code: public String getString(String strKey)
      {
          if (this.getMessageFieldDesc(strKey) != null)
              return this.getMessageFieldDesc(strKey).getString();
          Object data = this.getNative(strKey);
          if (data != null)
              return data.toString();
          return null;
      }
docs: Do any of the conversion that has to be done for all message types.
  Typically overidden to return correctly converted data.
  @param trxMessage The raw internal trx message.
  @return The new Internal message with additional/refined data.