Hi,
The problem description is in the comments above the solution below.

The problem belongs to Codility, you can try out it yourself on their platform.

/**
 * A non-empty array A consisting of N integers is given. 
 * The array contains an odd number of elements, and each element of the array can be paired with another element that has the same value, 
 * except for one element that is left unpaired.
 * 
 * For example, in array A such that:
 * A[0] = 9  A[1] = 3  A[2] = 9
 * A[3] = 3  A[4] = 9  A[5] = 7
 * A[6] = 9
 * the elements at indexes 0 and 2 have value 9,
 * the elements at indexes 1 and 3 have value 3,
 * the elements at indexes 4 and 6 have value 9,
 * the element at index 5 has value 7 and is unpaired.
 */
public class OddOccurence {
    public int solution(int[] A) {
        int xor = 0;
        for (int i = 0; i < A.length; i++) {
            xor ^= A[i];
        }
        return xor;
    }
}