private static int count = 0;

static void listDir(File dir) {
    if (count == 0) {
        System.out.println(dir.getName());
    }
    String prefix = "  ".repeat(count);
    File[] files = dir.listFiles();
    for (File file : files) {
        String fileName = prefix + file.getName();
        if (file.isDirectory()) {
            System.out.println(fileName + "/");
            count++;
            listDir(file);
            count--;
        } else {
            System.out.println(fileName);
        }
    }
}