CODEWITHSUNDEEP
javasortingjava-8java-stream
Md Johirul Islam
Md Johirul Islam

Reputation: 5162

How can I sort a Map based upon on the size of its Collection values?

I have a HashMap like this:

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

map.put("USA", Arrays.asList("CA","IA","IL"));
map.put("India", Arrays.asList("MUM","CAL"));
map.put("Canada", Arrays.asList("TOR"));

I want to sort the map depending on the size of the list value, in ascending order. How can I do that?

In this case, I would like the keys to be ordered Canada, India, USA.

Upvotes: 13

Views: 14050

Answers (4)

DD.
DD.

Reputation: 21981

public static <K,V extends Collection> Map<K,V> sortMap(Map<K,V> map){
        return map.entrySet().stream()
                .sorted((e1, e2) -> Integer.compare(e2.getValue().size(), e1.getValue().size()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
    }

Upvotes: 2

Uyi Oriaghan
Uyi Oriaghan

Reputation: 41

You can simply map the hash to list of Pair() and then sort the list

val map: MutableMap<String, List<String>> = HashMap()

map["USA"] = listOf("CA", "IA", "IL")
map["India"] = listOf("MUM", "CAL")
map["Canada"] = listOf("TOR")

val sortedPairs = map.map { Pair(it.key, it.value) }.sortedBy { it.second.size }.toMap()

println("sorted List: $sortedPairs")

Upvotes: 1

Misha
Misha

Reputation: 28133

HashMap does not have a guaranteed iteration order so you will need to collect to a LinkedHashMap in order for the sorting to be meaningful.

import static java.util.Comparator.comparingInt;
import static java.util.stream.Collectors.toMap;

Map<String, List<String>> sorted = map.entrySet().stream()
    .sorted(comparingInt(e -> e.getValue().size()))
    .collect(toMap(
        Map.Entry::getKey,
        Map.Entry::getValue,
        (a, b) -> { throw new AssertionError(); },
        LinkedHashMap::new
    ));

The AssertionError is thrown because a combiner function is only used on parallel streams, which we are not using.

You can also use comparingByValue if you find it more readable:

import static java.util.Map.Entry.comparingByValue;

Map<String, List<String>> sorted = map.entrySet().stream()
    .sorted(comparingByValue(comparingInt(List::size)))
    // ... as above

Upvotes: 24

bhspencer
bhspencer

Reputation: 13560

You have two problems.

  1. Map doesn't support sorting.

  2. SortedMap doesn't support sorting on values only sorting on keys.

As a result of this using a Map or SortedMap isn't going to help you. What you need to do is iterate over you map and put each Entry<String, ArrayList<String>> into a collection such as a List and then sort the list with a custom compare. See this example TreeMap sort by value or this example Sorting LinkedHashMap

Upvotes: 1

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