Case Study: Ignoring Nonalphanumeric Characters When Checking Palindromes

Palindrome.java, here considered all the characters in a string to check whether it is a palindrome. Write a new program that ignores nonalphanumeric characters in checking whether a string is a palindrome.
Here are the steps to solve the problem:

1. Filter the string by removing the nonalphanumeric characters. This can be done by creating an empty string builder, adding each alphanumeric character in the string to a string builder, and returning the string from the string builder. You can use the isLetterOrDigit(ch) method in the Character class to check whether character ch is a letter or a digit.
2. Obtain a new string that is the reversal of the filtered string. Compare the reversed string with the filtered string using the equals method.

The complete program is shown in the program below.

package demo;
import java.util.Scanner;

public class PalindromeIgnoreNonAlphanumeric {

    public static void main(String[] args) {
        // Create a Scanner
        Scanner input = new Scanner(System.in);

        // Prompt the user to enter a string
        System.out.print("Enter a string: ");
        String s = input.nextLine();

        // Display result
        System.out.println("Ignoring nonalphanumeric characters, \nis " + s + " a palindrome? " + isPalindrome(s));
    }

    /** Return true if a string is a palindrome */
    public static boolean isPalindrome(String s) {
        // Create a new string by eliminating nonalphanumeric chars
        String s1 = filter(s);

        // Create a new string that is the reversal of s1
        String s2 = reverse(s1);

        // Check if the reversal is the same as the original string
        return s2.equals(s1);
    }

    /** Create a new string by eliminating nonalphanumeric chars */
    public static String filter(String s) {
        // Create a string builder
        StringBuilder stringBuilder = new StringBuilder();

        // Examine each char in the string to skip alphanumeric char
        for(int i = 0; i < s.length(); i++) {
            if(Character.isLetterOrDigit(s.charAt(i))) {
                stringBuilder.append(s.charAt(i));
            }
        }

        // Return a new filtered string
        return stringBuilder.toString();
    }

    /** Create a new string by reversing a specified string */
    public static String reverse(String s) {
        StringBuilder stringBuilder = new StringBuilder(s);
        stringBuilder.reverse(); // Invoke reverse in StringBuilder
        return stringBuilder.toString();
    }

}

The filter(String s) method (lines 31–44) examines each character in string s and copies it to a string builder if the character is a letter or a numeric character. The filter method returns the string in the builder. The reverse(String s) method (lines 47–51) creates a new string that reverses the specified string s. The filter and reverse methods both return a new string. The original string is not changed.

The program in the linked post above checks whether a string is a palindrome by comparing pairs of characters from both ends of the string. The program above uses the reverse method in the StringBuilder class to reverse the string, then compares whether the two strings are equal to determine whether the original string is a palindrome