Example: Sort Array Elements by Frequency in Java

// Sort Array Elements by Frequency
import java.util.ArrayList;
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[] originalArray = {1, 2, 3, 2, 1, 4, 1, 5, 5, 5};

        System.out.println("Original Array:");
        printArray(originalArray);

        // Step 1: Convert int[] to List<Integer>
        List<Integer> list = new ArrayList<>();
        for (int num : originalArray) {
            list.add(num);
        }

        // Step 2: Calculate frequency of each element using a HashMap
        Map<Integer, Integer> frequencyMap = new HashMap<>();
        for (int num : list) {
            frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
        }

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

                // Primary sort: by frequency (descending)
                if (freq1 != freq2) {
                    return freq2 - freq1; // Higher frequency comes first
                } else {
                    // Secondary sort: by value (ascending) if frequencies are equal
                    return n1 - n2; // Smaller value comes first
                }
            }
        });

        System.out.println("\nArray sorted by Frequency:");
        printList(list);
    }

    // Helper method to print an int array
    public static void printArray(int[] arr) {
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + (i == arr.length - 1 ? "" : ", "));
        }
        System.out.println();
    }

    // Helper method to print a List of Integers
    public static void printList(List<Integer> list) {
        for (int i = 0; i < list.size(); i++) {
            System.out.print(list.get(i) + (i == list.size() - 1 ? "" : ", "));
        }
        System.out.println();
    }
}