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Engineering
Documenting use cases
The use case
• A use case diagram showing
– All use cases
– All actors
–
Relationshi
p
between them
p
Is very useful for reasons we have already
discussed.
• A use case on its own doesn’t tell us very
much…..
The complete use case
• Each use case must include details
about what has to be done to carry out
the process.
•
We need to consider:
We
need
to
consider:
– basic functionality
– any alternatives
– error conditions
– what must be true before starting
– what must be true when exiting
Standards
• UML gives a format for a use case including a
complete range of attributes that the designer
might want to include.
• This format has been adapted by many
desi
g
ners…..
g
• Basic Use Case documentation should include:
– preconditions
– flow of events (basic and alternatives)
– postcondition
Pre- and post-conditions
• They indicate what comes before and after
the use case
• Tell what state the system must be in at
start of use case (pre
condition)
start
of
use
case
(pre
-
condition)
• Tell what state the system must be in at
the end of the use case (post-condition)
• Post-condition must always be true
Pre- and post-condition
example
Deposit VAT Use case
Pre-condition: it is end of one our business
quarters
Flow of events:
1. The system determines the amount of VAT
payable for this quarter
2. The system send electronic VAT payment to
customs & excise
Post-condition: The government has
deposited our VAT and updated our records.
