Functional.java

// And finally a functional example of the find location with non-zero inventory for item example
public class Functional {
  
    // @Inject
    private ItemRepository itemRepository;
    // @Inject
    private InventoryLocationRepostitory inventoryLocationRepostitory;

    // Monadic approach, composed behaviors
    // "The monad represents computations with a sequential structure: a monad defines what it means to chain operations together."
    public Function<Optional<String>, Optional<InventoryLocation>> firstAvailableLocation = itemIdOpt ->
            itemIdOpt.flatMap(itemId ->
                itemRepository.findByItemId(itemId)
                    .flatMap(item ->
                        inventoryLocationRepostitory
                                .findByItemId(item)
                                .filter(inventoryLocation -> inventoryLocation.getInStock() > 0)
                                .findFirst()));
}
// a few less lines, no null checks, the cruft is basically gone. Didn't bother to unwrap the optiona, we used it!