I’ve a list of Order which I am fetching from some third part api.

List<Order> orders = fromApi(orderId);

Here is the Order class:

class Order{
     Long orderId;
     Product product
}

Similarly I fetch products also for those orders. Here the key is orderId.

List<Order> orderIds = orders.stream().map(Order::orderId).collect(Collectors.toList());
Map<Long,Product> userProductMap = fromAnotherApi(orderIds);

Here is Product class:

class Product{
        String productDesc;
}

I want to extract a map with orderId as a key and Order as value(given below), but meanwhile also want to update orders with the product(from userProductMap) in same stream iteration.

 Map<Long,Order> userOrderMap = orders.stream().collect(Collectors.toMap(order -> order.id, Function.identity()));

I want to provide some custom implementation instead of Function.identity() so that while putting Order as value in Map here, I take out product from the userProductMap and update in Order and use updated Order in Map as value.