How to sort a Map by Value's nested value (ArrayList) in Java?

Question

I have the following Map:

Map<Long, List<Address>> map = new HashMap<Long, List<Address>>();

which is filled with pairs of keys and values. For example: key = student id and

value = list of Address. In Address object I have country name(String). I want to sort the total map by the country name. I have tried many ways but not getting the Idea. Any ideas? Below is my tried code.

private static Map<Long, List<Address>> sortByValue(Map<Long, List<Address>> unsortMap) {

    // Convert Map to List of Map
        List<Map.Entry<Long, List<Address>>> unSortedList =
                new ArrayList<Map.Entry<Long, List<Address>>>(unsortMap.entrySet());

    // sort the List
        Collections.sort(unSortedList, new Comparator<Map.Entry<Long, List<Address>>>() {
            public int compare(Map.Entry<Long, List<Address>> object1,
                               Map.Entry<Long, List<Address>> object2) {
            // sort by country name
                return ???;
            }
        });

     // Loop the sorted list and put it into a new insertion order Map LinkedHashMap
        Map<Long, List<Address>> sortedMap = new LinkedHashMap<Long, List<Address>>();

            for (Map.Entry<Long, List<Address>> entry : unSortedList) {
            sortedMap.put(entry.getKey(), entry.getValue());

        }
    return sortedMap;
    }

Which one out of the list do you want to compare? And what's stopping you? — shmosel
– shmosel, Commented May 17, 2017 at 7:21
I want sort the total map by countryName.But key should be id — P S M
– P S M, Commented May 17, 2017 at 7:22
I think little modify in domain object it will become simple.Create student object has id and address and sort that list and then populate in a map.Or treeMap you can pass your customize Comparator. — gati sahu
– gati sahu, Commented May 17, 2017 at 7:22
Why do you have a list of addresses? What if they contain different countries for a single student? — shmosel
– shmosel, Commented May 17, 2017 at 7:25

Darshan Mehta · Accepted Answer · 2017-05-17 07:32:31Z

You can create a temporary TreeMap inside the method and store the reverse mappings (i.e. country -> keys) into it. Once done, you can iterate over it and fill the values in the result, e.g.:

public static Map<Long, List<Address>> sort(Map<Long, List<Address>> map){

    //Create temporary map, sorted by countries
    Map<String, List<Long>> countryMap = new TreeMap<>();

    map.entrySet().stream()
    .forEach(e -> {
        e.getValue()
        .stream()
        .map(a -> a.country)
        .forEach(c -> countryMap.computeIfAbsent(c, k -> new ArrayList<Long>()).add(e.getKey()));
    });

    //Iterate over treemap and populate the values in result
    Map<Long, List<Address>> sortedMap = new LinkedHashMap<>();
    countryMap.entrySet()
    .stream()
    .flatMap(e -> e.getValue().stream())
    .forEach(k -> sortedMap.put(k, map.get(k)));

    return sortedMap;
}

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