Palindrome Checker in Java

A palindrome is a word, phrase, number, or other sequences of characters which reads the same backward as forward (ignoring spaces, punctuation, and capitalization). For example, “madam” and “racecar” are palindromes.

This program checks if a given string is a palindrome using the Java programming language.

Java Program

/**
 * Palindrome Checker: Check if a given string is a palindrome.
 */
public class PalindromeChecker {
    /**
     * Method to check if the input string is a palindrome.
     * 
     * @param str the input string to check
     * @return true if the input string is a palindrome, false otherwise
     */
    public static boolean isPalindrome(String str) {
        // Removing all non-alphanumeric characters and converting to lower case
        String cleanedStr = str.replaceAll("[^a-zA-Z0-9]", "").toLowerCase();

        // Initializing pointers for start and end of the string
        int left = 0;
        int right = cleanedStr.length() - 1;

        // Comparing characters from start and end
        while (left < right) {
            if (cleanedStr.charAt(left) != cleanedStr.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }

        return true;
    }

    /**
     * Main method to run the Palindrome Checker.
     * 
     * @param args command-line arguments (not used)
     */
    public static void main(String[] args) {
        String[] testStrings = {
            "madam",
            "racecar",
            "A man, a plan, a canal, Panama",
            "No lemon, no melon",
            "Hello, World!"
        };

        for (String s : testStrings) {
            System.out.println("\"" + s + "\" is " + (isPalindrome(s) ? "" : "not ") + "a palindrome.");
        }
    }
}

Explanation

The program consists of two methods:

• isPalindrome: This method takes a string as input and checks if it is a palindrome. It does the following:
  • Removes all non-alphanumeric characters and converts the string to lowercase to ensure uniformity.
  • Initializes two pointers: one at the start (left) and one at the end (right) of the cleaned string.
  • Compares characters at these pointers. If they do not match, the string is not a palindrome. If they match, the pointers move towards the center.
  • If all characters match as the pointers move towards the center, the string is a palindrome.
• main: This method tests the isPalindrome method with several example strings, printing the results to the console.

Output

Running the above program will produce the following output:

"madam" is a palindrome.
"racecar" is a palindrome.
"A man, a plan, a canal, Panama" is a palindrome.
"No lemon, no melon" is a palindrome.
"Hello, World!" is not a palindrome.