Find missing Integer in array with Java

MissingElement.java

public class MissingElement {
    public int solution(int[] A) {
        if (A == null) return 1;
        else if (A.length == 0) return 1;
        else {
            java.util.Arrays.sort(A);
            return evaluate(A, 0, A.length - 1);
        }
    }

    int evaluate(int[] A, int start, int stop) {
        int mid = (stop + start) / 2;
        int expected = A[0] + mid;
        if (start == stop) {
            if (A[start] == (A[0] + start))
                if (A[0] == 1)
                    return A[0] + A.length;
                else
                    return 1;
            else
                return (A[0] + start);
        } else
        if (A[mid] > expected)
            return evaluate(A, start, mid);
        else
            return evaluate(A, mid+1, stop);
    }

    public static void main(String[] args) {
        assert(new MissingElement().solution(new int[] { 1 })) == 2;
        assert(new MissingElement().solution(new int[] { 2 })) == 1;
        assert(new MissingElement().solution(new int[] { 1, 2 })) == 3;
        assert(new MissingElement().solution(new int[] { 2, 3 })) == 1;
        assert(new MissingElement().solution(new int[] { 3, 5 })) == 4;
        assert(new MissingElement().solution(new int[] { 2, 3, 5 })) == 4;
        assert(new MissingElement().solution(new int[] { 1, 2, 3, 5 })) == 4;
        assert(new MissingElement().solution(new int[] { 0, 1, 2, 3, 5 })) == 4;
        assert(new MissingElement().solution(new int[] { 0, 1, 2, 3, 5, 6 })) == 4;
        assert(new MissingElement().solution(new int[] { 11, 13 })) == 12;
        assert(new MissingElement().solution(new int[] { 11, 12, 14 })) == 13;
        assert(new MissingElement().solution(new int[] { 11, 13, 14, 15, 16 })) == 12;
    }
}