How to sort this HashMap while flattening/grouping it, using streams?

ab11 :

I asked a question yesterday flattening/grouping a HashMap. I got a great response that suggested I do this.

return new ArrayList<>(map.entrySet().stream()
    .collect(Collectors.groupingBy(
        Map.Entry::getValue,
        Collectors.mapping(Map.Entry::getKey, Collectors.joining(","))))
    .values())

However, I would like my list of comma separated characters to be sorted alphabetically. The comments suggested I try using TreeMap::new as an argument to groupingBy(), but when I try this my list of characters remains unsorted:

return new ArrayList<>(map.entrySet().stream()
    .collect(Collectors.groupingBy(
        Map.Entry::getValue,
        TreeMap::new,
        Collectors.mapping(Map.Entry::getKey, Collectors.joining(","))))
    .values())

Below is my original question with a link to the post.

Given mapping of letters to numbers, I would like to return a list of Strings, where each String is a comma delimited list of the letters grouped by their associated number.

For this map

    Map<String, Integer> map = new HashMap<String, Integer>();
    map.put("A", 1);
    map.put("B", 2);
    map.put("C", 4);
    map.put("D", 1);
    map.put("E", 1);
    map.put("F", 2);

I would like to return a List containing:

"A,D,E" "B,F", "C"

Any suggestions how this can be accomplished using the 1.8 streaming functions?

How to flatten and group this HashMap, using streams?

Leo Aso :

Turn the map to a TreeMap before performing the operation, and the keys will be in order in the output.

//                         * here
return new ArrayList<>(new TreeMap(map).entrySet().stream()
.collect(Collectors.groupingBy(
    Map.Entry::getValue,
    TreeMap::new,
    Collectors.mapping(Map.Entry::getKey, Collectors.joining(","))))
.values())