How can I sort a Java HashMap by its values in ascending and descending order?

Sorting a HashMap by Values

In Java, a HashMap is an unsorted collection of key-value pairs. To sort a HashMap by its values, we can use external sorting algorithms, as the HashMap itself doesn't provide an in-built sorting method.

One common approach is to convert the HashMap into a linked list of its entries, sort the linked list by value, and then rebuild the HashMap. To ensure that the key-value pairs remain linked during this process, we use a comparator that compares values while preserving the original key-value ordering.

The following code snippet demonstrates this approach:

import java.util.*;

public class HashMapSorting {
    private static final boolean ASC = true;
    private static final boolean DESC = false;

    public static void main(String[] args) {
        // Creating dummy unsorted HashMap
        HashMap<string integer> unsortedMap = new HashMap();
        unsortedMap.put("B", 55);
        unsortedMap.put("A", 80);
        unsortedMap.put("D", 20);
        unsortedMap.put("C", 70);

        System.out.println("Before sorting...");
        printMap(unsortedMap);

        System.out.println("After sorting ascending order...");
        Map<string integer> sortedMapAsc = sortByValue(unsortedMap, ASC);
        printMap(sortedMapAsc);

        System.out.println("After sorting descending order...");
        Map<string integer> sortedMapDesc = sortByValue(unsortedMap, DESC);
        printMap(sortedMapDesc);
    }

    private static <k extends comparable>, V extends Comparable<v>> Map<k v> sortByValue(Map<k v> unsorted, final boolean order) {
        List<map.entry v>> list = new LinkedList(unsorted.entrySet());

        list.sort((o1, o2) -> order
                ? o1.getValue().compareTo(o2.getValue()) == 0
                  ? o1.getKey().compareTo(o2.getKey())
                  : o1.getValue().compareTo(o2.getValue())
                : o2.getValue().compareTo(o1.getValue()) == 0
                  ? o2.getKey().compareTo(o1.getKey())
                  : o2.getValue().compareTo(o1.getValue()));
        return list.stream().collect(
                Collectors.toMap(
                        Map.Entry::getKey, Map.Entry::getValue, (a, b) -> b, LinkedHashMap::new
                )
        );
    }

    private static void printMap(Map<string integer> map) {
        for (Map.Entry<string integer> entry : map.entrySet()) {
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}</string></string></map.entry></k></k></v></k></string></string></string>

This code demonstrates both ascending and descending order sorting by implementing a comparator that evaluates the values while preserving the original key-value relationships.

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