Positioning the File Position Pointer
Lines 47–48 position the put file-position pointer for object
outCredit to the byte location calculated by
( client.getAccountNumber() - 1 ) * sizeof( ClientData )
Because the account number is between 1 and 100, 1 is subtracted from the account number when calculating the byte location of the record. Thus, for record 1, the file-position pointer is set to byte 0 of the file.