public void testPlusFields() {
        Period test;
        test = Period.years(1).plusYears(1);
        assertEquals(new Period(2, 0, 0, 0, 0, 0, 0, 0, PeriodType.standard()), test);
        test = Period.months(1).plusMonths(1);
        assertEquals(new Period(0, 2, 0, 0, 0, 0, 0, 0, PeriodType.standard()), test);
        test = Period.weeks(1).plusWeeks(1);
        assertEquals(new Period(0, 0, 2, 0, 0, 0, 0, 0, PeriodType.standard()), test);
        test = Period.days(1).plusDays(1);
        assertEquals(new Period(0, 0, 0, 2, 0, 0, 0, 0, PeriodType.standard()), test);
        test = Period.hours(1).plusHours(1);
        assertEquals(new Period(0, 0, 0, 0, 2, 0, 0, 0, PeriodType.standard()), test);
        test = Period.minutes(1).plusMinutes(1);
        assertEquals(new Period(0, 0, 0, 0, 0, 2, 0, 0, PeriodType.standard()), test);
        test = Period.seconds(1).plusSeconds(1);
        assertEquals(new Period(0, 0, 0, 0, 0, 0, 2, 0, PeriodType.standard()), test);
        test = Period.millis(1).plusMillis(1);
        assertEquals(new Period(0, 0, 0, 0, 0, 0, 0, 2, PeriodType.standard()), test);
        
        test = new Period(0L, PeriodType.millis());
        try {
            test.plusYears(1);
            fail();
        } catch (UnsupportedOperationException ex) {}
    }