code: public static long getSize(File dir)
     {
        long size = 0;
        for (File file : dir.listFiles())
        {
           if (file.isFile())
           {
              size += file.length();
           }
           else
           {
              size += getSize(file);
           }
        }
  
        return size;
     }
docs: Returns the size of directory including all subfolders.