Example: SortArrayByFrequencyHashMap in Java

// SortArrayByFrequencyHashMap
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

// Main class containing the entry point of the program
public class Main {
    public static void main(String[] args) {
        int[] arr = {2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12};
        System.out.println("Original Array: " + Arrays.toString(arr));

        // Step 1: Count the frequency of each element
        Map<Integer, Integer> frequencyMap = new HashMap<>();
        for (int num : arr) {
            frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
        }

        // Step 2: Create a list of elements from the original array
        // This list will be sorted based on frequencies
        List<Integer> list = new ArrayList<>();
        for (int num : arr) {
            list.add(num);
        }

        // Step 3: Sort the list using a custom comparator
        Collections.sort(list, new Comparator<Integer>() {
            @Override
            public int compare(Integer n1, Integer n2) {
                // Get frequencies from the map
                int freq1 = frequencyMap.get(n1);
                int freq2 = frequencyMap.get(n2);

                // Compare frequencies in decreasing order
                if (freq1 != freq2) {
                    return freq2 - freq1; // For decreasing frequency
                } else {
                    // If frequencies are same, compare numbers in ascending order
                    return n1 - n2;
                }
            }
        });

        System.out.println("Sorted by Frequency (Decreasing): " + list);
    }
}