Htykuut

Error Message :

java.lang.RuntimeException: Could not deserialize object. Can't convert object of type java.util.ArrayList to type com.example.model.Cart (found in field 'cart')

I had already referred to many similar questions and all were using Firebase Realtime database, whereas I am using Firebase Firestore.

The below classes are the classes I use for the database.

User.java :

public class User implements Serializable {



    //  Data

    private String name, uid, mail, address;

    private Long phone;

    private ArrayList<Order> orders;

    private Cart cart;



    public User() {

        orders = new ArrayList<>();

        cart = new Cart();

    }

    /*  Getters and Setters for all variables */

}

Cart.java :

public class Cart {



    //  Data

    private ArrayList<SingleOrderItem> cart;



    public Cart() {

        cart = new ArrayList<>();

    }



    /*  Getters and Setters for all variables */

}

SingleOrderItem.java :

public class SingleOrderItem {



    //  Data

    private Product product;

    private int quantity;

    private Long cost;



    /*  Getters and Setters for all variables */

}

Product.java :

public abstract class Product {



    //  Data

    private String id, name, picUrl;

    private Long price; 



    /*  Getters and Setters for all variables */

}

Adding data to Firebase Firestore :

//  Initialize Firebase variables

FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();

FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();

FirebaseFirestore firebaseFirestore = FirebaseFirestore.getInstance();



//  Fetch item details from activity

SingleOrderItem singleOrderItem = new SingleOrderItem();

singleOrderItem.setProduct(sweet);

singleOrderItem.setQuantity(1);

singleOrderItem.setCost(sweet.getPrice());



//  Get cart from user

Cart cart = user.getCart();

if (cart == null) {

    cart = new Cart();

}

ArrayList<SingleOrderItem> cartProducts = cart.getCart();

cartProducts.add(singleOrderItem);

cart.setCart(cartProducts);



//  Update cart to user

firebaseFirestore.collection("users").document(firebaseUser.getUid())

        .set(cart, SetOptions.merge());

Retrieving data from Firebase Firestore :

//  Initialize firebase variables

FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();

FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();

FirebaseFirestore firebaseFirestore = FirebaseFirestore.getInstance();



if (firebaseUser != null) {

    //  Fetch data from firestore

    firebaseFirestore.collection("users").document(firebaseUser.getUid())

                .addSnapshotListener((documentSnapshot, e) -> {



                    if (documentSnapshot != null) {



                        //  Save fetched data

                        User user = documentSnapshot.toObject(User.class);

                        userMutableLiveData.setValue(user);

                    }

                });

}

I get the above error message in this particular line :

User user = documentSnapshot.toObject(User.class);

While retrieving data from Firebase Firestore.

What I am trying to do :
I am creating an e-commerce app and when the user clicks on Add To Cart button on any product, I would like to add it to the user's cart in firebase firestore.

Note :
Retrieving data is implemented in ViewModel.