Map getter not pulling items by index properly-CodePudding

I have a Map of <String, CartItem> which is a custom class, and I am trying to return specific values from that Map, but every time I do return them in the format item[I], I get a "Null check operator used on a null value." Is there a different way I should be calling this?

Details

I have a map of _items with a public getter that returns those items:

class Cart with ChangeNotifier {
  Map<String, CartItem> _items = {};

  Map<String, CartItem> get items {
    return {..._items};
  }

Followed by a function that adds new items to the Map:

  void addItem(
    String productId,
    double price,
    String title,
  ) {
    if (_items.containsKey(productId)) {
      _items.update(
        productId,
        (existingCartItem) => CartItem(
          id: existingCartItem.id,
          title: existingCartItem.title,
          price: existingCartItem.price,
          quantity: (existingCartItem.quantity   1),
        ),
      );
    } else {
      _items.putIfAbsent(
        productId,
        () => CartItem(
          id: DateTime.now().toString(),
          title: title,
          price: price,
          quantity: 1,
        ),
      );
    }
    print(items.length);
    print(items[0]);
    print(items);
    notifyListeners();
  }

When I call addItem, this is the output I get from those three print statements - it appears calling as items[0] returns null, even though the length and the full items list print properly:

Performing hot restart...                                               
Restarted application in 1,483ms.
flutter: 1
flutter: null
flutter: {p1: Instance of 'CartItem'}

Then whenever I call that Cart Provider looking for a specific item in the Map:

            child: ListView.builder(
                itemCount: cart.itemCount,
                itemBuilder: (ctx, i) => CartItem(cart.items[i]!)),
          )

I get a "Null check operator used on a null value" error. Why is this way of calling the items in the map returning null?

CodePudding user response：

You seem to assume that items[0] works like an array and returns the first element. It does not. A Map has an indexer by key. It returns the item, with the given key. That is the whole point of a map, being able to look up items by key. You set your productId as a key, so to find it you would need to use items[productId].

Some thoughts: does your cart items needs to be a map? If all you want is enumerating through it, it would work if you made it a simple list instead. Then the numeric indexer would work again.

If it needs to be a map, you can use items.values to iterate through all the values. But please note that a map is not ordered. Again, if you need an ordered list, it would be better to use a list in the first place.