Process Stock Adjustments (DML)

public void receiveShipment(Id warehouseStockId, Decimal quantityReceived) {
    Warehouse_Stock__c stock = [
        SELECT Id, Quantity_On_Hand__c FROM Warehouse_Stock__c WHERE Id = :warehouseStockId
    ];
    stock.Quantity_On_Hand__c += quantityReceived;
    update stock;
}

1. public void receiveShipment(Id warehouseStockId, Decimal quantityReceived) { — the method.
2. Warehouse_Stock__c stock = [ — reading the record first, exactly Module 19's "Update" lesson pattern.
3. SELECT Id, Quantity_On_Hand__c FROM Warehouse_Stock__c WHERE Id = :warehouseStockId — queries the current value.
4. ]; — closes the query.
5. stock.Quantity_On_Hand__c += quantityReceived; — += is Module 4's compound assignment operator, doing real inventory work, adding to the current value.
6. update stock; — saves the change.
7. } — closes the method.

public Boolean 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) {
        System.debug('Not enough stock to ship this quantity.');
        return false;
    }

    stock.Quantity_On_Hand__c -= quantityShipped;
    update stock;
    return true;
}

1. public Boolean shipOrder(Id warehouseStockId, Decimal quantityShipped) { — returns whether the shipment succeeded.
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) { — checking before subtracting prevents stock from ever going negative — a warehouse can't ship out more than it actually has.
7. System.debug('Not enough stock to ship this quantity.'); — logs a clear reason.
8. return false; — rather than silently creating an impossible negative quantity.
9. } — closes the if.
10. (blank line)
11. stock.Quantity_On_Hand__c -= quantityShipped; — only reached if there was enough stock.
12. update stock; — saves the change.
13. return true; — reports success.
14. } — closes the method.

// DON'T DO THIS
stock.Quantity_On_Hand__c = quantityReceived; // overwrites, doesn't add

1. // DON'T DO THIS — the label says it all.
2. stock.Quantity_On_Hand__c = quantityReceived; — setting the field directly to the shipment quantity would erase whatever stock was already there instead of adding to it — the read-then-modify-then-write pattern (query the current value, change it, update) is what makes an adjustment actually additive rather than destructive.