public void assertNonNegative() {
		if(value < 0)
			throw new IllegalStateException("non negative value required");
	}
@Test(expected=IllegalStateException.class)
    public void negativeQuantity_throwsIllegalStateExceptionOnCheckForNonNegative() throws Exception {
        Quantity<Metrics> negative = new Quantity<Metrics>(-1, Metrics.cm);
        negative.assertNonNegative();
    }