How to sort hash map based on number of keys for a value using flatmap java8?

Question

This is a follow-up of How to get the count of keys for values in a hash map using lambda. I have a HashMap and I want to find the number of keys for each value

Answer 1

You could have the following:

Map<Integer, List<Integer>> sorted = 
    map.entrySet()
       .stream()
       .sorted(comparing(e -> ex.get(e.getKey()), reverseOrder()))
       .collect(toMap(
           Map.Entry::getKey,
           e -> e.getValue().stream().sorted(comparing(ex::get, reverseOrder())).collect(toList()),
           (v1, v2) -> { throw new IllegalStateException(); },
           LinkedHashMap::new
       ));

This creates a Stream of the entries of the map, sorts them in reverse order according the count of keys for that entry's key and finally collects that into a map where the new value is sorted in reverse order with regard to the count of each integer (ex::get). The collecting map is a LinkedHashMap to preserve encounter order.

Output:

{1=[2, 3, 0], 2=[4, 1, 0], 3=[4, 1, 5], 4=[2, 3, 5], 0=[1, 2], 5=[4, 3]}

Static imports used:

import static java.util.Comparator.comparing;
import static java.util.Comparator.reverseOrder;
import static java.util.stream.Collectors.toList;
import static java.util.stream.Collectors.toMap;