@Override
    public double sd() {
        return 1 / lambda;
    }
@Test
    public void testSd() {
        System.out.println("sd");
        ExponentialDistribution instance = new ExponentialDistribution(1.0);
        instance.rand();
        assertEquals(1.0, instance.sd(), 1E-7);
        instance = new ExponentialDistribution(2.0);
        instance.rand();
        assertEquals(0.5, instance.sd(), 1E-7);
        instance = new ExponentialDistribution(3.0);
        instance.rand();
        assertEquals(0.3333333, instance.sd(), 1E-7);
        instance = new ExponentialDistribution(4.0);
        instance.rand();
        assertEquals(0.25, instance.sd(), 1E-7);
    }