static ArrayList array = new ArrayList();
    public static void visitAllFiles(File dir) {
        if (dir.isDirectory()) {
            String[] children = dir.list();
            for (int i = 0; i < children.length; i++) {
                visitAllFiles(new File(dir, children[i]));
            }
        } else {
            String name = dir.getName();
            String sub=name.substring(name.length()-3, name.length());
            if(sub.equals("txt"))
             array.add(dir);
        }
    }