code: Interval getInterval(int aId) {
  		Interval interval = intervalsById.get(aId);
  		if (interval == null) {
  			throw new UnknownIntervalException(aId);
  		}
  		return interval;
  	}
docs: This method retrieves an Interval with the given id.
  
  @param aId the Interval id
  @return the found Interval
  @throws UnknownIntervalException if no Interval with the given id could be found