@Override
    public double mean() {
        return (double) m * n / N;
    }
@Test
    public void testMean() {
        System.out.println("mean");
        HyperGeometricDistribution instance = new HyperGeometricDistribution(100, 30, 70);
        instance.rand();
        assertEquals(21, instance.mean(), 1E-7);
        instance = new HyperGeometricDistribution(100, 30, 80);
        instance.rand();
        assertEquals(24, instance.mean(), 1E-7);
        instance = new HyperGeometricDistribution(100, 30, 60);
        instance.rand();
        assertEquals(18, instance.mean(), 1E-7);
    }