UML Behavioural Models

Question 1

Mention the three UML behavioural models discussed in class

Answer 1

UML Activity Diagrams
UML State Machine Diagrams
UML Sequence Diagrams

Question 2

What is the UML Activity diagram a model of and what is its purpose

Answer 2

Modelling of step-wise activities and actions.

Purpose is to describe the behaviour in terms of processes and workflows.

An UML Activity Diagram is principally expressive enough to specify the system behaviour for a certain process completely.

Question 3

Activity diagram
Initial node

Answer 3

Start of execution
Exactly one
Filled out circle

Question 4

Activity diagram
Action

Answer 4

Active step
By system or actor

Round rectangle, description of action inside i.e. boil water

High level description of an active step that can be executed by an actor or by the system.

Not specified specifically, but can be by swim lanes.

Question 5

Activity diagram
Transition

Answer 5

Connects actions
An arrow from which to which action the transition leads.

Question 6

Activity diagram
Final node

Answer 6

End of execution
Circle filled in the middle.

Potentially many. But does not have different semantics, cannot be a distinction where one final node means ‘success’ and another ‘failure’.
Also possible to have no final node.

Question 7

Activity diagram
Decision

Answer 7

Empty diamond.
Control flow branch.
Similar to Java’s if or switch statements.

Question 8

Activity diagram
Guard

Answer 8

By the decision in [ ]
Boolean expression, must be true to execute.
For decisions, must not overlap.

Question 9

Activity diagram
Fork

Answer 9

Start of parallelism
Start of all outgoing transitions in parallel.
A fat vertical line, arrows going in and out of it.

Once the fork is hit by the execution of the activity diagram because of the incoming transition was completed, all outgoing transitions are executed in parallel, i.e., (conceptually) at the same time.

Question 10

Activity diagram
Join

Answer 10

End of parallelism
A fat vertical line, arrows going in and out of it.

Continue when all incoming transitions were completed.

Question 11

Activity diagram
Swim lanes

Answer 11

Attribute actions to actors.
Rectangle around activity diagram with name on top.

Question 12

What does the UML State Machine diagram model and what is the purpose?

Answer 12

Models protocols based on discrete system states.

The purpose is to describe reaction of system to triggers for current state.

Question 13

When should one use State Machine Diagrams over Activity diagrams? And the other way around

Answer 13

A UML state machine diagram is used when a system has many discrete states the system stays in for a relevant time.

Whereas UML activity diagram is used when states are perceived as almost transient and focus should be put on the actions leading to the change of states.

Question 14

State Machine Diagram
State

Answer 14

Current ‘mode’ of system
i.e. waiting, order approved, etc.

Rounded rectangle, with two sections. First sections is description of state.

Question 15

State Machine Diagram
Transition

Answer 15

Connects states.
Direction of arrow shows from which to which.

Question 16

State Machine Diagram
Initial state

Answer 16

Filled out circle.
Marks the start of execution.
exactly one.
Only outgoing transitions.

Question 17

State Machine Diagram
Final state

Answer 17

Marks the end of execution for the described state machine

Circle with filled out circle in the centre.

Possible to have multiple final states, but no different semantic.
Also possible to have no final state.

Question 18

State Machine Diagram
Trigger

Answer 18

Event that occurs to cause transition to fire.

text by transition arrow

i.e. endOfDay, so transition to final state.

A transition will only fire id the annotated trigger event occurs. If no trigger is annotated, the transition can principally fire immediately (but guards have to be true).

Question 19

State Machine Diagram
Guard

Answer 19

Condition under which transition can fire.

A guard is a boolean condition that has to be satisfied for the transition to be able to fire (once trigger occurs).

condition in square brackets

example [time < 22:45]

Question 20

State Machine Diagram
Action

Answer 20

Performed when transition fires.

has a leading / to mark action and not trigger

This may e.g. be a method that is called to set some internal variables.

Question 21

State Machine Diagram
Decision

Answer 21

hollow diamond with outgoing guards.

Selection of one of the outgoing
transitions via their guards.

Question 22

What is the UML sequence diagram modelling and what is the purpose?

Answer 22

Models calls in example scenarios.
Purpose is to describe detailed calls and their order on an example scenario.

Question 23

Sequence diagram
Lifeline

Answer 23

From top to bottom.
How long an entity (object) exists.
Box with :class
and then from that a Dotted line + activation box.

Question 24

Sequence diagram
Activation box

Answer 24

Active part of lifeline.
Can be nested.
Hollow rectangle.

Is the part of the lifeline where an entity is actively participating in the depicted scenario.

Question 25

Sequence diagram
Message

Answer 25

Active communication with other entity.

arrow to other entity with method called on top.

in Object oriented design usually means that the sending entity is calling a method on the receiving entity.
Possible to pass arguments (values for parameters)

Question 26

Sequence diagram
Return

Answer 26

Response to received message. Optional.
May contain descriptive text, e.g. the result of a calculation.
Dotted line back to the entity that sent a message in the first place, with text on top.

Question 27

Sequence diagram
Start node

Answer 27

Needed.
filled out circle, indicating start

Question 28

Sequence diagram
Termination

Answer 28

End of an entity’s lifeline.
Explicit signal that the entity is no longer used.

Cross on top of lifeline.

May be used to actively end the lifeline of an entity because it is no longer relevant to the scenario of the sequence diagram but also the system altogether.

Question 29

Sequence diagram
Synchronous message

Answer 29

Is depicted by a solid line with a solid tip. It means that, after sending the message, the sender is blocked until the operation completes at the receiver, i.e., the sender cannot do anything else on that thread.

Question 30

Sequence diagram
Nested activation box

Answer 30

An activation box may be nested, e.g. when an entity is still waiting for the return of a previous synchronous message while it receives another message.

Question 31

Sequence diagram
An asynchronous message

Answer 31

Is depicted by a solid line arrow with a pointed tip. It means, that after sending the message, the sender is not blocked until the operation completes the receiver.