public void removeProduct(Product product, int quantity) {
   InventoryItem inventoryItem = inventory.get(product.getId());
   inventoryItem.deplete(quantity);
 }
 @ReadOnly
 public boolean isProductAvailable(Product product, int quantity) {
   InventoryItem inventoryItem = inventory.get(product.getId());
   return inventoryItem.getQuantityOnHand() >= quantity;
 }
 public void addProduct(Product product, int quantity) {
   InventoryItem inventoryItem = inventory.get(product.getId());
   inventoryItem.replenish(quantity);
 }