Review and Refactor

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

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

Both methods query Warehouse_Stock__c by Id with the exact same field list — the same "fetch this one stock record" shape, written out twice.

private Warehouse_Stock__c getStockById(Id warehouseStockId) {
    return [SELECT Id, Quantity_On_Hand__c FROM Warehouse_Stock__c WHERE Id = :warehouseStockId];
}

public void receiveShipment(Id warehouseStockId, Decimal quantityReceived) {
    Warehouse_Stock__c stock = getStockById(warehouseStockId);
    stock.Quantity_On_Hand__c += quantityReceived;
    update stock;
}

public void shipOrder(Id warehouseStockId, Decimal quantityShipped) {
    Warehouse_Stock__c stock = getStockById(warehouseStockId);
    if (stock.Quantity_On_Hand__c < quantityShipped) {
        throw new InventoryException('Not enough stock to ship this quantity.');
    }
    stock.Quantity_On_Hand__c -= quantityShipped;
    update stock;
}

getStockById is private, exactly like the extracted helpers in every previous project module — a change to which fields are fetched now happens in one place, not two (or more, as the class grows further).

This closes out the third straight project module ending in the same "extract the repeated fetch-by-Id logic into a private helper" refactor — Student lookups in Module 12, Employee lookups in Module 17, Book__c lookups in Module 22, and now Warehouse_Stock__c lookups here. Recognizing this as a repeating pattern, not a coincidence, is itself a useful thing to notice: "fetch one record by Id, used by several methods" is common enough to reach for a shared private helper almost by default in real Apex classes.