code: public Participant getParticipant(String sessionId, String participantPrivateId) throws OpenViduException {
  		Session session = sessions.get(sessionId);
  		if (session == null) {
  			throw new OpenViduException(Code.ROOM_NOT_FOUND_ERROR_CODE, "Session '" + sessionId + "' not found");
  		}
  		Participant participant = session.getParticipantByPrivateId(participantPrivateId);
  		if (participant == null) {
  			throw new OpenViduException(Code.USER_NOT_FOUND_ERROR_CODE,
  					"Participant '" + participantPrivateId + "' not found in session '" + sessionId + "'");
  		}
  		return participant;
  	}
docs: Returns a participant in a session
  
  @param sessionId            identifier of the session
  @param participantPrivateId private identifier of the participant
  @return {@link Participant}
  @throws OpenViduException in case the session doesn't exist or the
  participant doesn't belong to it