enhance shopping-cart logic

src/main/java/edu/neu/neumall/repository/ShoppingCartRepository.java

@@ -5,6 +5,7 @@ import org.springframework.data.jpa.repository.JpaRepository;
 import org.springframework.stereotype.Repository;
 
 import java.util.List;
+import java.util.Optional;
 
 @Repository
 public interface ShoppingCartRepository extends JpaRepository<ShoppingCart, Long> {
@@ -12,4 +13,6 @@ public interface ShoppingCartRepository extends JpaRepository<ShoppingCart, Long
     public List<ShoppingCart> findAllByOwner_ID(long id);
 
     public List<ShoppingCart> removeByOwner_ID(long id);
+
+    public Optional<ShoppingCart> findByOwnerIDAndProductID(long ownerID, long productID);
 }

src/main/java/edu/neu/neumall/service/ShoppingCartService.java

@@ -26,6 +26,15 @@ public class ShoppingCartService {
         if (productOptional.isEmpty()) {
             return false;
         }
+        // cart item already exists
+        var cartExist = shoppingCartRepository.findByOwnerIDAndProductID(user.getID(), productID);
+        if (cartExist.isPresent()) {
+            var cart = cartExist.get();
+            cart.setCount(cart.getCount() + 1);
+            shoppingCartRepository.save(cart);
+            return true;
+        }
+        // item not exist, create a new one
         var product = productOptional.get();
         ShoppingCart shoppingCart = new ShoppingCart();
         shoppingCart.setCount(count);