Navigate in object with a lot of nulls

Question

I'm trying to implement a good design in my code. There is a Entity purchaseOrder and I need to navigate inside that object that has a list of stockOrder and each StockOrder has a list of StockOrderItem. There is no guarantee that a stockOrder or stockOrderItem exists so I needed to check a lot of nulls. Is there a elegant way to do that?

        PurchaseOrder purchaseOrder = purchaseOrderRepository.
            findByIdentifierAndBrandId(po.getPurchaseOrderIdentifier(), po.getBrandId());
        List<StockOrder> stockOrders = Optional.ofNullable(
            purchaseOrder.getStockOrders()).orElse(Collections.emptyList());
        for (StockOrder stockOrder : stockOrders) {
            for (StockOrderItem stockOrderItem : Optional.
                ofNullable(stockOrder.getStockOrderItems()).orElse(Collections.emptyList())) {
                InventoryDataLoad inventoryDataload = new InventoryDataLoad();
                inventoryDataload.setSku(stockOrderItem.getSku());
                inventoryDataLoadList.add(inventoryDataload);
            }
        }

Answer 1

It's not a good practice to return nulls for collection fields. Considering you have access to these classes, return an empty list from the get methods instead:

StockOrder.java

public List<StockOrderItem> getStockOrderItems() {
    return this.getStockOrderItems != null this.getStockOrderItems ? Collections.emptyList();
}

Or better, as @Ridiculon pointed out in the comments, initialize the collection fields with a new ArrayList() directly.

StockOrder.java

private final List<StockOrderItem> stockOrderItems;

public StockOrder() {
   this.stockOrderItems = new ArrayList();
}

With that in place, you can use a more elegant functional approach to populate your list, such as:

List<InventoryDataLoad> inventoryDataLoadList = purchaseOrderRepository
        .findByIdentifierAndBrandId(po.getPurchaseOrderIdentifier(), po.getBrandId())
        .getStockOrders()
        .stream()
        .map(StockOrder::getStockOrderItems)
        .flatMap(Collection::stream)
        .map(item -> {
            InventoryDataLoad inventoryDataload = new InventoryDataLoad();
            inventoryDataload.setSku(item.getSku());
            return inventoryDataload;
        })
        .collect(Collectors.toList());

If you don't have access to the classes to change the get method, you can create your own get methods:

private List<StockOrderItem> getStockOrderItems(StockOrder stockOrder) {
    return stockOrder.getStockOrderItems() != null ? stockOrder.getStockOrderItems() : Collections.emptyList();
}

private List<StockOrder> getStockOrders(PurchaseOrderRepository purchaseOrderRepository, PurchaseOrder po) {
    List<StockOrder> stockOrders = purchaseOrderRepository
            .findByIdentifierAndBrandId(po.getPurchaseOrderIdentifier(), po.getBrandId())
            .getStockOrders();
    return stockOrders != null ? stockOrders : Collections.emptyList();
}

And then do the functional approach using those:

List<InventoryDataLoad> inventoryDataLoadList = getStockOrders(purchaseOrderRepository, po)
        .stream()
        .map(this::getStockOrderItems)
        .flatMap(Collection::stream)
        .map(item -> {
            InventoryDataLoad inventoryDataload = new InventoryDataLoad();
            inventoryDataload.setSku(item.getSku());
            return inventoryDataload;
        })
        .collect(Collectors.toList());

EDIT: In both cases you can also improve it a little further by creating a separate method for creating the InventoryDataLoad object, such as:

private InventoryDataLoad createInventoryDataLoad(StockOrderItem item) {
    InventoryDataLoad inventoryDataload = new InventoryDataLoad();
                inventoryDataload.setSku(item.getSku());
                return inventoryDataload;
}

And then:

List<InventoryDataLoad> inventoryDataLoadList = getStockOrders(purchaseOrderRepository, po)
        .stream()
        .map(this::getStockOrderItems)
        .flatMap(Collection::stream)
        .map(this::createInventoryDataLoad)
        .collect(Collectors.toList());