Shopping Cart in Java

package shoppingcart.domain;

import kalix.javasdk.annotations.TypeName;

import java.util.List;
import java.util.stream.Collectors;

public record ShoppingCart(String cartId, List<LineItem> items, boolean checkedOut) {


  public record LineItem(String productId, String name, int quantity) {
    public LineItem increaseQuantity(int qty) {
      return new LineItem(productId, name, quantity + qty);
    }
  }

  public ShoppingCart addItem(LineItem item) {
    var itemToAdd =
      items.stream()
        .filter(it -> it.productId.equals(item.productId))
        .findFirst()
        .map(it -> it.increaseQuantity(item.quantity))
        .orElse(item);

    return removeItem(item.productId).addAsNew(itemToAdd);
  }

  public ShoppingCart removeItem(String productId) {
    if (hasItem(productId)) {
      var updatedItems =
        items.stream()
          .filter(it -> !it.productId.equals(productId))
          .collect(Collectors.toList());

      return new ShoppingCart(cartId, updatedItems, checkedOut);
    } else {
      return this;
    }
  }


  private ShoppingCart addAsNew(LineItem item) {
    items.add(item);
    return this;
  }

  private boolean hasItem(String productId) {
    return items().stream().anyMatch(it -> it.productId.equals(productId));
  }

  public ShoppingCart checkOut() {
    return new ShoppingCart(cartId, items, true);
  }

  public sealed interface Event {

    @TypeName("item-added")
    record ItemAdded(ShoppingCart.LineItem item) implements Event {
    }

    @TypeName("item-removed")
    record ItemRemoved(String productId) implements Event {
    }

    @TypeName("checked-out")
    record CheckedOut() implements Event {
    }
  }
}

import kalix.javasdk.EntityContext;
import kalix.javasdk.annotations.Id;
import kalix.javasdk.annotations.TypeId;
import kalix.javasdk.annotations.EventHandler;
import kalix.javasdk.eventsourcedentity.EventSourcedEntity;
import kalix.javasdk.eventsourcedentity.EventSourcedEntityContext;
import org.springframework.web.bind.annotation.*;
import shoppingcart.domain.ShoppingCart;
import shoppingcart.domain.ShoppingCart.Event.CheckedOut;
import shoppingcart.domain.ShoppingCart.Event.ItemAdded;
import shoppingcart.domain.ShoppingCart.Event.ItemRemoved;

import java.util.ArrayList;

@TypeId("shopping-cart") (1)
@Id("cartId") (2)
@RequestMapping("/cart/{cartId}") (3)
public class ShoppingCartEntity
  extends EventSourcedEntity<ShoppingCart, ShoppingCart.Event> { (4)

  final private String cartId;

  public ShoppingCartEntity(EventSourcedEntityContext entityContext) {
    this.cartId = entityContext.entityId();
  }

  @Override
  public ShoppingCart emptyState() { (5)
    return new ShoppingCart(cartId, new ArrayList<>(), false);
  }

  @PostMapping("/add") (6)
  public Effect<String> addItem(@RequestBody ShoppingCart.LineItem item) {
    if (currentState().checkedOut())
      return effects().error("Cart is already checked out.");

    if (item.quantity() <= 0) {
      return effects().error("Quantity for item " + item.productId() + " must be greater than zero.");
    }

    var event = new ItemAdded(item);

    return effects()
      .emitEvent(event) (7)
      .thenReply(newState -> "OK");
  }


  @PostMapping("/items/{productId}/remove") (6)
  public Effect<String> removeItem(@PathVariable String productId) {
    if (currentState().checkedOut())
      return effects().error("Cart is already checked out.");

    return effects()
      .emitEvent(new ItemRemoved(productId)) (7)
      .thenReply(newState -> "OK");
  }

  @PostMapping("/checkout") (6)
  public Effect<String> checkout() {
    if (currentState().checkedOut())
      return effects().error("Cart is already checked out.");

    return effects()
      .emitEvent(new CheckedOut()) (7)
      .thenReply(newState -> "OK");
  }

  @GetMapping() (6)
  public Effect<ShoppingCart> getCart() {
    return effects().reply(currentState());
  }

  @EventHandler (8)
  public ShoppingCart itemAdded(ItemAdded itemAdded) {
    return currentState().addItem(itemAdded.item());
  }

  @EventHandler (8)
  public ShoppingCart itemRemoved(ItemRemoved itemRemoved) {
    return currentState().removeItem(itemRemoved.productId());
  }

  @EventHandler (8)
  public ShoppingCart checkedOut(CheckedOut checkedOut) {
    return currentState().checkOut();
  }
}