Chapter 4 – Answers
[Question #1] What is UML?
[Your response] :
[Answer] : UML stands for Unified Modelling Language. UML is a standardised modelling language that
encompasses a large variety of diagram types that can be used to represent many aspects of software
development. In VCE Software Development, we only examine Use case diagrams and we only use these in the
Analysis phase of the PSM.
[Question #2] What is a Use case diagram?
[Your response] :
[Answer] : A Use case diagram is essentially a structured 'story' depicting the functional aspects of a system
within an organisation. It is used to depict the goals of the system and how people/organisations interact with the
system to achieve these goals. A Use case diagram depicts a number of 'use cases' - which are very similar to
processes in DFDs. A Use case diagram is intended to be very broad in scope and is the type of diagram that I
imagine a team of software developers constructing on a white board or a sheet of butchers paper - while
discussing an overview of the system.
[Question #3] Give two examples of possible actors in a use case diagram.
[Your response] :
[Answer] : customer, manager, supplier, entry clerk - the important point is that actors represent roles as
opposed to actual people. An information system that is interacting with a use case can also be represented as
an actor.
[Question #4] In a Use case diagram, what does a system boundary represent?
[Your response] :
[Answer] : A system boundary represents the confines of the system. The actors are drawn outside of the
system boundary. There is usually only one system boundary in a Use case diagram, but multiple boundaries can
be drawn to represent sub-systems.
[Question #5] In what ways is a DFD different to a Use case diagram?
[Your response] :
[Answer] : A DFD provides a more detailed analysis than a Use case diagram. A DFD focuses on data flows and
the ways in which data is processed and stored, whereas a Use case diagram simply indicates what associations
different actors have with particular processes (use cases).
[Question #6] Give examples of three physical objects that could be data stores in a DFD.
[Your response] :
[Answer] : filing cabinet, hard drive, in tray, time sheet, tape drive.
[Question #7] Why is it convention not to represent data flows between entities in a DFD?
[Your response] :
[Answer] : Entities in a DFD are by definition, outside of the organisation. Representing data flows between
entities that are outside of the organisation is beyond the scope of the processes being represented. What two
entities exchange between them is not the concern of the organisation being depicted in the DFD. What's more it would be difficult to work out what the purpose of the transfer is and what processing is taking place.
[Question #8] Why is it a problem when the data flows into a process are the same as the data flows out of a
process?
[Your response] :
[Answer] : A process by definition, changes data in some way. If the data that flows into the process is the same
as the data that flows out of the process, the question could be asked: 'what happened to the data'? It would
seem that the process didn't do anything at all in such a case.
[Question #9] What is the process known as DFD levelling?
[Your response] :
[Answer] : A DFD can be very complex. To keep it fairly simple, it is convention to restrict the number of
processes to no greater than about 8. This can mean that some of the processes being represented can be quite
complex. Complex processes can be used as the subject for a new DFD - and the process by which this is done
is known as levelling.
[Question #10] What are the main advantages of a context diagram over a DFD?
[Your response] :
[Answer] : A context diagram (also known as a Level 0 DFD) shows only the interactions of entities with the
organisation. None of the processes within the organisation are depicted. In this way, it is a simpler diagram and
summarises the data flows into and out of an organisation.