Java Program to Check if Input String is Palindrome

Introduction

A palindrome is a word, phrase, number, or other sequence of characters that reads the same forward and backward (ignoring spaces, punctuation, and capitalization). Checking if a string is a palindrome is a common task in text processing. In this blog post, we will explore various methods to check if an input string is a palindrome in Java.

Table of Contents

1. Using a Simple Loop
2. Using StringBuilder's reverse() Method
3. Using Java 8 Streams
4. Complete Example Program
5. Conclusion

1. Using a Simple Loop

One of the simplest methods to check if a string is a palindrome is by comparing characters from the start and end of the string moving towards the center.

Example:

public class PalindromeCheckUsingLoop {
    public static void main(String[] args) {
        String input = "Madam";
        boolean result = isPalindrome(input);
        System.out.println("Is the string \"" + input + "\" a palindrome? " + result);
    }

    public static boolean isPalindrome(String str) {
        str = str.toLowerCase().replaceAll("[^a-zA-Z0-9]", ""); // Normalizing the input
        int left = 0;
        int right = str.length() - 1;

        while (left < right) {
            if (str.charAt(left) != str.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }
}

Output:

Is the string "Madam" a palindrome? true

Explanation:

• The string is converted to lowercase and all non-alphanumeric characters are removed to normalize the input.
• A loop is used to compare characters from the start and end of the string moving towards the center.
• If any characters do not match, the string is not a palindrome.

2. Using StringBuilder's reverse()() Method

Another method to check if a string is a palindrome is by reversing the string and comparing it to the original string.

Example:

public class PalindromeCheckUsingStringBuilder {
    public static void main(String[] args) {
        String input = "Madam";
        boolean result = isPalindrome(input);
        System.out.println("Is the string \"" + input + "\" a palindrome? " + result);
    }

    public static boolean isPalindrome(String str) {
        str = str.toLowerCase().replaceAll("[^a-zA-Z0-9]", ""); // Normalizing the input
        String reversedStr = new StringBuilder(str).reverse().toString();
        return str.equals(reversedStr);
    }
}

Output:

Is the string "Madam" a palindrome? true

Explanation:

• The string is normalized by converting it to lowercase and removing non-alphanumeric characters.
• The StringBuilder's reverse() method is used to reverse the string.
• The reversed string is compared to the original string to determine if it is a palindrome.

3. Using Java 8 Streams

Java 8 Streams provide a modern and concise way to check if a string is a palindrome.

Example:

import java.util.stream.IntStream;

public class PalindromeCheckUsingStreams {
    public static void main(String[] args) {
        String input = "Madam";
        boolean result = isPalindrome(input);
        System.out.println("Is the string \"" + input + "\" a palindrome? " + result);
    }

    public static boolean isPalindrome(String str) {
        str = str.toLowerCase().replaceAll("[^a-zA-Z0-9]", ""); // Normalizing the input
        return IntStream.range(0, str.length() / 2)
                        .allMatch(i -> str.charAt(i) == str.charAt(str.length() - i - 1));
    }
}

Output:

Is the string "Madam" a palindrome? true

Explanation:

• The string is normalized by converting it to lowercase and removing non-alphanumeric characters.
• An IntStream is used to iterate over the first half of the string.
• The allMatch() method checks if all characters from the start and end of the string match.

4. Complete Example Program

Here is a complete program that demonstrates all the methods discussed above to check if a string is a palindrome.

Example Code:

import java.util.stream.IntStream;

public class PalindromeCheckExample {
    public static void main(String[] args) {
        String input = "A man, a plan, a canal, Panama";

        // Using a Simple Loop
        boolean resultLoop = isPalindromeUsingLoop(input);
        System.out.println("Using Simple Loop:");
        System.out.println("Is the string \"" + input + "\" a palindrome? " + resultLoop);

        // Using StringBuilder's reverse() Method
        boolean resultStringBuilder = isPalindromeUsingStringBuilder(input);
        System.out.println("\nUsing StringBuilder's reverse() Method:");
        System.out.println("Is the string \"" + input + "\" a palindrome? " + resultStringBuilder);

        // Using Java 8 Streams
        boolean resultStreams = isPalindromeUsingStreams(input);
        System.out.println("\nUsing Java 8 Streams:");
        System.out.println("Is the string \"" + input + "\" a palindrome? " + resultStreams);
    }

    public static boolean isPalindromeUsingLoop(String str) {
        str = str.toLowerCase().replaceAll("[^a-zA-Z0-9]", ""); // Normalizing the input
        int left = 0;
        int right = str.length() - 1;

        while (left < right) {
            if (str.charAt(left) != str.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    }

    public static boolean isPalindromeUsingStringBuilder(String str) {
        str = str.toLowerCase().replaceAll("[^a-zA-Z0-9]", ""); // Normalizing the input
        String reversedStr = new StringBuilder(str).reverse().toString();
        return str.equals(reversedStr);
    }

    public static boolean isPalindromeUsingStreams(String str) {
        str = str.toLowerCase().replaceAll("[^a-zA-Z0-9]", ""); // Normalizing the input
        return IntStream.range(0, str.length() / 2)
                        .allMatch(i -> str.charAt(i) == str.charAt(str.length() - i - 1));
    }
}

Output:

Using Simple Loop:
Is the string "A man, a plan, a canal, Panama" a palindrome? true

Using StringBuilder's reverse() Method:
Is the string "A man, a plan, a canal, Panama" a palindrome? true

Using Java 8 Streams:
Is the string "A man, a plan, a canal, Panama" a palindrome? true

5. Conclusion

Checking if a string is a palindrome can be accomplished in multiple ways in Java. The simple loop method is straightforward and easy to understand, while the StringBuilder's reverse() method provides a quick and concise solution. Java 8 Streams offer a modern, functional programming approach to the same problem.

By understanding these different methods, you can choose the one that best fits your needs and coding style. Happy coding!