Lecture 7: State Machines

Question 1

What is a state machine?

Answer 1

• Models the behaviour of an entity
• Specifies the state changes which an object performs triggered by states, events or transitions received by objects

Question 2

How many state machines for an enity?

Answer 2

• Contains a single state machine for a single entity
• Specifies: the reaction from events
• The action from states

Question 3

What is the Moore state machine?

Answer 3

• Moore machine uses **entry actions only **
• Output is produced when the state changes

Question 4

What is a Mearly state machine?

Answer 4

• Mealy machine uses input actions only

• Output is produced based on the input state

Question 5

What are the two different kinds of state machines?

Answer 5

• Behavioural – Defines the behaviour of a model element (the instance)
• Protocol - Model the protocol of a classifier
- Don’t have actions

Question 6

What are the states of an object determind by?

Answer 6

• The values of its attributes
• The relationship to other objects
• The activities it is performing

Question 7

What are the three elements of state machines?

Answer 7

• State
• Event
• Transition

Question 8

What is a state?

Answer 8

a condition during the life of an object during which it satisfies some condition to perform an activity or wait for an event “Roundtangles”

Question 9

What is an event?

Answer 9

An occurance that happens in a space and time

“Are written over the transition that they trigger”

Question 10

What is a transition?

Answer 10

Is the movement from one state to another triggering an event

“Paths/link”

Question 11

What are the four types of events?

Answer 11

1. Call event
2. Signal event
3. Change event
4. Time event

Question 12

What does call event do?

Answer 12

Triggered by a synchronous operation call

Question 13

What does a signal event do?

Answer 13

specifies communication of send request instances between objects.

Asynchronously between objects

• Receipt of a signalA signal is a classifier which

Question 14

What does a change event do?

Answer 14

The constraint is a Boolean expression on the attributes of the objects either true/false is fullfilled

Question 15

What does a time event do?

Answer 15

Time events occur when a time expression becomes true using the two key words: after and when

• *After**: after time period
• *When**: Some time condition becomes true

Specifies a particular time at which the event is triggered

Question 16

The events can be appear what?

Answer 16

• Can appear synchronously/asynchronously
• Cause transitions between states

Question 17

What does synchronously do?

Answer 17

Synchronous calls, which are associated with an operation, have a send and receive message

• Call event triggered by call
• Exception even triggered by an object

Question 18

What does asynchronously do (list the events)?

Answer 18

When a signal is interrupted by a error condition and there are no operation with the signal

• Signal event: signal sent by an object

• Change event: triggered by side effects on object attributes
• Time event: triggered by Boolean guard over time

Question 19

What is the syntax for transisiton?

Answer 19

• Event: triggers transition
• Guard condition: Boolean condition must be true before the transition occurs
• Junction pseudo code: Transition merges in a branch
• Choice pseudo-state: ~flows through the state according to its conditions

Question 20

What are the two special actions?

Answer 20

Entry and Exit

Question 21

When the transition has no event what’s the condition?

Answer 21

If transition has no event - automatic transition if the guard condition or precondition is true.