Lecture 6 - UML Sequence and State Diagrams

Question 1

What are UML Class Diagrams?

Answer 1

UML Class Diagrams describe the static structure of the system
– classes,
– class attributes,
– associations between classes,
– association roles and multiplicity

Question 2

Cinema Booking Example

Answer 2

REFER TO SLIDES

Question 3

What do Sequence Diagrams represent?

Answer 3

Sequence diagrams represent
- the interaction of participants (horizontally)
- over time (vertically)
- using messages to communicate

Question 4

What are Sequence Diagrams?

Answer 4

• Sequence diagrams represent the interaction of objects (horizontally) over time (vertically).
  *Showing how the behaviour of a use case is distributed amongst its participating objects allowing developers to find missing objects or resolve uncertainties

Question 5

What are the most important components in Sequence Diagrams?

Answer 5

1.The participating objects – what are the objects associated with the use case in consideration
2.The flow of events between these objects – what are the messages that might be sent from the sender object to the receiver objects?

Question 6

What are the conventions for Sequence Diagrams?

Answer 6

Layout
Object Creation
Object Access

Question 7

What are the conventions for Sequence Diagrams - Layout?

Answer 7

Layout:
1st column: corresponds to the actor who initiated the use case
2nd column: Should be a boundary object with which the actor interacts to initiate the case
3rd column: Should be the control object that manages the rest of the use case
4th column: An entity object representing long-lived info tracked

Question 8

What are the conventions for Sequence Diagrams - Object Creation?

Answer 8

Object Creation:
– Control objects are created by boundary objects initiating the use case
– Other boundary objects are created by control objects

Question 9

What are the conventions for Sequence Diagrams - Object Access?

Answer 9

Object Access:
– Entity objects are accessed by control and boundary objects,
– Entity objects should never access boundary or control objects:
This makes it easier to share entity objects across use cases
and makes entity objects resilient against technology-induced
changes in boundary objects.

Question 10

Order Pricing Example

Answer 10

REFER TO SLIDES

Question 11

Create New Online Library Account EXAMPLE

Answer 11

REFER TO SLIDES

Question 12

When do you use Sequence Diagrams?

Answer 12

• You should use sequence diagrams when you want to look at the behavior of several objects within a single use case.
• Sequence diagrams are good at showing collaborations among the objects;
• they are not so good at precise definition of the behavior.

Question 13

What are State Diagrams?

Answer 13

• A UML state diagram is a notation for describing the sequence of states an object goes through in response to external events.
• UML state machines are extensions of the finite state machine model.
• A state is a condition satisfied by the attributes of an object.
• An event is a potential trigger for a change of state

Question 14

What is a transition?

Answer 14

Indicates a move from one state to another

Question 15

What do State Machine Models show?

Answer 15

Show how individual objects change their state in response to events.
REFER TO SLIDES FOR DIAGRAMS

Question 16

In the context of behaviour modelling, 2 different characterizations of states must be considered, what are they?

Answer 16

1. the state of each class, and
2. the state of the system as observed from the outside as the system performs its function

Question 17

What are Statechart Diagrams?

Answer 17

• Graphs whose nodes are states and whose directed arcs are transitions labelled by events.
• States capture conditions which hold for a period of time
• Events change the state (except internal)
• Statechart diagrams represent behaviour from the perspective of a single object only– An object model with a set of objects must be represented by a set diagrams

Question 18

What is a state and what are its characteristics?

Answer 18

A state is a condition satisfied ny the attributes of an object
Example: State of a user interface screen
* logged in, logged out, active, idle
* active is an abstraction of all the user’s logged in activity
* State has duration

Question 19

What is an Event?

Answer 19

• Something that happens at a point in time (e.g. button press, mouse click)
• Triggers a transition– Internal transition (no state change)– External transition (change to different state)
• May result in an action being executed

Question 20

State Chart Diagram vs Sequence Diagram

Answer 20

• State diagrams help to identify:
  – Changes to objects over time
• Sequence diagrams help to identify
  – The relationships between objects over time
  – Sequence of operations as a response to one or more events

Question 21

When do you use state diagrams?

Answer 21

• State diagrams are good at describing the behavior of an object across several use cases.
• State diagrams are not very good at describing behavior that involves a number of objects collaborating.