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Код

public class MainTest {
    static SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy");

    public static void main(String[] args) throws IOException, ParseException {
        File rootFolder = new File(args[0]);
        Map<Date, Long> dateLongMap = new HashMap<Date, Long>();
        if (rootFolder.isDirectory()) {
            File[] files = rootFolder.listFiles();
            if (files != null) {
                for (File file : files) {
                    if (file.isFile()) {
                        Path path = Paths.get(file.getAbsolutePath());
                        BasicFileAttributes attributes = Files.readAttributes(path, BasicFileAttributes.class);
                        FileTime creationTime = attributes.creationTime();
                        Date formattedDate = sdf.parse(sdf.format(new Date(creationTime.toMillis())));
                        Long fileSize = dateLongMap.get(formattedDate);
                        if (fileSize != null) {
                            dateLongMap.put(formattedDate, fileSize + attributes.size());
                        } else {
                            dateLongMap.put(formattedDate, attributes.size());
                        }
                    }
                }
            }
        }
        System.out.println(dateLongMap.size());
        for (Map.Entry<Date, Long> fileSet : dateLongMap.entrySet()) {
            System.out.println(fileSet.getKey() + " : " + (fileSet.getValue()/1024));
        }
    }
}