Question

I’m having some trouble with a question from my programming II class and have hit a brick wall, was wondering if someone could help?

The question asks for a user to input a string, the program to reverse the input string and then to compare the reverse to the original, this must be done recursively.

So far I have:

public class question1 
{
public static void main(String args[])  
{

String input = JOptionPane.showInputDialog(null, "Please enter a sentence to determine if it is a palindrome.");
String backwardsinput = Reverse(input);
System.out.println(backwardsinput);
boolean Palindrome = PalindromeCheck(backwardsinput, input);

    if (Palindrome == true)

        {
        JOptionPane.showMessageDialog(null,"That is a palindrome!");
        }

    if (Palindrome == false)

        {
        JOptionPane.showMessageDialog(null,"That is not a palindrome"); 
        }

}

public static String Reverse (String input)
{
    if (input.length() <= 1)
    return input;

    else
    {
        char x = input.charAt(input.length()-1);               
        return x+Reverse(input.substring(0,input.length()-1));
    }

}



public static boolean PalindromeCheck (String backwardsinput, String input)
{

     if(input.length() == 0 || input.length() == 1)
            return true;

        if(backwardsinput.charAt(0) == input.charAt(input.length()-1))
            return PalindromeCheck(backwardsinput.substring(1, backwardsinput.length()-1), input.substring(1, input.length()-1));

        else
        return false;

       }    
}

My problem is, it tells me everything is a palindrome, I’ve looked at it over and over and can’t figure out why!

Answer 1

You’re doing the work twice (sort of).

if(backwardsinput.charAt(0) == input.charAt(input.length()-1))
                                            ^^^^^^^^^^^^^^^^

should be

if (backwardsinput.charAt(0) == input.charAt(0))
                                             ^

You almost got it 🙂

---

Also, another way of expressing

if (cond)
    return something;
else
    return false;

is

return cond && something;

thus your last lines could be written as

return backwardsinput.charAt(0) == input.charAt(0) &&
       palindromeCheck(backwardsinput.substring(1, backwardsinput.length() - 1),
                                input.substring(1, input.length() - 1));

---

Related question / answer:

• How to find out all palindromic numbers