Check if a String Is a Palindrome in Java

Last updated: March 17, 2024

Written by: baeldung

Java String

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1. Introduction

In this article, we’re going to see how we can check whether a given String is a palindrome using Java.

A palindrome is a word, phrase, number, or other sequences of characters which reads the same backward as forward, such as “madam” or “racecar”.

2. Check if a String Is a Palindrome

In the following sections, we’ll look at the various ways of checking if a given String is a palindrome or not.

2.1. A Simple Approach

We can simultaneously start iterating the given string forward and backward, one character at a time. If the there is a match the loop continues; otherwise, the loop exits:

public boolean isPalindrome(String text) {
    String clean = text.replaceAll("\\s+", "").toLowerCase();
    int length = clean.length();
    int forward = 0;
    int backward = length - 1;
    while (backward > forward) {
        char forwardChar = clean.charAt(forward++);
        char backwardChar = clean.charAt(backward--);
        if (forwardChar != backwardChar)
            return false;
    }
    return true;
}

2.2. Reversing the String

There are a few different implementations that fit this use case: we can make use of the API methods from StringBuilder and StringBuffer classes when checking for palindromes, or we can reverse the String without these classes.

Let’s take a look at the code implementations without the helper APIs first:

public boolean isPalindromeReverseTheString(String text) {
    StringBuilder reverse = new StringBuilder();
    String clean = text.replaceAll("\\s+", "").toLowerCase();
    char[] plain = clean.toCharArray();
    for (int i = plain.length - 1; i >= 0; i--) {
        reverse.append(plain[i]);
    }
    return (reverse.toString()).equals(clean);
}

In the above snippet, we simply iterate the given String from the last character and append each character to the next character, all the way through to the first character thereby reversing the given String.

Finally, we test for equality between the given String and reversed String.

The same behavior could be achieved using API methods.

Let’s see a quick demonstration:

public boolean isPalindromeUsingStringBuilder(String text) {
    String clean = text.replaceAll("\\s+", "").toLowerCase();
    StringBuilder plain = new StringBuilder(clean);
    StringBuilder reverse = plain.reverse();
    return (reverse.toString()).equals(clean);
}

public boolean isPalindromeUsingStringBuffer(String text) {
    String clean = text.replaceAll("\\s+", "").toLowerCase();
    StringBuffer plain = new StringBuffer(clean);
    StringBuffer reverse = plain.reverse();
    return (reverse.toString()).equals(clean);
}

In the code snippet, we invoke the reverse() method from the StringBuilder and StringBuffer API to reverse the given String and test for equality.

**2.3. Using Stream API**

We can also use an IntStream to provide a solution:

public boolean isPalindromeUsingIntStream(String text) {
    String temp  = text.replaceAll("\\s+", "").toLowerCase();
    return IntStream.range(0, temp.length() / 2)
      .noneMatch(i -> temp.charAt(i) != temp.charAt(temp.length() - i - 1));
}

In the snippet above, we verify that none of the pairs of characters from each end of the String fulfills the Predicate condition.

2.4. Using Recursion

Recursion is a very popular method to solve these kinds of problems. In the example demonstrated we recursively iterate the given String and test to find out whether it’s a palindrome or not:

public boolean isPalindromeRecursive(String text){
    String clean = text.replaceAll("\\s+", "").toLowerCase();
    return recursivePalindrome(clean,0,clean.length()-1);
}

private boolean recursivePalindrome(String text, int forward, int backward) {
    if (forward == backward) {
        return true;
    }
    if ((text.charAt(forward)) != (text.charAt(backward))) {
        return false;
    }
    if (forward < backward + 1) {
        return recursivePalindrome(text, forward + 1, backward - 1);
    }

    return true;
}

3. Check if a String Can Become a Palindrome by Rearranging

We’ll now check if we can rearrange a String so that it becomes a palindrome.

For instance, the String kkaay is an anagram of the palindrome kayak. The characteristics of a palindrome are that its first element is the same as its last element, its second element is the same as its penultimate element, and so on. The only possible character without a pair is the middle character if the String is of odd length.

In a nutshell, the number of occurrences of every character in a palindrome is even except for at most one character:

boolean hasPalindromePermutation(String text) {
    long charsWithOddOccurrencesCount = text.chars()
        .boxed()
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
        .values()
        .stream()
        .filter(count -> count % 2 != 0)
        .count();
    return charsWithOddOccurrencesCount <= 1;
}

In this code, we use the chars() method to convert the String into a stream of characters. Working with the integer representation is good enough for us because we don’t care about the concrete value. Then, we use Collectors.groupingBy() to get a map whose keys are the integer representations of the String characters, and the values are the number of occurrences of the character. Lastly, we remove all the even occurrence counts and finally check that there’s at most one odd occurrence count.

4. Conclusion

In this quick tutorial, we saw how to find out whether a given String is a palindrome or not. Additionally, we answered a common interview question: can a String become a palindrome by rearranging?

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.