code: private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
          s.defaultWriteObject();
          s.writeInt(length());
          s.writeObject(getValue());
      }
docs: Save the state of the {@code StringBuilder} instance to a stream
  (that is, serialize it).
  
  @serialData the number of characters currently stored in the string
  builder ({@code int}), followed by the characters in the
  string builder ({@code char[]}).   The length of the
  {@code char} array may be greater than the number of
  characters currently stored in the string builder, in which
  case extra characters are ignored.