Wednesday, December 26, 2012

How to check if a number is a palindrome or not in Java - Example

How to check if a number is a palindrome or not is a variant of  popular String interview question how to check if a String is a palindrome or not. A number is said to be a palindrome if number itself is equal to reverse of number e.g. 313 is a palindrome because reverse of this number is also 313. On the other hand 123 is not a palindrome because reverse of 123 is 321 which is not equal to 123, i.e. original number. In order to check if a number is a palindrome or not we can reuse the logic of How to reverse number in Java. Since in most of interview, you are supposed to solve this question without taking help from API i.e. only using basic programming construct e.g. loop, conditional statement, variables, operators and logic. I have also seen programmer solving this question by first converting integer to String and than reversing String using reverse() method of StringBuffer and than converting String back to Integer, which is not a correct way because you are using Java API. Some programmer may think that this is just a trivial programming exercise but it’s not. Questions like this or Fibonacci series using recursion can easily separate programmers who can code and who can’t. So it’s always in best interest to keep doing programing exercise and developing logic.

Java program to check if number is palindrome or not

How to check if a number is palindrome or not in Java with ExampleHere is a simple Java program which finds if a number is a palindrome or not. This program does not use any API method instead it uses division and remainder operator of Java programming language to determine if number is palindrome or not. Programming logic to reverse a number is encapsulate in reverse() method and isPalindrome(int number) reuse that logic to test if a number is palindrome or not.

import java.util.Scanner;
/**
 * This Java program takes an input number from command line and integer array
 * and check if number is palindrome or not. A number is called palindrome
 * if number is equal to reverse of number itself.
 *
 * @author Javin Paul
 */

public class PalindromeTest {
 
 
    public static void main(String args[]){
     
        Scanner scanner = new Scanner(System.in);      
        //int number = scanner.nextInt();
        int[] numbers = {1, 20, 22, 102, 101, 1221, 13321, 13331, 0, 11};
     
        for(int number: numbers){
            System.out.println("Does number : "
               + number +" is a palindrome? " + isPalindrome(number));
        }            
    }

    private static boolean isPalindrome(int number) {
        if(number == reverse(number)){
            return true;
        }
        return false;
    }
 
     
    private static int reverse(int number){
        int reverse = 0;
     
        while(number != 0){
          reverse = reverse*10 + number%10;
          number = number/10;
        }
             
        return reverse;
    }

}

Output
Does number : 1 is a palindrome? true
Does number : 20 is a palindrome? false
Does number : 22 is a palindrome? true
Does number : 102 is a palindrome? false
Does number : 101 is a palindrome? true
Does number : 1221 is a palindrome? true
Does number : 13321 is a palindrome? false
Does number : 13331 is a palindrome? true
Does number : 0 is a palindrome? true
Does number : 11 is a palindrome? true


That's all on how to check if a number is a palindrome or not. As I said this is a good programming exercise particularly for beginners who have just started learning Java programming language, as it teaches how to use division and remainder operator in Java. Once again Fibonacci series, Palindrome are classical coding question and should not be missed during preparation.

Other Java programming exercise for practice

1 comment :

Anonymous said...

I think is better to write one line:
return number == reverse(number);
instead of:
if(number == reverse(number)){
return true;
}
return false;

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