Java – Check Palindrome String

A string is said to be palindrome, if it is same as that of its reversed value.

In this tutorial, we shall check if a string is palindrome in two ways. Firstly, by reversing the string and comparing it with the original value. Secondly, we shall traverse through string characters from start and end of the string, and kind of meet in the middle, checking if the characters have equal value.

Check Palindrome String using StringBuilder

String could be a palindrome if it equals the reverse of it.

In the following example, we shall reverse the given string in Java using StringBuilder, and compare it with its original value. If both are equal, then we can say that the given string is a palindrome.

PalindromeString.java

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/**
 * Java Program - Check if String is Palindrome
 */

public class PalindromeString {

	public static void main(String[] args) {
		
		String str = "tattarrattat";
		
		//reverse the string
		String rev = (new StringBuilder(str)).reverse().toString();
		
		//check if str is palindrome
		if(str.equals(rev)) {
			System.out.println(str+" is Palindrome.");
		} else {
			System.out.println(str+" is not Palindrome.");
		}
	}
}

Output

tattarrattat is Palindrome.

Check if String is Palindrome by Checking at Character Level

Instead of reversing the string and checking for equality, we can check if the first and last characters are equal and progress to the middle of the string. If at any point, the characters are not equal, then it is not a palindrome.

Algorithm

We shall implement following algorithm in Java and write a program to check if given string is palindrome.

  1. Start.
  2. Take string in str. We need to check if this is palindrome string or not.
  3. Take a boolean variable isPalindrome to store if the string is palindrome or not. Initialize it with true.
  4. Initialize variable i with 0.
  5. Check if i is less than half the length of string str. If yes, go to step 6, else go to step 8.
  6. Check if character in str at index i is equal to that of at length-1-i. If not set isPalindrome to false and go to step 8.
  7. Increment i. Go to step 5.
  8. Based on the value of isPalindrome, print the result.
  9. Stop.

PalindromeString.java

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/**
 * Java Program - Check if String is Palindrome
 */

public class PalindromeString {

	public static void main(String[] args) {
		
		String str = "tattarrattat";
		
		boolean isPalindrome = true;
		
		//check if ith character is same from start and end
		for(int i=0;i<str.length()/2;i++) {
			if(str.charAt(i)!=str.charAt(str.length()-1-i)) {
				isPalindrome = false;
				break;
			}
		}
		
		//check if str is palindrome
		if(isPalindrome) {
			System.out.println(str+" is Palindrome.");
		} else {
			System.out.println(str+" is not Palindrome.");
		}
	}
}

Output

tattarrattat is Palindrome.

Conclusion

In this Java Tutorial, we have written Java program using different techniques on how to check if given string is a palindrome or not.