronization nodes
  • History states
  • Entry/Exit points

Initial States

• Have no incoming edges, typically one outgoing edge to a real state.
• Guard conditions for multiple outgoing edges ensure only one target state is reached.
• Transition to the next state is immediate.

Terminate and Final States

• Reached when the state machine terminates.
• The modeled object ceases to exist or transitions to a permanent final state.

State Machine Transitions

• Represent movement between states.
• Triggered by events.
• Include guard conditions to determine state changes, actions associated with a transition.
• Protocols are indicated by a specific keyword in front of the state name.

Transition Elements

• Events: zero or more of these trigger a transition.
• Guard conditions: one optional guard condition that must evaluate to true before a transition.
• Actions: zero to many actions performed when the transition occurs.
• Protocol transitions: no actions but pre/post-conditions to specify the protocol.

Automatic Transitions

• Transitions without events.
• Occur when guard conditions or preconditions are true.

Junction Pseudo-states

• Represent transition merging or branching points.
• Define merging or branching points for multiple incoming and outgoing transitions.

Mutually Exclusive Guard Conditions

• Ensure only one transition is executed.
• Used in conjunction with junction pseudo-states.

Choice Pseudo-states

• Direct the flow of a state machine based on conditions.
• A state that tests a decision and leads to different transitions (states)

Memory in State Machines (History psuedo-states)

• Shallow history pseudo-states: remembers the last substate at the same "level" in the composite state.
• Deep history pseudo-states: remembers the last substate at the same level (or a lower level).

Examples of Events in State Machines

• Call events: request for a specific operation invoked on an object instance.
• Signal events: one-way asynchronous communication between objects, conveying information.
• Change events: trigger changes whenever a Boolean expression transitions from false to true.
• Time events: triggered by time-based criteria, like time duration.

Composite States

• Grouping multiple substates into a larger, more complex composite state.
• Substates inherit the transitions of the composite state.
• Final states for composite sub-machines only apply within their own region.

Simple vs. Orthogonal Composite States

• Simple composite states have a single region.
• Orthogonal composite states have multiple regions that operate concurrently.

Entry/Exit Points

• Entry points for initiating execution of the composite state,
• Hiding mechanisms of internal operation.
• Simplifying interactions within a state machine.

Submachine States

• Referring to a separate state machine, providing re-usability of state machines (often in composite states)