Sort the Object's value inside the Hashmap

Question

I want to sort an Hashmap by the object's value. In this case, by country code.

  KEY              OBJECT                        
  String        LoyaltyCountry
                     - country name
                     - country code
                     - country loc

My code is as follows:

public static HashMap<String, LoyaltyCountry> loyaltyCountrySortMap(HashMap<String, LoyaltyCountry> loyaltyCountryMap) {

            if (loyaltyCountryMap != null) {
                List keys = new ArrayList();
                keys.addAll(loyaltyCountryMap.keySet());
                Collections.sort(keys, new Comparator<LoyaltyCountry>() {
                    public int compare(LoyaltyCountry o1, LoyaltyCountry o2) {
                        return o1.getCountryName().compareTo(o2.getCountryName());
                    }
                });
            }

            return loyaltyCountryMap;
        }

How can I do this correctly?

You're trying to sort the HashMap itself? Or are you trying sort the keys? Or values? Please clarify. Your code seems to imply you want to sort the keys. — lrAndroid, Apr 18 '12 at 16:53
possible duplicate of http://stackoverflow.com/q/10167278/1332690 — Charles Menguy, Apr 18 '12 at 16:55
What, exactly, are you trying to sort in it? HashMaps are structured to provide fast put/get, and changing the sorting methods in the HashMap defeats the purpose of using a map in the first place. — lrAndroid, Apr 18 '12 at 16:55
I am trying to sort the Country Name... And by sorting it, the whole hashmap will be sorted accordingly... — newbie, Apr 18 '12 at 16:57
I think you're filling a map with a list of sorted/unsorted values and then you want to sort it by value. I'll recommend to get the list already sorted and then use a `LinkedHashMap` — Luiggi Mendoza, Apr 18 '12 at 17:01

score 7 · Accepted Answer · answered Apr 18 '12 at 16:58

7

Here is a method that returns a Set containing the Map.Entry items sorted by their value.

public static <K, V extends Comparable<? super V>> SortedSet<Map.Entry<K, V>> entriesSortedByValues(Map<K, V> map) {
    SortedSet<Map.Entry<K, V>> sortedEntries = new TreeSet<Map.Entry<K, V>>(
            new Comparator<Map.Entry<K, V>>() {
                @Override
                public int compare(Map.Entry<K, V> e1, Map.Entry<K, V> e2) {
                    return e1.getValue().compareTo(e2.getValue());
                }
            });
    sortedEntries.addAll(map.entrySet());
    return sortedEntries;
}

answered Apr 18 '12 at 16:58

Sarel Botha

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I just happened to run into this recently so I just had to copy/paste code I already had :) – Sarel Botha Apr 18 '12 at 17:09
How can i change the SortedSet to HashMap? – newbie Apr 18 '12 at 17:52
You can probably extend HashMap and when entrySet() is called return the sorted version instead. – Sarel Botha Apr 19 '12 at 01:48

score 1 · Answer 2 · answered Apr 18 '12 at 16:53

Create a Comparator<Map.Entry<String,LoyaltyCountry>>, get the Map.entrySet(), put it into a List<Map.Entry<String,LoyaltyCountry>> and sort it.

Clarification: You say you want to sort the HashMap by values -- that isn't going to happen. A HashMap is an unsorted implementation of a Map. Moreover, the sorted implementations of Map all sort on keys, so even a TreeMap won't help you.

The closest you can come to it (short of making your own map implementation) is produce a list of the map entries, which is what I described how to do above.

score 1 · Answer 3 · answered Apr 18 '12 at 16:58

You can't sort HashMap.

Looks like you want to return an ordered list of map's values, sorted by country name.

For that change return type of your method to List, e.g

public static List<LoyaltyCountry> loyaltyCountrySortMap(
    HashMap<String, LoyaltyCountry> loyaltyCountryMap
  ) 
{

    if (loyaltyCountryMap != null) {
        List<LoyaltyCountry> values = new ArrayList();
        values.addAll(loyaltyCountryMap.values());
        Collections.sort(values, new Comparator<LoyaltyCountry>() {
            public int compare(LoyaltyCountry o1, LoyaltyCountry o2) {
                return o1.getCountryName().compareTo(o2.getCountryName());
            }
        });

        return values;
    }

    return null;
}

but how can i relate it with the key? – newbie Apr 18 '12 at 17:01 — newbie, Apr 18 '12 at 17:01
@newbie. Follow the advice of Sarel Botha. – Alexander Pogrebnyak Apr 18 '12 at 17:05 — Alexander Pogrebnyak, Apr 18 '12 at 17:05

Sort the Object's value inside the Hashmap

3 Answers3

Linked