Review

class MannWhitneyUTest {
    /** Concatenate the samples into one array.
     * @param x first sample
     * @param y second sample
     * @return concatenated array
     */
    private double[] concatenateSamples(final double[] x, final double[] y) {
        final double[] z = new double[x.length + y.length];

        System.arraycopy(x, 0, z, 0, x.length);
        System.arraycopy(y, 0, z, x.length, y.length);

        return z;
    }

}