State-based Design

Question 1

What is meant by a state-based system?

Answer 1

reactive system that behaves differently to evetns depending on what state its in

Question 2

How do we describe a reacitve system?

Answer 2

• Events; requests handled by system
• Actions; performed in repsonse to events
• States; affect how system responds
• Transitions; routes that exit/enter states

Question 3

What is meant by the alphabet of a state machine?

Answer 3

• Set of all events

Question 4

What is meant by “Completing a Machine”?

Answer 4

Add in to a state machine diagram all missing transitions from the alphabet

Question 5

What can be specified by a state machine?

Answer 5

Can capture different levels of abstraction: high level control and low level behaviour

Question 6

What is contained in a UML Frame label?

Answer 6

frame type code

name of UML element

Question 7

What is the difference between a Behavioural State MAchine and a Protocol State MAchine?

Answer 7

Behavioural:

• illustrates reactive behaviour
• shows how different ations are triggered by events
• missing transitions are ignored events

Protocol:

• illustrate protocol of class/interface
• shows allowed sequence of messages
• missing transitions are disallowed messages

Question 8

Why is nondeterminism an issue? WHat is a fix?

Answer 8

Means to next state is unknown, could choose arbitrary transition out of options

Use preconditions; add guards to each conflicting tranisition

Question 9

How are events presented to a UML state machine?

Answer 9

one at a time

Question 10

How are transitions enabled in a UML state machine?

Answer 10

• transitions exiting active state are enabled if event trigger matches the event
• once enabled the transition is then fired, event is consumed and a new event is presented

Question 11

What is meant by “Run-to-Completion” when refering to state machine semantics?

Answer 11

All triggered events must be processed before the triggering event is considered finished

Question 12

What are the 2 methods of composing state machines?

Answer 12

• have substate machine inside a superstate

- concurrent state machines; 2 independent state machines inside a state.

Question 13

With nested UML state machines, which transitions take precedence if there is a name coflict? substate or superstate

Answer 13

Substate; to do with inheritance & overriding

Question 14

Where do we enter/exit a substate?

Answer 14

Enter at default initial state

Can exit from any/all active substates

Question 15

How can a child subclass be compatible with its parent superclass?

Answer 15

• must exist in same states as parent
• must repsond to same transitions as parent
• new states must be substates of parent states

Question 16

What is the difference between a state diagram and an activity diagram?

Answer 16

State: all activity happens on transitions
states are quiescent (nothing happens in states)

Activity: all activity happens in nodes
flows represent transfer of control