java - How to change the sequence of entries in TreeMap based on the 'Key'

Question

I have a Map<String,List<UserVO>> studentsByCountry = new TreeMap<String,List<UserVO>>();

It displays like :

key=CountryA , value = List<UserVO>
key=CountryB , value = List<UserVO>
key=CountryC , value = List<UserVO>

Can I reshuffle this Map so that I display according to the displayOrder of the countries. Display order is countryC=1, countryB=2 & countryA=3. I want to display like

key=CountryC , value = List<UserVO>
key=CountryB , value = List<UserVO>
key=CountryA , value = List<UserVO>

One option is to append the sortOrderNumber to the key. For example 1_CountryC, 2_CountryB & 3_CountryA

Can I do it via conventional Java?

Answer 1

You should use new TreeMap<String, List<UserVO>>();

Map<String, List<UserVO>> sortedMap = new TreeMap<>(Collections.reverseOrder());
sortedMap.putAll(myMap);

Inspired from: Sorting Descending order: Java Map

If you want an unordered map you should use HashMap

TreeMap in Java is one of the implementations of the Map interface. How it differs from the other implementation HashMap is that unlike HashMap which is unordered, TreeMap is Sorted.