code: public static byte[] join(List<byte[]> arrays) {
      int maxlength = 0;
      for (byte[] array : arrays) {
        maxlength += array.length;
      }
      byte[] rs = new byte[maxlength];
      int pos = 0;
      for (byte[] array : arrays) {
        System.arraycopy(array, 0, rs, pos, array.length);
        pos += array.length;
      }
      return rs;
    }
docs: join multi array
  
  @param arrays
  @return