Getting the List

Question

I need to sort every value of a Map with multiple objects in each value, but I am struggling to see how. My Map looks like this:

HashMap<String, List<Carrier>> carriers_list;

I already have a Custom Comparator set up to compare a List of my objects.

static class CarrierComparator implements Comparator<Carrier> {
        @Override
        public int compare(Carrier c1, Carrier c2) {
            return c1.get_name().compareTo(c2.get_name());
        }
    }

I can use this Comparator like so:

List<Carrier> carrierList = getAllCarriers();
Collections.sort(carrierList, new CustomComparators.CarrierComparator());

How can I get each List for every key in my Map?

Don't you rather mean "How can I get the `List` containing all `Carrier`s from my `Map>` ? — Jeremy Grand, May 31 '17 at 11:48
Then please edit the question, the current name is misleading.I'll answer it in a bit. — Jeremy Grand, May 31 '17 at 11:50
Sorry, seeing the answer you validated, it seems that I had misunderstood your issue and made you edit the question inappropriately. You actually simply wanted to iterate on the map's values, no ? — Jeremy Grand, May 31 '17 at 12:44

score 4 · Answer 1 · answered May 31 '17 at 11:57

To get all the values combined in one big list:

List<Carrier> values = map.values().stream()
                                   .flatMap(List::stream)
                                   .collect(Collectors.toList());

You can do the sorting in the same expression:

List<Carrier> values = map.values().stream()
                                   .flatMap(List::stream)
                                   .sorted(comparator)
                                   .collect(Collectors.toList());

Jeremy Grand · Answer 2 · 2017-05-31T12:00:24.103

1

What you want to do is around the likes of :

Map<Object,List<Object>> multimap = new HashMap<>();
    multimap.values().stream()                     // stream the values
    .flatMap(List::stream)                         // stream on the items in every list
    .sorted(CarrierComparator::compare)            // use your comparator
    .collect(Collectors.toList());                 // collect in a single List

edited May 31 '17 at 12:00

answered May 31 '17 at 11:53

Jeremy Grand

2,300
12
18

score 1 · Accepted Answer · answered May 31 '17 at 11:55

1

You can iterate over the map's values:

CustomComparators.CarrierComparator comparator = new CustomComparators.CarrierComparator();
for (List<Carrier> l : carriers_list.values())
    Collections.sort(l, comparator);

answered May 31 '17 at 11:55

Stefan Warminski

1,845
1
9
18

If that is the answer, everyone else misunderstood the question. – Jeremy Grand May 31 '17 at 12:42

score 0 · Answer 4 · answered May 31 '17 at 12:07

Here is a basic code which shows how to extract all Carrier objects into one list and then sort them.

Set<String> keys = map.keySet();
    List<Carrier> carriers = new ArrayList<Carrier>();
    for (String key : keys) {
        List<Carrier> temp = map.get(key);
        for (Carrier carrier : temp) {
            carriers.add(carrier);
        }
    }
    carriers.sort(new CarrierComparator());

"carriers" will contain sorted list of objects.

4 Answers4