public final PeriodList add(final PeriodList periods) {
        if (periods != null) {
            final PeriodList newList = new PeriodList();
            newList.addAll(this);
            newList.addAll(periods);
            return newList.normalise();
        }
        return this;
    }
public final PeriodList add ( final PeriodList periods ) { if ( periods != null ) { final PeriodList newList = new PeriodList ( ) ; newList . addAll ( this ) ; newList . addAll ( periods ) ; return newList . normalise ( ) ; } return this ; }
A convenience method that combines all the periods in the specified list to
this list. The result returned is a new PeriodList instance, except where
no periods are specified in the arguments. In such cases this instance is returned.

Normalisation is also performed automatically after all periods have been added.

@param periods a list of periods to add
@return a period list instance