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

Question

I have a HashMap like this:

Map> map = new HashMap<>();

map.put(\"USA\", Arrays.asList(\"CA\",\"IA\",\"I         


        

Answer 1

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