MeanMedianMode.java

package by.andd3dfx.numeric;

import java.util.Arrays;
import java.util.Objects;
import java.util.stream.Collectors;

/**
 * @see <a href="https://youtu.be/nqalmvHE_-A">Video solution</a>
 */
public class MeanMedianMode {

    public static double mean(int[] items) {
        return Arrays.stream(items).sum() / (double) items.length;
    }

    public static double median(int[] items) {
        Arrays.sort(items);

        int n = items.length;
        if (n % 2 == 0) {
            return (items[n / 2 - 1] + items[n / 2]) / 2.0;
        }
        return items[n / 2];
    }

    public static double mode(int[] items) {
        var freqMap = Arrays.stream(items)
                .boxed()
                .collect(Collectors.groupingBy(
                        i -> i, Collectors.counting()
                ));

        int mode = items[0];
        for (var key : freqMap.keySet()) {
            if (Objects.equals(freqMap.get(key), freqMap.get(mode)) && key < mode) {
                mode = key;
            }
            if (freqMap.get(key) > freqMap.get(mode)) {
                mode = key;
            }
        }
        return mode;
    }

    public static double quartile1(int[] items) {
        var n = items.length;
        int[] left = new int[n / 2];

        Arrays.sort(items);
        System.arraycopy(items, 0, left, 0, n / 2);
        return median(left);
    }

    public static double quartile2(int[] items) {
        return median(items);
    }

    public static double quartile3(int[] items) {
        var n = items.length;
        int[] right = new int[n / 2];

        Arrays.sort(items);
        int pos = (n % 2 == 0) ? (n / 2) : (n / 2 + 1);
        System.arraycopy(items, pos, right, 0, n / 2);
        return median(right);
    }
}