Java 8 Stream API: Unexpected Behavior When Grouping by Multiple Fields
I'm getting frustrated with I'm working on a personal project and I'm encountering an unexpected behavior while using Java 8's Stream API to group a list of custom objects by multiple fields... I'm trying to group a list of `Order` objects by both `customerId` and `orderDate`, but the results aren't what I anticipated. Here's my `Order` class for reference: ```java public class Order { private String customerId; private LocalDate orderDate; private double amount; // Constructor, getters, and setters } ``` I have a list of `Order` objects that I want to group as follows: ```java List<Order> orders = Arrays.asList( new Order("C001", LocalDate.of(2023, 10, 1), 100.0), new Order("C001", LocalDate.of(2023, 10, 1), 150.0), new Order("C002", LocalDate.of(2023, 10, 2), 200.0), new Order("C001", LocalDate.of(2023, 10, 3), 300.0) ); ``` My grouping logic looks like this: ```java Map<String, Map<LocalDate, List<Order>>> groupedOrders = orders.stream() .collect(Collectors.groupingBy(Order::getCustomerId, Collectors.groupingBy(Order::getOrderDate))); ``` However, when I try to access the grouped results, I get some unexpected outputs. For example, when I retrieve the orders for `customerId C001`, I only see one entry for the date `2023-10-01`, but I expected to see two entries, one for each order amount of 100.0 and 150.0. Here's how I'm accessing the data: ```java Map<LocalDate, List<Order>> c001Orders = groupedOrders.get("C001"); System.out.println(c001Orders); ``` The output is: ``` {2023-10-01=[Order(customerId=C001, orderDate=2023-10-01, amount=100.0)]} ``` I believe the grouping by `customerId` and `orderDate` is correct, but I must be missing something in how the `Order` objects are being represented. Is it possible that the way `Order` implements the `equals` and `hashCode` methods is affecting the grouping? What should I implement in these methods to ensure that orders on the same date with different amounts are treated as separate entries? Any help would be greatly appreciated! This is part of a larger API I'm building. Is there a better approach? I'm developing on CentOS with Java.