Sort the Object's value inside the Hashmap<String, Object>

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?

Solution 1

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;
}

Solution 2

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.

Solution 3

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;
}