Handle Errors

public class InventoryException extends Exception {}

1. public class InventoryException extends Exception {} — exactly Module 22's LibraryException pattern, reused for a different domain — a dedicated exception type lets inventory-specific failures be caught distinctly from generic Apex exceptions.

public void shipOrder(Id warehouseStockId, Decimal quantityShipped) {
    Warehouse_Stock__c stock = [
        SELECT Id, Quantity_On_Hand__c FROM Warehouse_Stock__c WHERE Id = :warehouseStockId
    ];

    if (stock.Quantity_On_Hand__c < quantityShipped) {
        throw new InventoryException(
            'Cannot ship ' + quantityShipped + ' units — only ' + stock.Quantity_On_Hand__c + ' available.'
        );
    }

    stock.Quantity_On_Hand__c -= quantityShipped;
    update stock;
}

1. public void shipOrder(Id warehouseStockId, Decimal quantityShipped) { — the method.
2. Warehouse_Stock__c stock = [ — queries the current stock.
3. SELECT Id, Quantity_On_Hand__c FROM Warehouse_Stock__c WHERE Id = :warehouseStockId — the current value.
4. ]; — closes the query.
5. (blank line)
6. if (stock.Quantity_On_Hand__c < quantityShipped) { — this upgrades the earlier lesson's return false; into a throw.
7. throw new InventoryException( — with a specific, informative message.
8. 'Cannot ship ' + quantityShipped + ' units — only ' + stock.Quantity_On_Hand__c + ' available.' — a genuine business-rule violation now stops the operation loudly rather than just signaling failure through a return value.
9. ); — closes the throw.
10. } — closes the if.
11. (blank line)
12. stock.Quantity_On_Hand__c -= quantityShipped; — only reached if there was enough stock.
13. update stock; — saves the change.
14. } — closes the method.

public Warehouse_Stock__c getStockRecord(Id productId, String warehouseName) {
    List<Warehouse_Stock__c> matches = [
        SELECT Id, Quantity_On_Hand__c
        FROM Warehouse_Stock__c
        WHERE Product__c = :productId AND Warehouse_Name__c = :warehouseName
    ];

    if (matches.isEmpty()) {
        throw new InventoryException('No stock record exists for this product at this warehouse.');
    }

    return matches[0];
}

1. public Warehouse_Stock__c getStockRecord(Id productId, String warehouseName) { — the method.
2. List<Warehouse_Stock__c> matches = [ — queries by product and warehouse together.
3. SELECT Id, Quantity_On_Hand__c — the fields needed.
4. FROM Warehouse_Stock__c — the object.
5. WHERE Product__c = :productId AND Warehouse_Name__c = :warehouseName — the exact combination.
6. ]; — closes the query.
7. (blank line)
8. if (matches.isEmpty()) { — rather than assuming a Warehouse_Stock__c record always exists for any product/warehouse combination, checks matches.isEmpty() (Module 15) first.
9. throw new InventoryException('No stock record exists for this product at this warehouse.'); — receiving stock for a genuinely new warehouse should create a fresh record, not silently fail while looking for one that was never set up.
10. } — closes the if.
11. (blank line)
12. return matches[0]; — safe now, since the empty case was already handled.
13. } — closes the method.