So - come on (as Jeremy Paxman would say on University Challenge) - what should a book exchange system do ?
What are all the different jobs you would expect it to do ?
What are the steps involved in each one ?
The different situations that might need to be dealt with ? Are you going to deal with everything, or just show a subset of the system, for simplicity ?
My earlier post showed I had not read your first posting !
Basically for your required use cases:
- lend a book
- return a book
- remove a book
- add customer
- update or remove customer
you have to itemise the stages in each case, and show who/what is involved (Actors).
From this you then need to identify the class objects (e.g. customer, book ?) and what details (customer name, book name ...) and operations (add, lend, return ...) can be specified for each one - hence the class description, showing any relationships between the classes, if any.
Then your sequence diagram will show a typical set of operations occurring between two class instances, e.g. to lend a book - validate customer, add book to customer's account if borrowing limit not reached, etc.
Don't take any of those above paragraphs above literally, you will need to do your own investigation, and reading of a UML tutorial.
