Whats the most elegant way to add two numbers that are Optional

Question

I need to perform an add operation on two big decimals that are wrapped optionals:

Optional ordersTotal;
Optional newOrder;
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Answer 1

Optional and Stream here do not fit together elegantly.

The best in java 8 is:

ordersTotaI = !ordersTotaI.isPresent() ? newOrder
         : !newOrder.isPresent() ? ordersTotaI
         : Optional.of(ordersTotaI.get().add(newOrder.get()));

However the good news is, that java 9 will add some nice (and also ugly) functionality to Optional.

Answer 2

Considering that you want to reassign to ordersTotal, you'll notice that ordersTotal only changes if newOrder is present.

You can thus start with that check, and write it as:

if (newOrder.isPresent()) {
    ordersTotal = newOrder.map(ordersTotal.orElse(ZERO)::add);
}

_{(This could be considered as a simplification of Stuart Marks' second solution. This is also a case where the method reference cannot be converted back to a lambda, due to ordersTotal not being effectively final)}

If you start by this check, there is also another possible “clever” approach:

if (newOrder.isPresent()) {
    ordersTotal = ordersTotal.map(o -> newOrder.map(o::add)).orElse(newOrder);
}

where the intermediate map() returns an Optional<Optional<BigDecimal>> whose inner Optional cannot be empty. I wouldn't consider it a good solution due to its bad readability, and I would thus recommend the first option or one of Stuart's solutions.