public static boolean isDigit(final byte value)
    {
        return value >= 0x30 && value <= 0x39;
    }
@Test
    void isDigitReturnsFalseForAnyOtherByte()
    {
        for (int i = Byte.MIN_VALUE; i <= Byte.MAX_VALUE; i++)
        {
            final byte value = (byte)i;
            if (value >= 0x30 && value <= 0x39)
            {
                continue;
            }
            assertFalse(isDigit(value), () -> Integer.toHexString(value));
        }
    }