How can I sort a HashMap by its values while preserving key-value pairs in Java?

Sorting Hashmap by Values

Problem:

We need to sort a HashMap based on the values it contains, and maintain the key-value pairings during sorting.

Solution:

Sorting a HashMap by values can be accomplished using a generic approach. The following steps outline the process:

1. Create a Linked List: Convert the HashMap entries into a LinkedList, ensuring that the insertion order is preserved.
2. Custom Comparator: Define a custom comparator to compare the values of the entries. It should consider both ascending and descending order options.
3. Sort the List: Use the custom comparator to sort the LinkedList based on the values. Since the entries are linked, the keys will also be sorted.
4. Convert to Sorted HashMap: Use the sorted LinkedList to construct a new LinkedHashMap, where the keys and values are associated as they were in the original HashMap.
5. Custom Version: A tailored version of the sorting method can be created, allowing for specific ascending or descending value ordering.

Example Implementation:

The following Java code implements the sorting algorithm:

import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

public class SortMapByValue {

    public static final boolean ASC = true;
    public static final boolean DESC = false;

    public static void main(String[] args) {

        // Create dummy HashMap
        Map<Integer, String> unsortedMap = new HashMap<>();
        unsortedMap.put(1, "froyo");
        unsortedMap.put(2, "abby");
        unsortedMap.put(3, "denver");
        unsortedMap.put(4, "frost");
        unsortedMap.put(5, "daisy");

        // Sort in ascending order
        Map<Integer, String> sortedMapAsc = sortByValue(unsortedMap, ASC);

        // Sort in descending order
        Map<Integer, String> sortedMapDesc = sortByValue(unsortedMap, DESC);

        // Print sorted maps
        System.out.println("Sorted Ascending:");
        printMap(sortedMapAsc);
        System.out.println("Sorted Descending:");
        printMap(sortedMapDesc);
    }

    private static Map<Integer, String> sortByValue(Map<Integer, String> map, boolean order) {

        List<Entry<Integer, String>> list = new LinkedList<>(map.entrySet());

        // Custom comparator for values
        Collections.sort(list, new Comparator<Entry<Integer, String>>() {
            public int compare(Entry<Integer, String> o1, Entry<Integer, String> o2) {
                if (order) {
                    return o1.getValue().compareTo(o2.getValue());
                } else {
                    return o2.getValue().compareTo(o1.getValue());
                }
            }
        });

        // Return sorted LinkedHashMap
        Map<Integer, String> sortedMap = new LinkedHashMap<>();
        for (Entry<Integer, String> entry : list) {
            sortedMap.put(entry.getKey(), entry.getValue());
        }

        return sortedMap;
    }

    public static void printMap(Map<Integer, String> map) {
        for (Entry<Integer, String> entry : map.entrySet()) {
            System.out.println(entry.getKey() + " - " + entry.getValue());
        }
        System.out.println();
    }
}

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