public function getRound($roundId)
    {
        $this->_ensureRounds();

        if (! isset($this->_rounds[$roundId])) {
            return null;
        }
        if (! isset($this->_roundObjects[$roundId])) {
            $this->_roundObjects[$roundId] = $this->tracker->createTrackClass('Round', $this->_rounds[$roundId]);
        }
        return $this->_roundObjects[$roundId];
    }