substring() method in a Palindrome

In a well know recursion isPalindrome method

  public static boolean isPalindrome(String s){

        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;
        }
 }

there is one line that I dont quite understand. If, for example we pass the string anna to the isPalindrome method, what does this line of code

return isPalindrome(s.substring(1, s.length()-1));

do to the string when s has a value of nn ?

In my understanding number 1 (index 1) is for second letter n, and s.length()-1 is equal 2-1 = 1, but not including that index position, so that must be index 0 ??

Does it return an empty string or something else ?

Solution

When the value of s is nn, as we step through the statements, this will happen:

s.length() is 2, so the first if condition doesn't match
s.charAt(0) is n, and s.charAt(1) is n, so the second if matches
Return the result of isPalindrome with parameter s.substring(1, 1), which is the text range from position 1 until before the position 1, in other words, an empty string
In the recursive call with empty string as input, isPalindrome will match the first condition on length, and return true

For the record, this is a very inefficient way to check a palindrome in Java, because substring creates new strings, which is slow. A more efficient recursive solution is possible by adding start and end index parameters, and moving them inward, until their difference becomes 0 or 1.