按值排序HashMaps

Question

When I need to sort a HashMap by value, the advice seems to be to create the HashMap and then put the data into a TreeMap which is sorted by value.

For example: Sort a Map<Key, Value> by values (Java)

My question: why is it necessary to do this? Why not create a TreeMap(which is sorted by keys) and then sort it in place by value?

Answer 1

Because you can't reorder the entries of a TreeMap manually. TreeMap entries are always sorted on the keys.

I'm going to throw out Map that could be iterated in the order of values as another answer to "How to do it," though...specifically, a solution which doesn't return a map that chokes (by throwing exceptions) on queries to keys not in your original map.

Answer 2

If you know your values to be unique, you can use Guava's BiMap (bidirectional map) to store the data. Create a HashBiMap as you would your HashMap , then create a new TreeMap from its inverse:

new TreeMap<>(biMap.inverse());

That map will then be sorted by the values. Remember that what you're thinking of as "keys" and "values" will be swapped.

If your values are not unique, you can create a multimap of the inverse. A multimap is essentially a mapping from each key to one or more values. It's usually implemented by making a map from a key to a list. You don't have to do that though, because Google did it for you. Just create a multimap from your existing map, and ask Guava to invert it for you into a TreeMultimap , which, as you can guess, is a TreeMap that can hold multiple values per key.

Multimaps.invertFrom(Multimaps.forMap(myMap), new TreeMultimap<V, K>());

Multimap documentation is provided.

Answer 3

I have this very small code which is working fine:

public class SortMapByValues {
    public static void main(String[] args) {

        Map<Integer, String> myMap = new LinkedHashMap<Integer, String>();

        myMap.put(100, "hundread");
        myMap.put(500, "fivehundread");
        myMap.put(250, "twofifty");
        myMap.put(300, "threehundread");
        myMap.put(350, "threefifty");
        myMap.put(400, "fourhundread");

        myMap = sortMapByValues(myMap);

        for (Map.Entry<Integer, String> entry : myMap.entrySet()) {
            System.out.println(entry.getKey() + " " + entry.getValue());
        }

    }

    public static Map<Integer, String> sortMapByValues(
            Map<Integer, String> firstMap) {
        Map<String, Integer> SecondyMap = new TreeMap<String, Integer>();

        for (Map.Entry<Integer, String> entry : firstMap.entrySet()) {
            SecondyMap.put(entry.getValue(), entry.getKey());
        }
        firstMap.clear();
        for (Map.Entry<String, Integer> entry : SecondyMap.entrySet()) {
            firstMap.put(entry.getValue(), entry.getKey());
        }
        return firstMap;
    }

}

Output:

500 fivehundread  
400 fourhundread  
100 hundread  
350 threefifty  
300 threehundread  
250 twofifty 

Answer 4

I wrote the following one-liner using Java 8 Stream API to sort any given map by value:

List<Map.Entry<String, String>> sortedEntries = map.entrySet().stream()
  .sorted((o1, o2) -> o1.getValue().compareTo(o2.getValue())).collect(Collectors.toList());