Mode.java

package com.thealgorithms.maths;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;

/*
 * Find the mode of an array of numbers
 *
 * The mode of an array of numbers is the most frequently occurring number in the array,
 * or the most frequently occurring numbers if there are multiple numbers with the same frequency
 */
public class Mode {

    public static void main(String[] args) {

        /* Test array of integers */
        assert (mode(new int[]{})) == null;
        assert Arrays.equals(mode(new int[]{5}), new int[]{5});
        assert Arrays.equals(mode(new int[]{1, 2, 3, 4, 5}), new int[]{1, 2, 3, 4, 5});
        assert Arrays.equals(mode(new int[]{7, 9, 9, 4, 5, 6, 7, 7, 8}), new int[]{7});
        assert Arrays.equals(mode(new int[]{7, 9, 9, 4, 5, 6, 7, 7, 9}), new int[]{7, 9});
    }

    /*
   * Find the mode of an array of integers
   *
   * @param numbers array of integers
   * @return mode of the array
     */
    public static int[] mode(int[] numbers) {

        if (numbers.length == 0) {
            return null;
        }

        HashMap<Integer, Integer> count = new HashMap<>();

        for (int num : numbers) {
            if (count.containsKey(num)) {

                count.put(num, count.get(num) + 1);

            } else {

                count.put(num, 1);
            }
        }

        int max = Collections.max(count.values());
        ArrayList<Integer> modes = new ArrayList<>();

        for (int num : count.keySet()) {
            if (count.get(num) == max) {
                modes.add(num);
            }
        }
        return modes.stream().mapToInt(n -> n).toArray();
    }
}