How can I get an ArrayList from an ArrayList sorted by id

I have mList2 with values. There are values with the same id. How can I get a List or ArrayList in which objects with the same id are grouped and add it to mLists?

mList = ArrayList<DataPost>()
            mLists = ArrayList<ArrayList<DataPost>>()
            var f= mList2[0].ids_post
            for ((b,o) in mList2.withIndex()){
                if (o.ids_post.equals(f)){
                    mList.add(o)
                }else{
                    mLists.addAll(listOf(mList))
                    mList.clear()
                    mList.add(o)
                    f = o.ids_post
                }
            }

The program below should meet your requirements.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

class Main {
    public static void main(String args[]) {
                // Sample items
        String arrId[] = { "India", "England", "France", "China", "India", "France", "India", "England", "Germany" };
        List<String> mList2 = Arrays.asList(arrId);

                // Output list
        List<List<String>> output = new ArrayList<List<String>>();

                // List to track items that have been already grouped
        List<String> itemsAlreadyGrouped = new ArrayList<String>();

                // Loop through all items in the sample list
        for (int i = 0; i < mList2.size(); i++) {
                        // Temporary list to hold identical items
            List<String> groupList = new ArrayList<String>();
                        
                        // Reset the flag for each iteration
            boolean groupCandidateFound = false;
                        
                        // If the item at the current index is not found in the 
                        // list to track items that have been already grouped
            if (!itemsAlreadyGrouped.contains(mList2.get(i))) {
                for (int j = 0; j < mList2.size(); j++) {
                                        // If the item at the current index of 
                                        // the inner loop is the same as the 
                                        // item at the current index of the 
                                        // outer loop, add it to the temporary
                                        // list and mark the flag true                                       
                    if (mList2.get(i).equals(mList2.get(j))) {
                        groupList.add(mList2.get(i));
                        groupCandidateFound = true;
                    }
                }
                                // If the flag is true
                if (groupCandidateFound) {
                    itemsAlreadyGrouped.add(mList2.get(i));
                }
            }
            if (groupList.size() > 0) {             
                output.add(groupList);
            }
        }
        
        //Let's test the logic
        for (List<String> group : output) {
            System.out.println(group);
        }
    }
}

Output for the given sample of items:

[India, India, India]
[England, England]
[France, France]
[China]
[Germany]