Solution: The Email Sanitizer

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.Objects;
public class EmailSanitizer {
    public static List<String> sanitizeEmails(List<String> rawEmails) {
        return rawEmails.stream()
            // Remove nulls first to avoid NullPointerException
            .filter(email -> email != null)
            // Trim whitespace from the remaining strings
            .map(String::trim)
            // Remove strings that became empty after trimming
            .filter(email -> !email.isEmpty())
            // Convert to lowercase for consistency
            .map(String::toLowerCase)
            // Collect the results back into a List
            .collect(Collectors.toList());
    }
    public static void main(String[] args) {
        List<String> rawData = Arrays.asList(
            "  john.doe@Example.com ", 
            null, 
            "", 
            "JANE.SMITH@work.org", 
            "   ", 
            "admin@Internal.net"
        );
        List<String> cleanData = sanitizeEmails(rawData);
        System.out.println("Original list size: " + rawData.size());
        System.out.println("Clean list size: " + cleanData.size());
        System.out.println("Cleaned Emails: " + cleanData);
    }
}