Check files for checksum

Check.java

public static void checkFilesForChecksum(List<File> files1, List<File> files2)
    {
        Map<String, File> files2Map = new TreeMap<String, File>();
        files2.stream().filter(p -> p.isFile()).forEach(p -> files2Map.put(checksum(p), p));

        files1.forEach(file -> {
            if (file.isDirectory())
            {
                Optional<File> opt = files2.stream().filter(p -> file.isDirectory() && p.getName().equals(file.getName())).findFirst();
                if (opt.isPresent())
                    checkFilesForChecksum(Arrays.asList(file.listFiles()), Arrays.asList(opt.get().listFiles()));
                else
                    FILE.info("Dir:" + file.getName());
            }
            else if (file.isFile())
            {
                if (!files2Map.containsKey(checksum(file)))
                    FILE.info("File:" + file.getName());
            }
            else
            {
                throw new RuntimeException(file.getName() + " is not a FILE and is not a directory");
            }
        });
    }