Palindrome Program in Java

August 4, 2023

Check Whether or Not the Number is a Palindrome in Java

Given an integer input as the number, the objective is to check whether or not the given number is a palindrome. To do so, we’ll first reverse the string input using loops and recursion and check if it matches the original number.

Example
Input : 121
Output : Palindrome
palindrome or not in java

Check Whether or Not the Number is a Palindrome in Java Language

Given an input integer as a number, the objective is to check whether or not the given number integer is a Palindrome or not in Java Language. To do so we’ll reverse the the number using the modulo and divide operators and check if the reversed number matches the original number. Here are few methods to Check Whether or Not the Number is a Palindrome in Java Language,

  • Method 1: Using Iteration
  • Method 2: Using Recursion

Let’s discuss the above mentioned methods in detail in the upcoming sections. Don’t forget to check the blue box mentioned below for better understanding of the problem.

Checking for palindrome

Method 1: Using Iteration

Working

In this method we’ll use loops to break down the number into individual digits and the reassemble them in reverse order. We’ll use the modulo operator to extract the digits and divide operator to shorten the number. Finally we’ll compare if the reversed number matches the original number.

For a given integer number variable, we perform the following operations,

  • Run a while loop until the condition temp variable is not 0.
    • Extract the digits using modulo operator.
    • Using the formula reverse = reverse * 10 + last digit, we’ll keep updating the reverse variable.
    • We’ll shorten the number using divide operator.
  • Check if the reversed number matches the original number.

Let’s implement the above mentioned logic in Java Language.

Table of Contents

Java Code

Run 
public class Main
 {
   public static void main (String[]args)
   {
     //variables initialization
     int num = 12021, reverse = 0, rem, temp;

       temp = num;
     //loop to find reverse number
     while (temp != 0)
       {
     	rem = temp % 10;
     	reverse = reverse * 10 + rem;
     	temp /= 10;
       };

     // palindrome if num and reverse are equal
     if (num == reverse)
       System.out.println (num + " is Palindrome");
     else
       System.out.println (num + " is not Palindrome");
   }
 }

Output

12021 is Palindrome

Method 2: Using Recursion

Working

In this method we’ll use recursion to break down the number into individual digits and then reassemble them in reverse order. In the end we’ll check if the reversed number matches the original number.

For a given integer input as number, we perform the following,

  • Define a recursive function which takes the number as an argument.
    • Set the base case as number ==0.
    • Set the recursive step call as getReverse(num/10, rev).
  • Check if the returned value matches the original number.

Let’s implement the above mentioned logic in Java Language.

Java Code

Run 
public class Main
{
  public static void main (String[]args)
  {
    //variables initialization
    int num = 12021, reverse = 0, rem, temp;

    // palindrome if num and reverse are equal
    if (getReverse(num, reverse) == num)
     System.out.println (num + " is Palindrome");
    else
      System.out.println (num + " is not Palindrome");
  }
  
  static int getReverse(int num, int rev){
    if(num == 0)
        return rev;
    
    int rem = num % 10;
    rev = rev * 10 + rem;
    
    return getReverse(num / 10, rev);
}
}

Output

21021 is Palindrome

Advanced Methods

The above programs are algorithmic methods that will work with numbers.

The below programs may be a little advanced but will work with both numeric and string inputs

  • Method 3 : Using StringBuilder
  • Method 4: Using for Loop and charAt
  • Method 5: Using toCharArray
  • Method 6: Using Stack

Method 3: Using StringBuilder

Explanation of the code mentioned in the comments below –

Run 
class Main {

    public static void main(String[] args) {

        // convert these strings to either lowercase or uppercase to create consistency
        System.out.println(isPalindrome("radar".toLowerCase()));

        // Naman wouldn't have been palindrome if toLowerCase method wasn't used
        System.out.println(isPalindrome("Naman".toLowerCase()));
        System.out.println(isPalindrome("12321".toLowerCase()));
        System.out.println(isPalindrome("12345".toLowerCase()));
    }

    private static boolean isPalindrome(String string) {

        String reversed = new StringBuilder(string).reverse().toString();
        return string.equals(reversed);
    }
}

Output

true
true
true
false

Method 4: Using for loop and charAt

Explanation of the code mentioned in the comments below –

Run 
class Main {

    public static void main(String[] args) {

        // convert these strings to either lowercase or uppercase to create consistency
        System.out.println(isPalindrome("radar".toLowerCase()));

        // Naman wouldn't have been palindrome if toLowerCase method wasn't used
        System.out.println(isPalindrome("Naman".toLowerCase()));
        System.out.println(isPalindrome("12321".toLowerCase()));
        System.out.println(isPalindrome("12345".toLowerCase()));
    }

    private static boolean isPalindrome(String original) {

        String reversed = "";

        int len = original.length();

        for (int i = len - 1; i >= 0; i--) {

            reversed = reversed + original.charAt(i);
        }

        return original.equals(reversed);
    }
}

Output

true
true
true
false

Method 5: Using with toCharArray

Explanation of the code mentioned in the comments below –

Run 
class Main {

    public static void main(String[] args) {

        // convert these strings to either lowercase or uppercase to create consistency
        System.out.println(isPalindrome("radar".toLowerCase()));

        // Naman wouldn't have been palindrome if toLowerCase method wasn't used
        System.out.println(isPalindrome("Naman".toLowerCase()));
        System.out.println(isPalindrome("12321".toLowerCase()));
        System.out.println(isPalindrome("12345".toLowerCase()));
    }

    private static boolean isPalindrome(String original) {

        char[] data = original.toCharArray();

        int i = 0;
        int j = data.length - 1;

        while (j > i) {

            if (data[i] != data[j]) {
                return false;
            }

            ++i;
            --j;
        }

        return true;
    }
}

Output

true
true
true
false

Method 6: Using Stack

Explanation of the code mentioned in the comments below –

Please check DSA Blogs of PrepInsta here

Run 
import java.util.Stack;

class Main {

    public static void main(String[] args) {

        // convert these strings to either lowercase or uppercase to create consistency
        System.out.println(isPalindrome("radar".toLowerCase()));

        // Naman wouldn't have been palindrome if toLowerCase method wasn't used
        System.out.println(isPalindrome("Naman".toLowerCase()));
        System.out.println(isPalindrome("12321".toLowerCase()));
        System.out.println(isPalindrome("12345".toLowerCase()));
    }

    private static boolean isPalindrome(String original) {

        char[] data = original.toCharArray();

        Stack stack = new Stack<>();

        for (char c: data) {

            stack.push(c);
        }

        char[] data2 = new char[data.length];

        int len = stack.size();

        for (int i = 0; i < len; i++) {

            data2[i] = stack.pop();
        }

        var reversed = new String(data2);

        return original.equals(reversed);
    }
}

Output

true
true
true
false

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Working with Numbers

4 comments on “Palindrome Program in Java”


  • Satendra

    Class satendr{
    Public static void main (String[]args]{
    String s=” raja” ;
    String rev=” “;
    for (int i= s.length()-1; I>=0; i- -){
    rev= rev+ s.charAt(I);
    }
    if(s equals (rev)){
    System.out.println(” this is palindrome char: ” + rev);
    }
    else{
    System.out.println(” not palindrome char: “+ s );
    }}}


  • Shristy

    this there’s something wrong in for loop
    it should be like this
    for(int i=length-1;i>=0;i–)