Method merge

base/src/main/java/net/time4j/Duration.java:3197–3291

(
    	Duration<U> duration,
    	TimeSpan<? extends U> timespan
    )

    }

    private static <U extends IsoUnit> Duration<U> merge(
    	Duration<U> duration,
    	TimeSpan<? extends U> timespan
    ) {

        if (duration.isEmpty()) {
            if (isEmpty(timespan)) {
                return duration;
            } else if (timespan instanceof Duration) {
                return cast(timespan);
            }
        }

        Map<U, Long> map = new HashMap<>();

        for (int i = 0, n = duration.count(); i < n; i++) {
            Item<U> item = duration.getTotalLength().get(i);
            map.put(
                item.getUnit(),
                Long.valueOf(
                    MathUtils.safeMultiply(
                        item.getAmount(),
                        (duration.isNegative() ? -1 : 1)
                    )
                )
            );
        }

        boolean tsign = timespan.isNegative();

        for (int i = 0, n = timespan.getTotalLength().size(); i < n; i++) {
            TimeSpan.Item<? extends U> e = timespan.getTotalLength().get(i);
            U unit = e.getUnit();
            long amount = e.getAmount();

            // Millis und Micros ersetzen
            Item<U> item = replaceFraction(amount, unit);

            if (item != null) {
                amount = item.getAmount();
                unit = item.getUnit();
            }

            // Items aktualisieren
            if (map.containsKey(unit)) {
                map.put(
                    unit,
                    Long.valueOf(
                        MathUtils.safeAdd(
                            map.get(unit).longValue(),
                            MathUtils.safeMultiply(amount, (tsign ? -1 : 1))
                        )
                    )
                );
            } else {
                map.put(
                    unit,
                    MathUtils.safeMultiply(amount, (tsign ? -1 : 1))