State Machines

Question 1

What is the purpose of a state machine diagram?

Answer 1

To model:
- possible states of a system/object
- transitions between states triggered by events
- behaviour in each state

Question 2

What activities can be defined within a state?

Answer 2

• entry/Activity: runs on entering the state
• exit/Activity: runs on exiting the state
• do/Activity: runs while in the state

Question 3

What are the components of a transition?

Answer 3

1. Event (trigger)
2. Guard (boolean condition)
3. Effect (actions during transition)

Question 4

Internal vs External transitions

Answer 4

• Internal: No state change (e.g. event1 triggers Activity3 in state1)
• External: State change (e.g. event1 moves from state1 to state2)

Question 5

Name 3 types of events

Answer 5

1. Signal event (e.g. rightMouseDown)
2. Call event (e.g. register(exam))
3. Time event (e.g. after(5 secs))

Question 6

What is an initial state?

Answer 6

A pseudo-state with:
- no incoming edges
- immediate transition to next state
- guards on outgoing edges must be mutually exclusive

Question 7

Final state vs Terminate node

Answer 7

Final state: object remains here (end of lifecycle)
Terminate node: object is deleted

Question 8

How does a decision node work?

Answer 8

Splits transitions into alternative paths with guards (e.g. [x > 0])

Question 9

What does a parallelisation node do?

Answer 9

Splits control flow into concurrent sub-flows (1 input -> N outputs)

Question 10

What is a synchronisation node?

Answer 10

Merges concurrent sub-flows into one (N inputs -> 1 output)

Question 11

Change events vs Guards

Answer 11

Change event: continuously checked (e.g. when(x > y))
Guard: checked only when event occurs

Question 12

Model a LectureHall with state Free and Occupied

Answer 12

[Free] –(occupy)–> [Occupied]
[Occupied] –(release)–> [Free]

Question 13

Sketch states for exam registration

Answer 13

[Unregistered] –(register)–> [Registered]
[Registered] –(cancel)–> [Unregistered]
[Registered] –(pass)–> [Completed]

Question 14

States

Answer 14

Represents conditions or situations during the lifecycle of an object

Question 15

Transitions

Answer 15

Represents the movement from one state to another, triggered by events, guards or activities

Question 16

Events

Answer 16

Triggers that cause a transition

Question 17

Guards

Answer 17

Boolean conditions that must be true from a transition to occur

Question 18

Activities

Answer 18

Actions executed when entering, exiting or during a state

Question 19

Transition syntax

Answer 19

event[guard]/effect e.g. e1[x>5]/x++

Question 20

When is a do-activity aborted?

Answer 20

When an event triggers a transition with no guard condition

Question 21

What is a time event?

Answer 21

An event like when(date=31.12.2001) that triggers at a specific time