Recursive palindrome test cant include special characters

Question

I have written a recursive method to check if a word is a palindrome or not.

But I now also wish to check for any special characters (*?!< in the string. So if the string is ab.ba it is not a palindrome.

Here is what I have and the isAlphanuemerical() seems to be working ok when stepping through it, but it still registers as a palindrome if I enter ab.ba

public static boolean isPal(String s)
{
    if(s.length() == 0 || s.length() == 1)
        return true;

    // Check if String contains any alphanumeric characters
    if (isAlphanumeric(s))
        return false;


    if(s.charAt(0) == s.charAt(s.length()-1))
        return isPal(s.substring(1, s.length()-1));//if its not the case than string is not.
    return false;
}

And my isAlphaneumerical() method

public static boolean isAlphanumeric(String str) {
    for (int i = 0; i < str.length(); i++) {
        char c = str.charAt(i);
        if (!Character.isLetter(c) && !Character.isDigit(c))
            return false;
    }
    return true;
}

Answer 1

if (isAlphanumeric(s))
        return false;

Should be

if (!isAlphanumeric(s))
        return false;

Also, you don't need to call isAlphanumeric(s) EVERYTIME. You could call it once, or change it so it only checks the first/last characters. You're checking something that was already checked before.

public static boolean isPal(String s)
{
    if (!isAlphanumeric(s))
        return false;

    return _isPal(s);
}

private static boolean _isPal(String s)
{
    if (s.length() == 0 || s.length() == 1)
        return true;

    if (s.charAt(0) == s.charAt(s.length()-1))
        return _isPal(s.substring(1, s.length()-1));

    return false;
}

private static boolean isAlphanumeric(String str) 
{
    for (int i = 0; i < str.length(); i++) 
    {
        char c = str.charAt(i);

        if (!Character.isLetter(c) && !Character.isDigit(c))
            return false;
    }
    return true;
}

Answer 2

Your code is doing exactly what you told it to do. The isAlphanumeric method works fine.

But your isPal returns false if it is alphanumeric.

If you don't wan't non-alphanumeric strings to be palindromes, then reverse your logic:

if (!isAlphanumeric(s))
    return false;

This uses the ! operator to negate the boolean result.