Serializable     (this object will be written to a file)
                    ---------------------------------------------------------------------------
                             ^
                             |
                       Borrowing <abstract>
   
                          - bookTitle  :String
                          - borrowerID :String
                          - issueDate  :Date          (my own Date class, which ive created)

                          + toString()   :String      
                          + calculateReturnDate() <abstract>  :Date

                    ------------------------------------------------------------------------------

                         ^                |        ^
                         |                |        |
                    ShortLoan (extends)   |     LongLoan (extends)
                      +tostring()         |        +toString()