Best way to categorize a list

Mehdi :

I have an Order object

public class Order{
  private Date orderDate;
  //other fields + getters & setters
}

And a List of Orders (List< Orders>). Now I want to group this list of orders based on similar orderDate (similar in here means, same year, month and day) and produce a map like this:

Map<Date, List<Order>> orderMap = new HashMap<>();

How to do this in Java 8+ ?

Take note we might have similar date but different timestamp which means we want to based it on "year, month, day"

Scorix :

I don't really support the point of doing everything with streams, but you can do it with a hard convertion way.

 List<Order> orders = ... your list ...
 Map<Date, List<Order>> orderMap = orders.stream().collect(Collectors.groupingBy(order->{
      //Converting from Date to Instant
      Instant dateInstant = order.getOrderDate().toInstant();
      //Converting Instant to LocalDateTime and setting it to the start of the day
      LocalDateTime dateTime = dateInstant.atZone(ZoneId.systemDefault()).toLocalDate().atStartOfDay();
      //Converting it back to an instant
      Instant startOfDayInstant = dateTime.atZone(ZoneId.systemDefault()).toInstant();
      //Returning the util date
      return Date.from(startOfDayInstant);
    }));

This is under the circumstance that you need util date. Otherwise I support the answer from michalk