pairwiseSum

package ch.claude_martin.playground;

import java.util.*;

public final class SomeClass {
    public static void main(String[] args) {
        int[] a = { 2, 1, 18, -5 };
        int[] pairwiseSum = pairwiseSum(a);
        System.out.println(Arrays.toString(pairwiseSum));
    }

    public static int[] pairwiseSum(int[] a) {
        if (Objects.requireNonNull(a, "array can't be null").length % 2 != 0)
            throw new IllegalArgumentException("length must be even");
        int[] result = new int[a.length / 2];
        for (int i = 0, k = 0; k < result.length; i = i + 2) {
            result[k++] = a[i] + a[i + 1];
        }
        return result;
    }

}