Java8 to sort a Map value within Map

Question

I have a map in this format: Map<String, Map<Integer, Long>> and content like

{BCD={321=2}, ABC={123=1}, DEF={798=3}, CDE={564=1, 456=1}, GHI={908=2}}`

Is it possible sort the Long value reverse, then String and Integer?

It's not yet clear, what you are up to: You provide a datastructure which should be sorted. First question: what should be sorted? Please provide an example of the expected output. What entries should be eventually in the sorted array/list? Maybe you code an object which contains the named contents of your map to get a better understanding on your problem. — SirFartALot
– SirFartALot, Commented Jun 27, 2019 at 12:29

SirFartALot · Accepted Answer · 2019-06-27 12:43:26Z

1

No. You cannot sort contents of a Map.

Sorting is only possible on things, which retain a sorting, like List, TreeMap or TreeSet.

If you want the Map contents to be sorted, just implement a Comparator (e.g. Comparator<Map.Entry<String, Map<Integer, Long>>>) which is capable of returning an integer representing the order of two entries and feed all contents of your Map into a List<Map.Entry<String, Map<Integer, Long>>>, which can then be sorted.

   private static final Comparator<Map.Entry<String, Map<Integer, Long>>> COMPI = new Comparator<>() {
          @Override
          public int compare(Map.Entry<String, Map<Integer, Long>> obj1,
               Map.Entry<String, Map<Integer, Long>>(obj2)) {
                   ... return 0, if obj1 equal to obj2
                   ... return 1, if obj1 lower than obj2
                   ... return -1, if obj1 greater than obj2
          }

          public static List<Map.Entry<String, Map<Integer, Long>>> sortMyMap(Map<String, Map<Integer, Long>> myMap) {
                List<Map.Entry<String, Map<Integer, Long>>> l = new java.util.ArrayList<>(myMap.entrySet());
                Collections.sort(l, COMPI);
                return l;
           }
}

The most difficult part would be to implement the comparator correctly...

edited Jun 27, 2019 at 12:43

answered Jun 27, 2019 at 12:00

SirFartALot

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3 Comments

rsp Over a year ago

Or a TreeMap maybe?

Zabuzard Over a year ago

Just as a note, there is the general interface SortedMap which sums up suchs maps, mainly consisting of TreeMap. Theres also LinkedHashMap which retains insertion order (but not general sorting).

SirFartALot Over a year ago

Well: ... and many other datastructures like Arrays, LinkedLists and other things. But especially a java.util.Map is not sortable per se, but only by transforming it somehow...

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