Introduction

In this tutorial I will check how to verify a string is palindrome or not using Java programming language.

A palindrome is a technique, where a word, phrase, or sequence that reads the same backwards as forwards. For example, ehcache, madam, etc are examples of palindrome.

Prerequisites

Java

Check Palindrome

I will use here few techniques to present how to check a string is palindrome or not.

1st Approach

In first approach, I will use while loop to extract each char from the given string to test for palindrome.

public static boolean isPalindromUsingIndex(final String str) {
	int firstIndex = 0;
	int lastIndex = str.length() - 1;

	boolean result = true;

	while (true) {

		if (firstIndex >= lastIndex) {
			break;
		}

		char first = str.charAt(firstIndex);
		char last = str.charAt(lastIndex);

		if (first != last) {
			result = false;
			break;
		}

		firstIndex++;
		lastIndex--;

	}

	return result;
}

2nd Approach

In second approach, I will use while loop and charAt() method as I have used in the previous example but in different way.

public static boolean isPalindromUsingCharAt(final String str) {
	int right = 0;
	int left = 1;

	int len = str.length();

	while (str.charAt(right) == str.charAt(len - left) && left < (len / 2)) {
		left++;
		right++;
	}

	if (str.charAt(right) == str.charAt(len - left)) {
		return true;
	}

	return false;
}

3rd Approach

In this approach, I will use length(), charAt() and for loop to determine palindrome.

public static boolean isPalindromUsingLength(final String str) {
	int l = str.length();
	char[] chars = new char[l];

	for (int j = l - 1; j > -1; j--) {
		chars[l - j - 1] = (str.charAt(j));
	}

	if (str.equals(String.valueOf(chars))) {
		return true;
	}

	return false;
}

4th Approach

I will determine whether a string is palindrome or not by iterating only half of the given string’s length.

public static boolean isPalindromUsingHalfLength(final String str) {
	int len = str.length();

	for (int i = 0; i < len / 2; i++) {
		if (str.charAt(i) != str.charAt(len - i - 1)) {
			return false;
		}
	}

	return true;
}

5th Approach

Now I will verify using the reverse() method of the StringBuffer or StringBuilder class.

Remember String class does not have reverse() method as it is a final class.

public static boolean isPalindromUsingReverse(final String str) {
	StringBuilder sb = new StringBuilder(str);
	// StringBuffer sb = new StringBuffer(str);

	if (str.equals(sb.reverse().toString())) {
		return true;
	}

	return false;
}

Testing Palindrome

Now creating a main method in the class and executing the main method by calling the above methods for checking palindrome will give you the expected output.

public static void main(String[] args) {
	System.out.println(isPalindromUsingIndex("ehcache"));
	System.out.println(isPalindromUsingCharAt("ehcache"));
	System.out.println(isPalindromUsingLength("ehcache"));
	System.out.println(isPalindromUsingReverse("ehcache"));
	System.out.println(isPalindromUsingHalfLength("ehcache"));
}

Running the above code will give you below output:

true
true
true
true
true

Source Code

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