code: public static byte[] toByteArray(Writable... writables) {
          final DataOutputBuffer out = new DataOutputBuffer();
          try {
              for (Writable w : writables) {
                  w.write(out);
              }
              out.close();
          } catch (IOException e) {
              throw new RuntimeException("Fail to convert writables to a byte array", e);
          }
          return out.getData();
      }
docs: Convert writables to a byte array