code: public void setData(byte[] data, int offset, int length) {
          if (immutable) {
              throw new IllegalStateException(ERR_MSG_IMMUTABLE);
          }
          this.data = new byte[length];
          System.arraycopy(data, offset, this.data, 0, length);
      }
docs: Sets the message payload data.
  
  @param data the payload data
  @param offset the beginning offset to copy from
  @param length the length to copy