Java Program to Check if Input String is Palindrome

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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!