public static Ip4Prefix valueOf(int address, int prefixLength) {
        return new Ip4Prefix(Ip4Address.valueOf(address), prefixLength);
    }
@Test(expected = NullPointerException.class)
    public void testInvalidValueOfNullString() {
        Ip4Prefix ipPrefix;
        String fromString;

        fromString = null;
        ipPrefix = Ip4Prefix.valueOf(fromString);
    }