public static IpPrefix valueOf(int address, int prefixLength) {
        return new IpPrefix(IpAddress.valueOf(address), prefixLength);
    }
@Test(expected = NullPointerException.class)
    public void testInvalidValueOfNullString() {
        IpPrefix ipPrefix;
        String fromString;

        fromString = null;
        ipPrefix = IpPrefix.valueOf(fromString);
    }