Clean Up a Contact Import

A CRM in Toronto imports contacts that people paste from spreadsheets and old systems: one email address per line, with stray spaces (some of them Unicode spaces copied from documents), empty lines, comment lines starting with #, and the same person in different capitals.

Write Contacts.importEmails(text) so that it returns the clean list:

1. Split the text into lines and remove surrounding white space from each line, including Unicode spaces such as the em space (U+2003).
2. Skip lines that are blank (empty or only white space) and lines that start with #.
3. Lowercase each address with Locale.ROOT.
4. Remove duplicates, sort alphabetically, and return an unmodifiable list (adding to it must throw UnsupportedOperationException).

The driver prints how many contacts there are, each one in brackets (so stray spaces would show), and whether the list could be changed.

Example

Input:  Ana@Example.com
        (an empty line)
          ben@example.com
        # exported from the old CRM
        ana@example.com
Output: 2 contacts
        [ana@example.com]
        [ben@example.com]
        the list is read-only

Java 11's String methods and Java 16's Stream.toList() make this short.

• up to 1,000 lines
• lines may contain Unicode white space