Substring index out of bounds

Question

I am writing a simple program to find the longest palindrome in a string. What I'm doing is checking if each substring is a palindrome and then checking its length. If the length is greater than the previous length, then I have the new longest substring. For example, "babad", would return "bab" or "aba", either is fine. But my issue is that I get index out of bounds on my substring call and I can't figure out why.

public class LongestPal{
    public static void main(String[] args)
    {
        String test = new String("babad");
        String result = longestPalindrome(test);

    }
    public static String longestPalindrome(String s) {
        int length = 0;
        String answer = new String();

        for(int i = 0;i<=s.length();i++)
        {
            for(int j = 0; j<= s.length();j++)
            {
                String subStr = s.substring(i,j); // Get all substrings
                System.out.println(subStr); // Checking to see if all are printed

                boolean result = isPalindrome(subStr); //Check for palindrome
                if(result)
                {
                    if(length < subStr.length()) //If length of new substr is greater than the old one, 
                                                //the new answer will be longer substring
                    {
                        answer = subStr;   
                    }
                }
            }
        }
        return answer;
    }
    public static boolean isPalindrome(String s) //Recursive palindrome checker
    {
        if(s.length() == 0 || s.length() == 1)
            return true; 
        if(s.charAt(0) == s.charAt(s.length()-1))
            return isPalindrome(s.substring(1, s.length()-1));
        return false;
    }
}

When I print out, I get all the substring combinations, until "babbad" after that is when the error occurs.

Answer 1

Part A of the problem - Compiler error

1. Change your outer loop from for(int i = 0;i<=s.length();i++) to for(int i = 0;i<s.length();i++) as others have pointed out.
2. Change your inner loop from for(int j = 0; j<= s.length();j++) to for(int j = i; j< s.length();j++).

Your program is giving you out of bound exception because in the second iteration of your outer loop, your i becomes 1 and j starts from 0. And then you call the String's substring method as s.substring(1,0); which obviously gives you this exception.

Make the corrections as suggested and go from there on. Let us know if you need more help.

Part B of the problem - Faster algorithm

Here's a more efficient algorithm. I believe this should be the fastest one too.

    public static void main(String[] args) {        
    String s = "babad";

    int starter = 0;

    if(s.length() % 2 == 0) 
        starter = 1;

    String answer = "";
    Boolean found = Boolean.FALSE;
    while(starter < s.length() - 1) {
        for(int i=0; i<=starter; i++) {
            if(isPelindrom(answer = s.substring(i, i+ s.length() - starter))) {
                found = Boolean.TRUE;
                break;
            }
        }
        if(found) break;
        starter = starter + 1;          
    }
    if(!found) answer = String.valueOf(s.charAt(0));
    System.out.println("The answer is " + answer);

}

public static Boolean isPelindrom(String s) {
    return s.equalsIgnoreCase(new StringBuilder(s).reverse().toString());
}

Let me know if you need additional help.