Method plus

base/src/main/java/net/time4j/Duration.java:1302–1375

(
        long amount,
        U unit
    )

     * @see     #with(long, IsoUnit) with(long, U)
     */
    public Duration<U> plus(
        long amount,
        U unit
    ) {

        if (unit == null) {
            throw new NullPointerException("Missing chronological unit.");
        }

        long originalAmount = amount;
        U originalUnit = unit;
        boolean negatedValue = false;

        if (amount == 0) {
            return this;
        } else if (amount < 0) {
            amount = MathUtils.safeNegate(amount);
            negatedValue = true;
        }

        // Millis und Micros ersetzen
        List<Item<U>> temp = new ArrayList<>(this.getTotalLength());
        Item<U> item = replaceFraction(amount, unit);

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

        if (this.isEmpty()) {
            temp.add((item == null) ? Item.of(amount, unit) : item);
            return new Duration<>(temp, negatedValue);
        }

        // Items aktualisieren
        int index = this.getIndex(unit);
        boolean resultNegative = this.isNegative();

        if (index < 0) { // Einheit nicht vorhanden
            if (this.isNegative() == negatedValue) {
                temp.add(Item.of(amount, unit));
            } else { // mixed signs possible => last try
                return this.plus(Duration.of(originalAmount, originalUnit));
            }
        } else {
            long sum =
                MathUtils.safeAdd(
                    MathUtils.safeMultiply(
                        temp.get(index).getAmount(),
                        (this.isNegative() ? -1 : 1)
                    ),
                    MathUtils.safeMultiply(
                        amount,
                        (negatedValue ? -1 : 1)
                    )
                );

            if (sum == 0) {