mutate method

private Candidate mutate(Candidate c) {
    int swapPos1, swapPos2;
    swapPos1 = rnd.nextInt(numElements);
    swapPos2 = rnd.nextInt(numElements);
    while(swapPos1 == swapPos2) {
        swapPos2 = rnd.nextInt(numElements);
    }

    int temp = c.genotype[swapPos1];
    c.genotype[swapPos1] = c.genotype[swapPos2];
    c.genotype[swapPos2] = temp;

    return c;
}