Question

I have the following function which should return a map of dates to longs such that you iterate through the entries in reverse chronological order.

counts is a LinkedHashMap, so the entries are stored in the order they were inserted (which is chronological in this case):

public Map<Date, Long> getCountsChronological() {
   Map<Date, Long> chronologicalMap =
     new TreeMap<Date, Long>(Collections.reverseOrder());
   chronologicalMap.putAll(counts);
   return chronologicalMap;
}

This function does not work, though it seems in theory, that it should. When I iterate through the map that it returns, I am still getting the entries in chronological order.

Answer 1

Unable to reproduce – works fine for me:

import java.util.*;

public class Test {

    public static void main(String[] args) {
        Map<Date, Long> map = new HashMap<Date, Long>();

        map.put(new Date(100), 100L);
        map.put(new Date(400), 400L);
        map.put(new Date(300), 300L);
        map.put(new Date(200), 200L);

        map = reverseChronological(map);
        for (Map.Entry<Date, Long> entry : map.entrySet()) {
            System.out.println(entry.getKey().getTime());
        }
    }    

    public static Map<Date, Long> reverseChronological(Map<Date, Long> input) {
        Map<Date, Long> chronologicalMap =
            new TreeMap<Date, Long>(Collections.reverseOrder());
        chronologicalMap.putAll(input);
        return chronologicalMap;
    }
}

Are you sure you’re not iterating over the original map instead of the returned one?