Sort List in place by a variable deep down in the structure in a complex Object

SG Tech Edge :

I have a complex "Order" object parsed from json. Somewhere deep in the structure i have the orderDate. I can extract it only as a STRING! And now I try to sort a List of Orders in place.

I came with the Idea of creating a new List, where the array inside consist two elements, first the Order Object itself, second the Date parsed to A Date object. eg. new Object[]{order, new Date(order.getOrderDate())}. Then sort by second element and then parse back to a List and return. But this creates two new Lists and is not in place.

The other idea is to create a custom Comparator that sorts like this

    orders.sort(new Comparator<Order>() {
        @Override
        public int compare(Order o1, Order o2) {
            return new Date(o1.getOrderDate()).compareTo(new Date(o2.getOrderDate()));
        }
    });

But the second variant will create a lot of new Date Objects. Worst case a lot times for every entry.

Is there a more beautiful way around it?

oleg.cherednik :

It's possible to create Date instance for unique date - using Map.

final Comparator<Order> sortByDateAsc = new Comparator<Order>() {
    private final Map<String, Date> map = new HashMap<>();

    @Override
    public int compare(Order o1, Order o2) {
        Date d1 = map.computeIfAbsent(o1.getOrderDate(), Date::new);
        Date d2 = map.computeIfAbsent(o2.getOrderDate(), Date::new);
        return d1.compareTo(d2);
    }
};

orders.sort(sortByDateAsc);