Java 8 Streams - Checking Whether String is Palindrome Or Not

1. Overview

In this tutorial, We'll learn how to check the given string is palindrome or not in java 8 programming.

Palindrome means when the string is reversed, it is as same as the original string.

Example: Input string is abcba and when it is revered abcba. Here both are same so this string is called as palindrome.

2. Java Check String Palindrome - Two Pointers

This is a simple solution without using java 8 concepts. Here we take two pointer right and left. 

right value is initialed with 0 and left is with string length -1.

We take these two indexes values from string using charAt(index) method and compares them inside a while loop. If these two are not same then the given string is not a palindrome.

If equals, then increment right by 1 and decrement left by 1.

Repeat the comparisons steps until the right >= left or for any unmatch.

Look at the below example.

package com.javaprogramto.programs.strings.palindrome;

public class StringCheckPalindrome1 {

	public static void main(String[] args) {

		String input1 = "hello";
		boolean isPalindrome = isPalindrome(input1);
		System.out.println("Is " + input1 + " palindrome? " + isPalindrome);

		String input2 = "abcba";
		isPalindrome = isPalindrome(input2);
		System.out.println("Is " + input2 + " palindrome? " + isPalindrome);
	}

	private static boolean isPalindrome(String input) {

		int right = 0;
		int left = input.length() - 1;

		while (right <= left) {

			if (input.charAt(right) != input.charAt(left)) {
				return false;
			}

			right++;
			left--;
		}

		return true;
	}

}

Output:

Is hello palindrome? false
Is abcba palindrome? true

3. Java Check String Palindrome - for loop

Can we reduce no of lines from the above code? Yes by using for loop.

package com.javaprogramto.programs.strings.palindrome;

public class StringCheckPalindrome2 {

	public static void main(String[] args) {

		String input1 = "hello";
		boolean isPalindrome = isPalindromeWithForLoop(input1);
		System.out.println("Is " + input1 + " palindrome? " + isPalindrome);

		String input2 = "abcba";
		isPalindrome = isPalindromeWithForLoop(input2);
		System.out.println("Is " + input2 + " palindrome? " + isPalindrome);
	}

	private static boolean isPalindromeWithForLoop(String input) {
		
		int length = input.length();
		for (int i = 0; i < length; i++) {

			if (input.charAt(i) != input.charAt(length - i - 1)) {
				return false;
			}

		}

		return true;
	}

}

This code generates the same about as same as 2 pointer example.

4. Java Check String Palindrome - StringBuilder

Next, Can we reduce the above code to single line ? Answer is yes. It is done with the help of StringBuilder class and using its reverse() method.

reverse() method is to reverse the given input string and then compares it with the original input string.

	private static boolean isPalindromeWithStringBuilder(String input) {

		return input.equals(new StringBuilder(input).reverse().toString());
	}

This code also work without any errors and produces the same output.

5. Java 8 Program To Check String Palindrome 

Next, We'll see how to check the string palindrome in java 8 functional style programming.

Call IntStream.range() method with 0 to length/2. This is like running a loop from string index 0 to its length/2.

Next, call noneMatch() method with the predicate condition. This method returns true if no value is matched to the given predicate.

The solution can be implemented in a single line to make the code clean.

Look at the below code.

package com.javaprogramto.programs.strings.palindrome;

import java.util.stream.IntStream;

public class StringCheckPalindrome4 {

	public static void main(String[] args) {

		String input1 = "hello";
		boolean isPalindrome = isPalindromeInJava8(input1);
		System.out.println("Is " + input1 + " palindrome? " + isPalindrome);

		String input2 = "abcba";
		isPalindrome = isPalindromeInJava8(input2);
		System.out.println("Is " + input2 + " palindrome? " + isPalindrome);
	}

	private static boolean isPalindromeInJava8(String input) {

		return IntStream.range(0, input.length() / 2)
				.noneMatch(index -> input.charAt(index) != input.charAt(input.length() - index - 1));

	}

}

6. Conclusion

In this article, We've seen how to check the string is palindrome or not in older java and new java 8 streams api.

GitHub

while loop

StringBuilder

IntStream

How to reverse the words in the string?