How can I sort a Map according to the parameters of its values? [duplicate]

Answer 1

Map.Entry.comparingByValue() will only work if the values in the map are Comparable, and List is not Comparable. The error from the compiler is telling you that.

You need to provide a custom Comparator which knows how to sort lists by their size, e.g.

Map.Entry.comparingByValue((list1, list2) -> list1.size() - list2.size())

Answer 2

This may be helpful to you.

I changed the type of result map to LinkedHashMap to respect insertion order.

public static void main(String[] args) {
    final Map<String, List<Integer>> map = new HashMap<>();
    map.put("k1", Arrays.asList(new Integer[]{1, 2, 3, 4, 5}));
    map.put("k2", Arrays.asList(new Integer[]{1, 2, 3, 4, 5, 6}));
    map.put("k3", Arrays.asList(new Integer[]{1, 2, 3}));
    System.out.println(getMapSortedByListSize(map));
}

public static <K, V> Map<K, List<V>> getMapSortedByListSize(final Map<K, List<V>> map) {
    return map.entrySet().stream()
            .sorted((e1, e2) -> e1.getValue().size() - e2.getValue().size())
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (a, b) -> a, LinkedHashMap::new));
}

Answer 3

Map.Entry.comparingByValue() cannot serve as the comparator.

This is a comparator for Stream<T> sorted(Comparator<? super T> comparator):

Stream<Entry<String, List<Object>>> sortedStream = index.entrySet().stream()
        .sorted(new Comparator<Map.Entry<String,List<Object>>>() {
    @Override
    public int compare(Entry<String, List<Object>> o1, Entry<String, List<Object>> o2) {
        return o1.getValue().size()-o2.getValue().size();
    }
});