public static boolean isHexString(String s) {
        if (s.length() % 2 != 0) {
            return false;
        }
        for (char ch : s.toCharArray()) {
            if (!('0' <= ch && ch <= '9' || 'a' <= ch && ch <= 'f')) {
                return false;
            }
        }
        return true;
    }
@Test
    public void testWrongHexString() {
        Assert.assertFalse(Util.isHexString("a"));
        Assert.assertFalse(Util.isHexString("akl"));
    }