code: public static void writeStringFixedSize(final DataOutput out, String s,
        int size) throws IOException {
      byte[] b = toBytes(s);
      if (b.length > size) {
        throw new IOException("Trying to write " + b.length + " bytes (" +
            toStringBinary(b) + ") into a field of length " + size);
      }
  
      out.writeBytes(s);
      for (int i = 0; i < size - s.length(); ++i) {
        out.writeByte(0);
      }
    }
docs: Writes a string as a fixed-size field, padded with zeros.