Palindrome using stack

Question

I wrote a palindrome using stacks in java. But, due to some reason while popping the stack the program is not working as expected. I have also tried using the top and iterate using this variable. Either ways it is not working.

import java.util.Stack;

class Palindromer {

Stack<Character> stack = null;
int top = -1;

public boolean isPalindrome(String s) {
    for (int i = 0; i < s.length(); i++) {
        stack = new Stack<Character>();
        stack.push(s.charAt(i));
        top++;
    }

    System.out.println("value of top is " + top);
    String returnString = "";

    while (!stack.isEmpty()) {
        returnString += stack.pop();
    }
    System.out.println(returnString);

    return returnString.equals(s);
}
}

public class PalindromeChecker {
public static void main(String[] args) {
    Palindromer palin = new Palindromer();
    if (palin.isPalindrome("BananaB")) {
        System.out.println("is palindrome");
    } else {
        System.out.println("not a palindrome");
    }
}
}

Answer 1

You should place the new Stack<Character>(); outside the loop.

The way you do it:

    for (int i = 0; i < s.length(); i++) {
        stack = new Stack<Character>();
        stack.push(s.charAt(i));
        top++;
    }

the stack variable is reassigned each loop and does contain only one character after the loop. Change it into

    stack = new Stack<Character>();
    for (int i = 0; i < s.length(); i++) {
        stack.push(s.charAt(i));
        top++;
    }

BTW: Better declare stack and top in the isPalindrome method. So you get rid of the error in top with further calls.