Exception in thread "main" java.lang.NullPointerException
        at java.lang.String.<init>(String.java:602)
        at sun.nio.fs.UnixUserPrincipals.fromGid(UnixUserPrincipals.java:129)
        at sun.nio.fs.UnixFileAttributes.group(UnixFileAttributes.java:187)
        at GroupsTest.main(GroupsTest.java:9)

To reproduce, create file with numerical owner (group id 12345 is not in /etc/group) and try to read attributes:
$ touch file
$ chown :12345 file
$ java -cp . GroupsTest

===
import java.io.*;
import java.nio.file.*;
import java.nio.file.attribute.*;

public class GroupsTest {
        public static void main(String[] args) throws IOException {
                File f = new File("file");
                PosixFileAttributes attrs = Files.readAttributes(f.toPath(), PosixFileAttributes.class, LinkOption.NOFOLLOW_LINKS);
                System.out.println(attrs.group());
        }
}
===