Palindrome Question

Question

I had a question regarding a basic program I was writing said whether a word such as racecar is a palindrome or not.

All my methods which reverse the string, strip the punctuation work but the one that determines if it is a palindrome does not.

/**
* Determines if a series of letters makes a palinedrome
* 
* @param  str   All punctuation and spaces have been removed 
*               before this method is called.  
* @return true  if phrase is a palindrome,
*         false otherwise.
*/
public boolean isPalindrome(String str)
{
   String d = reverseString (str); 
   return( str.equals (reverseString (str) ) ); 

}

Answer 1

If string reverseString(String string), and all whitespace was removed then checking if something is a palindrome should be

public boolean isPalindrome(String string)
{
    return string.equals(reverseString(string));
}

Granted this is case sensitive so if your palindrome definition does not care about casing, then use equalsIgnoreCase instead.

If this does not work, then you may want to check your stripping and reverseString methods again.

Answer 2

Okay, I'm not sure what purpose d is meant to serve in your function since it's never used but, if you want to see why your function's not working, just add debug code:

public boolean isPalindrome (String str) {
    System.out.println ("DEBUG: original string = '" + str + "'");
    System.out.println ("DEBUG: reverse string = '" + reverseString (str) + "'");
    if (str.equals (reverseString (str)))
        System.out.println ("DEBUG: returning true");
    else
        System.out.println ("DEBUG: returning false");
    return str.equals (reverseString (str));
}

I'd bet money on there being something wrong with your reverseString function (but not much money). These debug statements should give you enough to figure out where the problem lies.

Answer 3

Your problem is the reverse string method you have not shown. If that method is working properly then your isPalindrome method should work. All you need to do is fix your reverse string method.

Java does not have a native reverse string method, and I highly recommend you write your own.

Java does, however, have a reverse method for StringBuffer and StringBuilder. StringBuilder is preferred over StringBuffer.

Use the equals method to compare your reversed string to the original string

Answer 4

The code should be like this:

String d = reverseString (str); 
return( str.equals (d) );

You don't have to call twice reverseString()

P.S.: StringBuffer has a method that reverses a String.