Sort a Hashmap by Value in Java 8

Question

I've got a HashMap that I need sorting by value and I'm trying to keep it concise, so I'm using Java 8. However various methods aren't working and I'm unsure why. I've tried this:

followLikeCount.values()
  .stream()
  .sorted(Map.Entry.comparingByValue())
  .collect(Collectors.toList());

Which throws this compile time exception:

Main.java:65: error: no suitable method found for sorted(Comparator<Entry<Object,V#1>>)
  .sorted(Map.Entry.comparingByValue())

I can't see why there is a mismatch from observation. I've also tried using the comparator:

Comparator<Map.Entry<Integer, Integer>> byValue =
          Map.Entry.<Integer, Integer>comparingByValue();

Which yields a similar error. Please could you advise why the comparators aren't valid?

Does `values()` return the `Entry` with key/value pairs, or just the values? — markspace, Sep 16 '17 at 16:31
Ah true, just the values, but I can't turn the HashMap followLikeCount into a stream directly. Is a different method altogether required? — Bryn, Sep 16 '17 at 16:34
Ah, perfect. So the comparator compile-time error was due to expecting a comparator for just the value part. Thank you! — Bryn, Sep 16 '17 at 16:37

score 7 · Accepted Answer · answered Sep 16 '17 at 16:36

You try to use a Comparator<Map.Entry<Integer, Integer>> on List<Integer> returned by values()

followLikeCount.values()
   .stream()
   .sorted(Map.Entry.comparingByValue())
   .collect(Collectors.toList());

You need to use this comparator on a Set<Map.Entry<Integer, Integer>> which can be returned by entrySet() :

List<Map.Entry<Integer, Integer>> list = followLikeCount.entrySet()
                                                    .stream()
                                                    .sorted(Map.Entry.comparingByValue())
                                                    .collect(Collectors.toList());

If you want to get only the values, sorted, you can change the Comparator and get back a List<Integer> :

List<Integer> list = followLikeCount.values()
            .stream()
            .sorted(Comparator.naturalOrder())
            .collect(Collectors.toList());

If you only want to get the sorted (`Comparable`) values, you can even do with a comparator, `followLikeCount.values() .stream() .sorted() .collect(Collectors.toList());`. Or without Streams: `List list = new ArrayList<>(followLikeCount.values()); list.sort(null);` — Holger, Sep 18 '17 at 07:04

Sort a Hashmap by Value in Java 8

1 Answers1