In this project you will extend your mail-system from the Email Attachment Project to use File
I/O to write email to a file and read it from a file.

Part 1

Add two methods to your MailServer class:

public void shutdown() throws ShutdownException

This method should save the details of all the MailItems (excluding
attachments) stored on the MailServer to a file named mail.csv in the
project directory. You may borrow the methods makeAbsoluteFilename and
getProjectFolder from the address-book-io project. Each MailItem should be
written to one line in the file, with the fields in comma-separated value
(csv) format (you can assume that none of the fields in the MailItem contain
commas):

from,to,subject,message

Make sure you handle exceptions appropriately. You must create the
ShutdownException class, which should contain a String describing why the
shut down procedure failed.

private void restore() throws RestoreException

This method should read the details of the MailItems stored in mail.csv and
restore the state of the MailServer. The MailItems for each user should be
queued in the same order as they were received by the MailServer before the
MailServer was shut down.

Make sure you handle exceptions appropriately. You must create the
RestoreException class, which should contain a String describing why the
restore procedure failed.

Now modify the MailServer class so that when the MailServer is created, it
checks to see if it needs to restore its state from mail.csv.

Part 2

Now update the shutdown and restore methods so that they handle attachments.
Do this by adding a fifth field to each line of the mail.csv file, denoting
the number of attachments on the MailItem:

from,to,subject,message,num_of_attachments

Then write the details of each Attachment on the following lines, one to a
line. You will need to figure out an appropriate format for writing
attachments.