code: public static String[] concat(String[] ...aa) {
      int l = 0;
      for (String[] a : aa) l += a.length;
      String[] r = new String[l];
      l = 0;
      for (String[] a : aa) {
        System.arraycopy(a, 0, r, l, a.length);
        l += a.length;
      }
      return r;
    }
docs: Concatenate given list of arrays into one long array.
  
  <p>Expect not null array.</p>
  
  @param aa list of string arrays
  @return a long array create by concatenation of given arrays.