It’s known that emails can have attachments, some even have other emails as attachments. This allows them to have attachments at any level of nesting. Emails can be stored in so called Internet Message Format, files in this format usually have .eml extension.
Let’s assume that someone could execute the following operations for arbitrary number of times in a random order:
- Attach EML or ZIP file to the email
- Archive list of emails to the root folder of ZIP archive
So the sequence of operations is a storing format for the original set of files. For example ZIP archive can contain emails with ZIP archive as attachment and the last one ZIP archives contain emails with email as attachment (see samples provided).
You need to write a CLI program that extracts files from the provided file based on the provided format. See sample output for the sample above. You must not guess input file format and the program should allow the user to specify input format in some way, as a command line argument for example. Program should raise an error if an input file cannot be converted based on the provided format. For EML files handling it is highly recommended to use Java Mail library ( https://javaee.github.io/javamail ), MimeMessage, Multipart,BodyPart classes in particular. For ZIP files handling it is recommended to use java.util.zip package from Java SE. Solution should have quite good quality, which means it should be covered by tests, be able to work with real-life emails (as this one) and quite large ZIP archives.